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El Niño Southern Oscillation in a Changing Climate
Chapter · November 2020
DOI: 10.1002/9781119548164.ch19
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19
ENSO Impact on Marine Fisheries and Ecosystems
Patrick Lehodey1, Arnaud Bertrand2, Alistair J. Hobday3, Hidetada Kiyofuji4, Sam McClatchie5,
Christophe E. Menkès6, Graham Pilling7, Jeffrey Polovina8, and Desiree Tommasi9
ABSTRACT
El Niño events were first perceived several centuries ago as a dramatic change in the marine resources along the
Peruvian coast. It is now recognized as part of the world’s largest natural climate fluctuation: the El Niño
Southern Oscillation (ENSO). There is a rapidly growing body of scientific literature showing that ENSO has
physical and ecological impacts throughout the Pacific Ocean and more broadly across the other oceanic basins
through atmospheric teleconnections. This review details a range of these examples in all major ecosystems
impacted by ENSO in the Pacific Ocean. Teleconnections with other basins are also discussed, as are the diversity of changes associated with ENSO phases and their consequences on fisheries sustained by these ecosystems.
Information is provided on the emerging complexity of the connection between ENSO and the ocean ecosystems, and particularly the diversity of El Niño types, characterized by eastern and central spatial patterns and
differences in intensity. As these mechanisms become better understood, useful predictive capacity for ecosystem
and fisheries management will result. However, growing evidences suggest that climate change may have already
started interacting with ENSO dynamics and effects, complicating mechanistic understanding.
19.1. INTRODUCTION
Ocean ecosystems and fish stocks dynamics are strongly
affected by multiple scales of climate variability (e.g.,
Lehodey et al., 2006; Drinkwater et al., 2010). Impacts on
marine species can be on movements and migration patterns or on biological and environmental conditions that
affect the survival of individuals, either directly (e.g.,
starving larvae and juveniles) or through cascading and
delayed events in the food web. Effects on growth,
reproduction, and behavior are also important and ubiquitous. The El Niño Southern Oscillation (ENSO) originates in the equatorial Pacific, where it is the dominant
mode of interannual variability. It leads to physical and
ecological impacts throughout the Pacific basin, with
important connections in the other oceanic basins (chapters 14 and 15).
El Niño was originally recognized by South American
fishers in the 1600s as an unusual warm ocean current off
the coast of Peru (Garcia‐Herrera et al., 2008; Grove &
Collecte Localisation Satellite, Ramonville St Agne, France
Institut de Recherche pour le Développement (IRD),
MARBEC, Univ Montpellier, CNRS, Ifremer, IRD, Sète,
France
3
CSIRO Oceans and Atmosphere, Hobart, TAS, Australia
4
National Research Institute of Far Seas Fisheries, Japan Fisheries
Research and Education Agency, Shimizu, Shizuoka, Japan
5
38 Upland Rd, Huia, Auckland, 0604, New Zealand
6
Institut de Recherche pour le Développement (IRD),
ENTROPIE (IRD/CNRS/Univ. La Réunion) Nouméa, New
Caledonia
7
The Pacific Community (SPC), Noumea, New Caledonia
8
196 Pauahilani Pl., Kailua, HI, USA
9
Institute of Marine Sciences, University of California Santa
Cruz, Santa Cruz, CA, USA, and NOAA Southwest Fisheries
Science Center, La Jolla, CA, USA
1
2
El Niño Southern Oscillation in a Changing Climate, Geophysical Monograph 253, First Edition.
Edited by Michael J. McPhaden, Agus Santoso, and Wenju Cai.
© 2021 American Geophysical Union. Published 2021 by John Wiley & Sons, Inc.
429
430
EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE
Adamson, 2018). The name (Christ Child in Spanish),
first used in scientific literature in 1893, was linked to the
time of year (around December) during which these
warm water events tended to occur. The understanding
that El Niño was a basin‐scale phenomenon involving an
alternate La Niña phase and coupling between the
atmosphere and the ocean came later (Bjerknes, 1966
chapter 2).
Analyses of the spatial patterns of El Niño events
reveals two main types. Eastern Pacific (EP) El Niños
were first recognized, while central Pacific (CP, also
sometimes called Modoki) El Niños have increased in frequency since the 1980s. Although the development of
each ENSO phase involves common ocean‐atmosphere
feedback processes, each event varies in intensity and
impact (Timmermann et al., 2018). Consequently, the
impacts of ENSO on the biology and ecology of marine
species, including many exploited species, are diverse,
complex, and often difficult to distinguish from other
drivers of variability, e.g. the Pacific Decadal Oscillation
(PDO) or Indian Ocean Dipole (Saji et al., 1999; Webster
et al., 1999). This chapter provides a series of regional
case studies covering the main large Pacific ecosystems
and highlights the diversity of changes associated with
ENSO phases and their consequences on the fisheries
sustained in each region. Key questions and future needs
to progress understanding of this complex multidisciplinary research are then discussed, with particular
attention to developing useful predictive capacity for ecosystem and fisheries management.
19.2. THE HUMBOLDT CURRENT SYSTEM
It is in the northern Humboldt Current System (HCS),
off Peru, that the impacts of canonical (EP) El Niño
events on ecosystem and fisheries are most notable
(Chavez et al., 2008). Located along the coast, this
upwelling region presently supports more fish catch per
unit area than any other region in the oceans; it represents 0.1% of the world ocean but produces up to 10% of
the world fish catch (Chavez et al., 2008). For a long time,
it was thought that the impact of El Niño events on
important fishery resources was straightforward, with a
negative impact on anchovy and a positive one on sardine, as demonstrated by the variability in catch landings
(e.g. Bakun & Broad, 2003). More recently, this simple
vision has been questioned because various factors occurring at different spatiotemporal scales need to be considered to understand the impact of each El Niño (Bertrand
et al., 2004). First, it is important to discriminate between
the eastern Pacific (EP) and the central Pacific (CP) El
Niños. The former can dramatically affect the HCS; however, the latter does not always result in HCS impacts.
Surface temperature anomalies can even be negative in
coastal Peru during CP El Niño. For this reason, in this
section we will only focus on EP El Niño, as a clear link
with the HCS has been demonstrated.
El Niño events are typically triggered by anomalies in
the wind field in the western equatorial Pacific. These
anomalies generate downwelling Kelvin waves that propagate eastward and subsequently poleward along the
coasts of North and South America (see chapters 14 and
15), deepening the thermocline and making coastal
upwelling “inefficient” in terms of nutrient enrichment.
The usually colder and nutrient‐enriched surface waters
in the HCS are replaced by warmer and nutrient‐depleted
waters. The area of productive coastal water is reduced
dramatically. The weaker upwelling allows the less productive oceanic ecosystem to extend toward the coast
(Bertrand et al., 2008; Chavez et al., 2002) (Figure 19.1).
The HCS ecosystem responses can be very different
across different ENSO events. Upwelling and the associated biological system recovered rapidly after the 1997–
1998 El Niño, suggesting weaker ecological impacts than
those observed in the seasons after the 1972–1973 and
1982–1983 events (Bertrand et al., 2004; Escribano et al.,
2004). For instance, in the case of pelagic fish, several
phenomena at different spatiotemporal scales are thought
to affect the diversity of responses (Bertrand et al., 2004),
including, by decreasing order on the time scale: (i) the
decadal regime; (ii) the frequency, intensity and duration
of ENSO events; (iii) the conditions of species populations before the event, especially those that are heavily
exploited; (iv) the fishing pressure and other mortality
sources, e.g. predation; (v) reproductive behavioural
responses by the fish, including timing and location of
spawning; and (vi) the presence of local efficient
upwellings.
19.2.1. Impact on Benthic Species
The “tropicalization” of the Humboldt Current during
strong El Niño events has different impacts on benthic
species, with some winners and some losers. For crustaceans, high temperature during El Niño events can lead
to mass mortality of the brachurian crabs Romaleon seto­
sus and Platyxanthus oribigny off Peru and a migration
to deeper waters of Cancer porter, with negative impacts
on the artisanal crab fishery (Fischer & Thatje, 2016). In
contrast, the range of penaeid shrimps (e.g. Xiphopenaeus
riveti and Penaeus stylirostris) and the spiny lobster
Panuliris gracilis extends southward. Peruvian fishers
adjusted their fishing methods to catch greater quantities
of shrimp after the El Niño of 1982–1983 (Barber &
Chavez, 1986; Arntz et al., 2006; Thatje et al., 2008).
However, this tropicalization effect was less pronounced
during the two following extreme El Niño events in 1997–
1998 and 2015–2016.
Col
d
Coa
stal
Wa
te
r
Epipelagic layer
Oceanic water
Thermocline
ental
n
Conti
shelf
1
Oxygen minimum zone
Relative scale
Nutrients
Phytoplankton
Zooplankton
0
Distance from the shore (km)
250
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Oceanic water
0
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wa
ter
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ental
n
Conti
shelf
Oxygen minimum zone
1
250
Relative scale
Nutrients
Phytoplankton
Zooplankton
Distance from the shore (km)
0
0
Figure 19.1 Conceptual model of pelagic ecosystem changes associated with El Niño in the Humboldt Current
Ecosystem. In non–El Niño years (top panel), the thermocline is shallow, so the wind‐driven upwelling is highly
efficient to supply nutrients. The coastal ecosystem exhibits high biomass and primary productivity that extend far
from shore. It is dominated by large phytoplankton, supports a food web with large zooplankton, small pelagic
fish, seabirds, marine mammals, and fishers. An oceanic low biomass and primary productivity ecosystem is
found offshore of the coastal ecosystem when nutrients are depleted. It is dominated by picophytoplankton,
whose grazers are protists with similar growth rates, creating an efficient recycling system. A complex food web
evolves with a smaller proportion of the primary production reaching the upper trophic levels composed of
­oceanic species such as tuna or dolphin fish. Note that the presence of mesoscale to submesoscale eddies can
lead to the presence of several peaks of nutriments or phytoplankton. During eastern Pacific El Niño years (bottom
panel) the productive coastal area is reduced dramatically, and the oceanic ecosystem impinges close to the
shore. (Inspired from Chavez et al., 2002, and Guevara‐Carrasco & Bertrand, 2017)
432
EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE
Strong El Niño events are synonymous with high temperatures, floods, and heavy river discharges of sediment‐
laden freshwater in northern Peru. These impacts
negatively affect mollusc species, for example, reducing
biomass of the scallop Agropecten purpulatus. In contrast, in the south, a drastic (1982–1983), significant
(1997–1998), or moderate (1991–1992) increase in scallop
stock biomass was roughly proportional to the intensity
of warm anomalies in sea surface temperatures observed
during these events. These increases had a positive net
effect for Peruvian scallop fisheries that compensated for
the economic loss in the north.
The impact of La Niña events is opposite; however, the
increase in scallop landings in northern Peru does not
compensate for the decrease in the south (Badjeck et al.,
2009). Similarly, El Niño events are associated with
reduced surf clam Mesodesma donacium landings in Peru
and northern Chile but increased landings in southern
Chile (Ortega et al., 2016). Finally, in Peru and northern
Chile, increasing sea surface temperature (SST) enhances
the recruitment and availability of octopus prey items
during El Niño events, which leads to a dramatic increase
in Octopus mimus abundance (Arntz et al., 2006).
19.2.2. Impact on Demersal and Pelagic Species
The Peruvian hake (Merluccius gayi peruanus) is the
most abundant commercially exploited demersal fish in
the northern HCS. During an El Niño event, its availability, in particular juvenile abundance, increases near the
Peruvian coast. This is due to higher oxygen content and
possibly a temperature effect (Guevara‐Carrasco &
Lleonart, 2008). During El Niño events, the oxygen content
increases in coastal waters (Figure 19.1) due to the deepening of the oxygen minimum zone upper limit under the
action of coastal trapped waves forced by intense downwelling equatorial Kelvin waves (Gutierrez et al., 2008,
Espinoza Morriberón et al., 2018). Also, the El Niño conditions allow warmer and more oxygenated waters from
the equatorial region to reach the coast of Peru (Montes
et al., 2011; Espinoza Morriberón et al., 2018).
However, this intrusion into the coastal domain is
accompanied by a drastic increase in metabolic cost associated with warmer waters and thus a decrease in female
hake fecundity, with an overall negative impact on the
Peruvian hake population (Ballón et al., 2008). By contrast, El Niño events have a positive effect on the recruitment of common Merluccius gayi gayi and southern
Merluccius australis hake in central‐southern Chile (Payá
& Ehrhardt, 2005).
Strong EP El Niños affect the distribution of small and
medium pelagic fish (anchovy, sardine, mackerel, and jack
mackerel). They generally concentrate closer to the coast
during El Niños, avoid the warm waters in Northern Peru,
and in some cases, move into deeper waters (Barber &
Chávez, 1986; Alheit & Niquen, 2004; Bertrand et al.,
2004). The increased accessibility to fish close to the coast
results in elevated catches at the beginning of the events
(Alheit & Niquen, 2004). Furthermore, El Niño events
were reputed to produce massive die‐offs of anchovy
(Engraulis ringens) off the shores of Peru and northern
Chile but to favor other pelagic species such as sardine
(Sardinops sagax) or jack mackerel (Trachurus murphyi)
(Pauly & Tsukayama, 1987; Arntz & Fahrbach, 1996;
Bakun & Broad, 2003). However, more recent studies have
nuanced this classical view. In most cases, EP El Niño
events effectively produced a reduction in anchovy biomass, but subsequent recovery did not always follow the
same pattern (Bakun & Broad, 2003; Alheit & Niquen,
2004). Recovery was slow after El Niño events in 1972–
1973, 1977–1978, and 1982–1983, but rapid after the El
Niño of 1987 and 1997–1998. Finally, the El Niños of
1991–1992 or 2002–2003 had no perceptible impact on
anchovy biomass (Bertrand et al., 2004). Similarly, if sardine catch can increase during El Niños, it does not mean
that these events are favorable for the sardine population.
There is no clear spatial pattern emerging from the various
observed events (Gutiérrez et al., 2012). Overall, there is
apparently rather negative effects due to a reduction in fish
condition factor and gonosomatic index (Barber & Chávez,
1986; Bertrand et al., 2004; Cárdenas, 2009).
Finally, oceanic predators such as the bonito Sarda
chilensis, the dolphin fish Coryphaena hippurus, and the
yellowfin tuna Thunnus albacares follow the warm water
and move closer to the coast, increasing their availability
to fishers (Barber & Chávez, 1986). In central‐south
Chile, El Niño impacts are less clear for most species. For
instance, catch variability in the anchovy fishery is seemingly not related with ENSO (Cubillos et al., 2007),
although El Niños seem to negatively impact the recruitment of the common sardine Strangomera bentincki in
this region (Cubillos & Arcos, 2002).
To conclude, while ENSO events can dramatically
affect pelagic fish populations of the HCS and their fisheries, they do not seem to play a major role in their long‐
term dynamics, which instead seem to be controlled by
decadal ocean conditions and fishing pressure (Alheit &
Niquen, 2004; Bertrand et al., 2004; Gutierrez et al.,
2007; Salvatteci et al., 2018).
19.3. THE EQUATORIAL PACIFIC AND TROPICAL
TUNA FISHERIES
The equatorial and tropical Pacific Ocean support the
world’s largest tuna fisheries, dominated by four tuna
species: skipjack (Katsuwonus pelamis), yellowfin
(Thunnus albacares), bigeye (T. obesus), and albacore
(T. alalunga). These represent >90% of the total catch
ENSO Impact on Marine Fisheries and Ecosystems 433
taken by industrial fleets. Skipjack and yellowfin spend
their entire lives in tropical waters, or tropical waters
transported into more temperate latitudes by currents
(Kuroshio and East Australian currents). They are fast
growing and reach first maturity early (~11 and
20 months, respectively, for skipjack and yellowfin). In
contrast, bigeye tuna and albacore have longer life spans
and mature later (~3 and 4.5 years respectively) and are
thus less productive stocks and respond on longer time
scales to environmentally driven variability in juvenile
recruitment. They have extended habitats ranging from
equatorial waters, where they spawn, to temperate waters,
with a well‐observed feeding migration for albacore.
Skipjack and yellowfin are caught mainly by surface
fishing gear (pole‐and‐line and purse seine), while large
yellowfin, bigeye, and albacore are targets of longline
fishing gear in subsurface waters.
El Niño or La Niña events directly affect horizontal
movements and vertical distributions of these tuna
species. Spatial distributions of purse seine catch and tagging data in the central western Pacific have revealed a
spatial shift in abundance that follows the eastward
extension of the warm pool during an El Niño event
(Lehodey et al., 1997). These large east‐west displacements of skipjack in the equatorial region associated with
ENSO can be simulated with the spatially explicit ecosystem and fish population dynamics model SEAPODYM
(Lehodey et al., 2008; Senina et al., 2008). Model results
are consistent with the observed changes in purse seine
fishing grounds and tagging data (Figure 19.2). They suggest that the eastward extension (westward contraction)
of the species and fisheries distributions during El Niño
(La Niña) phases are driven by changes in temperature,
prey, and dissolved oxygen concentration. As discussed
previously, not all ENSO events are equal. For example,
despite being one of the largest events (Figure 19.2), the
2015–2016 El Niño did not strongly impact primary production in the eastern Pacific Ocean, as did other (EP) El
Niño events.
The strong temperature gradient of the thermocline is
a physical barrier for skipjack and juvenile yellowfin and
bigeye tuna, while adult yellowfin and bigeye tuna can
dive below the thermocline to chase mesopelagic prey.
Therefore, changes in the vertical thermal structure of
the ocean associated with ENSO can potentially impact
the catchability of tuna species by different fishing gears.
Purse seiners targeting surface tuna use the top of the
thermocline as a lower barrier to trap tuna schools.
Typically, the thermocline in the western equatorial
Pacific is shallower (deeper) during El Niño (La Niña)
than in neutral conditions. The opposite pattern occurs
in the eastern Pacific. With modern purse‐seine nets that
can reach depths greater than 200 m, these changes in
thermocline depths have now limited impact. However, it
was not the case in the 1980s when the purse‐seine fleet
started to develop and explore the western Pacific with
nets adapted to the eastern region. The fishermen discovered that tuna schools were able to escape below their
shallow nets. The contraction and extension of adult
bigeye and yellowfin vertical habitat associated with
ENSO phases and the depth of the thermocline has been
shown from electronic tagging data (e.g., Brill et al.,
1999; Schaefer & Fuller, 2002). The impact on their
catchability by longline has been observed or detected
from analyses of catch data (Bertrand et al., 2002;
Bigelow et al., 2002) but may be also linked to change in
horizontal habitats and migrations associated with
ENSO.
The environmental variability associated with ENSO
can impact the survival of tuna larvae and produce high
and low peaks in their abundance. Unfortunately, there
are no direct abundance surveys, such as the eggs and
larvae sampling commonly used for coastal small pelagic
stocks, to monitor such large‐scale variability of tropical
tuna species larval densities. However, this variability
propagates through the population structure and can be
detected with some delay in the exploited stock, either
through the analysis of catch rates and size frequencies of
catch or as inferred from model and stock assessment
analyses. For long‐living species, e.g. bigeye and albacore,
the decreasing growth rate with age and its natural variability over time and space leads to a cohort (age) signal
more and more difficult to detect in larger fish. Therefore,
the recruitment variability associated with ENSO, i.e. low
or high peaks of abundance in the first cohort, is
smoothed and damped while it combines with the internal
dynamic processes of the species propagating toward the
older cohorts (Lehodey et al., 2010; Sibert et al., 2012;
Senina et al., 2017). Thus, the signal is easier to detect in
short‐living species like skipjack.
The first evidence of an ENSO impact on skipjack
larvae recruitment was obtained from simulations with
the model SEAPODYM (Lehodey et al., 2003, 2006;
Senina et al., 2008). El Niño events were shown to be
favorable to strong larvae recruitment as illustrated by a
(negative) correlation with the Southern Oscillation
Index (SOI) (www.cpc.ncep.noaa.gov/data/indices/soi).
In this model, the spawning and subsequent recruitment
of surviving larvae are predicted from the local biomass
of adult fish (spawners) with a density dependence
function (Beverton & Holt) and environmental conditions that can be more or less favorable for the concerned
species. This is expressed using a spawning index that
combines the species water temperature preference and
the abundances of prey (i.e. zooplankton) and predators
(micronekton) of larvae. The parameters that control
these processes are estimated through a quantitative
approach using catch data, size frequencies of catch, and
4.0
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(moving average)
R MFCL detrended
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mid-Dec 2015 (El Niño)
45
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195
Skipjack biomass in metric tonnes per km2
0
0.5
1
Figure 19.2 Impact of ENSO on Pacific skipjack tuna population and fisheries. (a) Comparison of skipjack tuna
recruitment index estimated with the model MUTIFAN‐CL (MFCL) for the Western Central Pacific Fisheries
Commission (WCPFC) stock assessment study (McKechnie et al., 2016) and the SOI index (reversed axis). Note
that the recruitment index has been detrended as the estimate indicates a linear increase. (b) Biomass distribution
of skipjack tuna predicted with SEAPODYM (t per km2) and observed catch (black circles) during typical La Niña
and El Niño situations.
ENSO Impact on Marine Fisheries and Ecosystems 435
tagging data (Senina et al., 2008). This ENSO‐larvae link
is confirmed by an independent estimate of recruitment
(Figure 19.2) of the Western Central Pacific Fisheries
Commission (McKechnie et al., 2016). In that case, the
recruitment series is estimated from catch and tagging
data, without any oceanographic information.
Similar positive effects of El Niños on early life stages
were detected with SEAPODYM in bigeye and yellowfin
tuna species, mainly in the eastern Pacific Ocean.
Favorable conditions for larvae survival increase during
El Niño events in the eastern Pacific Ocean and decrease
in the central region. These species with longer life spans
are also more susceptible to present decadal regimes of
high and low productivity due to the accumulation of
successive low or high peaks of recruitment driven by the
decadal modulation of ENSO. A dominance of either El
Niño or La Niña events is observed over multiyear
periods, possibly in correlation with the Pacific‐scale
Interdecadal Pacific Oscillation (IPO).
ENSO also impacts the far western Pacific tropical oceanic ecosystem (East of Indonesia, Philippines, and
Vietnam), where the variability is complicated by the additional influence of the Indian Ocean Dipole (IOD),
another interannual mode of variability developing in the
India Ocean (Saji et al., 1999; Webster et al., 1999). IOD
and ENSO are partially independent climate modes. Their
different phases occur together about 50% of the time
(Meyers et al., 2007). A positive IOD is associated with a
cold SST anomaly in the southeastern equatorial Indian
Ocean.
In the Indonesian region, the recent years have seen
contrasting climatic conditions. In 2014, ENSO and
IOD conditions were neutral. Then in 2015, an El Niño
developed in parallel with a positive phase of IOD. The
peak intensity in both El Niño and the positive IOD at
the end of 2015 coincided with a strong cold anomaly in
the southern Indonesian region, associated with a productive coastal upwelling south of Java and Sumatra
and in the Banda Sea. In early 2016, a strong negative
IOD developed together with a weak La Niña. The temperature anomaly south of Java and Sumatra became
positive, and the primary productivity became very
weak in the absence of coastal upwelling. The impact of
this variability on the bigeye larval recruitment has been
explored with the model SEAPODYM (Lehodey et al.,
2018). Although there are no observations to validate
the results, the simulation suggested that oceanographic
conditions during the combined warm phases of ENSO
and IOD in September 2015 were highly favorable to
tuna larvae survival but reversed to become unfavorable
in September 2016 during the cold phase. Future simulations and stock assessment studies may reveal if such
low and high peaks of recruitment can be detected in
the catch and adult population cohorts.
19.4. THE CENTRAL NORTH PACIFIC
The central North Pacific includes most of the North
Pacific Subtropical Gyre, a clockwise rotating gyre with
warm, low‐nutrient surface waters, bounded on the east
by the California Current, the south by the North
Equatorial Current, the west by the Kuroshio Current,
and the north by the Kuroshio Extension and North
Pacific currents (Howell et al., 2012). In spite of its low
productivity, it supports complex pelagic and insular
marine ecosystems, including tunas, billfishes, cetaceans,
sea turtles, seabirds, and coral reef fishes. The ecosystem
supports fisheries harvests by commercial longline vessels, local recreational fishers, and artisanal subsistence
fishers. The human communities in the region are located
in the center of the gyre in the State of Hawaii and in the
west in Micronesia, a region composed of five nations:
the federated states of Micronesia, Palau, Kiribati,
Marshall Islands, and Nauru, and three U.S. territories:
the Northern Mariana Islands, Guam, and Wake Island.
The physical impacts from El Niño events vary spatially in this region. The southern portion of the gyre,
south of about 15°N latitude, experiences the typical
equatorial ENSO impacts. The northern portion of the
gyre, north of about 25°N latitude, is influenced by midlatitude teleconnections that during an El Niño consist of
an intensified and southward‐shifted Aleutian Low
pressure system, especially during winter and spring,
resulting in a southward expansion of the westerly winds
and productive subtropical fronts (Howell et al., 2012).
During a La Niña, the Aleutian Low weakens, and westerlies and the subtropical fronts shift northward (Howell
et al., 2012). The Pacific Decadal Oscillation is a climate
mode that also manifests as an intensification or weakening of the Aleutian Low with a similar spatial structure
to ENSO in the northern gyre but operating on decadal
scales (Mantua et al., 1997; Di Lorenzo & Ohman, 2013;
Newman et al., 2016).
One of the longest time series for an apex species in the
Hawaiian Archipelago consists of population and demographic estimates for the endangered Hawaiian monk
seal (Monachus schauinslandi). Monk seal pup survival
estimates from 1984 to the present show the greatest
interannual variations occuring in populations located at
the northern islands and atolls (Lisianski Island, Pearl
and Hermes Reef, and Midway and Kure atolls) in the
northern portion of the Hawaiian Archipelago impacted
by the north‐south expansion or contraction of the westerly winds and productive fronts (Baker et al., 2007). In
particular, the latitude of the Transition Zone Chlorophyll
Front (TZCF; Polovina et al., 2017), a productive front
marking the center of a sharp north‐south gradient in
surface chlorophyll, shifts south, reaching these northern
atolls during the winter season impacted by a strong El
436
EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE
Niño, but it remains north of these northern atolls during
neutral or La Niña conditions (Baker et al., 2007). During
neutral or La Niña conditions, when the TZCF was north
of the atolls, annual new pup survival 1–2 years later was
about 50% compared with 80%–90% 1–2 years after the
TZCF reached the atolls during a strong El Niño (Baker
et al., 2007). When the TZCF reaches the northern atolls,
it may enhance the recruitment of prey for the pups that
forage on benthic fishes, while when the TZCF remains
north of the atolls, prey recruitment is low (Baker et al.,
2007). The 1‐to‐2‐year time lag between the position of
the TZCF and monk seal pup survival is thought to be
the time required between enhanced productivity and
prey recruitment (Baker et al., 2007). Increased entanglement of monk seals in marine debris and subsequent
mortality has been observed during El Niños (Donohue
& Foley, 2007). This observation can be linked to the
increased southward Ekman transport from the more
intense and southward‐shifted westerlies during strong El
Niños that deliver marine debris from the convergence
zone north of Hawaii to the Hawaiian Archipelago
(Howell et al., 2012).
The Hawaii longline fishery for tuna and swordfish
largely operates between 15°N and 30°N, north of the
equatorial ENSO impacts and mostly south of the midlatitude ENSO teleconnection impacts, which likely
explains why ENSO impacts in the fishery have generally
not been observed. However, during the 1997–1998 El
Niño, unusually high catches for bigeye tuna were documented at Palmyra Atoll (6°N latitude), a region that is
normally not part of the fishing grounds (Howell &
Kobayashi, 2006). These catches may have been due to
tuna moving eastward from the western Pacific and foraging in favorable habitat that developed around Palmyra
during El Niño conditions (Howell & Kobayashi, 2006).
In the populated islands and atolls of Micronesia,
impacts on local communities from a strong El Niño can
be severe (Rupic et al., 2018). During the developing
phase of El Niños, the region experiences strong westerly
wind bursts, heavy rainfall, and an increase in tropical
cyclones, together with a drop in SST and sea level
(Figure 19.3). During the mature and decaying phase,
easterlies return, drought conditions develop, and SST
and sea level increase (Figure 19.3). The more frequent
cyclones with heavy rain and strong storm surge damage
infrastructure and increase coastal erosion and saltwater
intrusion. During the decaying phase, drought conditions
may result in water shortages impacting human communities and agriculture (Rupic et al., 2018). During the
2014–2016 El Niño, drought conditions resulted in declarations of states of emergencies in Palau, Marshall
Islands, Guam, and the Northern Mariana Islands (Rupic
et al., 2018). The drop in sea level exposed shallow coral
reefs to the air, and intense solar radiation resulted in
coral mortality, while warmer SST resulted in coral
bleaching (Rupic et al., 2018).
19.5. THE CALIFORNIA CURRENT ECOSYSTEM
The California Current Ecosystem (CCE), between the
southernmost point of California and Oregon, is a moderately productive upwelling current system that supports
important fisheries. Historically they were dominated by
small pelagic species (anchovies and sardines) that exhibit
pronounced fluctuations in biomass over decadal periods.
In the 1990s, Pacific hake and market squid fisheries
developed and became dominant fisheries after the recent
collapse of the sardine stock. Superimposed on decadal
trends, high‐frequency variability is driven by ENSO
events that can have dramatic effects. They have been
reviewed recently (McClatchie, 2014) and synthesized in
this section.
El Niño events in the CCE are associated with anomalous higher temperature and lower salinity, deepening of
the mixed layer, increase in coastal dynamic height,
broadening and intensifying of northward coastal flow in
the Southern California Bight, temporary reversal of
the net southward flow, movement of the core of the
California Current further offshore, and reduction in the
intensity of coastal upwelling when trade winds weaken
(e.g., Chelton et al., 1982; Lynn & Bograd, 2002). Both
the offshore extent and the gradient in dynamic height
(i.e. the strength of this flow) vary between El Niño events
(Hayward, 2000). The anomalies associated with El Niño
off southern California and Baja California can be spatially patchy and variable from one event to another. For
example, during the 1982–1983 El Niño event, surface
temperature anomaly off southern California was in the
range of 0.5°C–3°C warmer on January 1983 but more
than 4°C warmer closer to the coast (Fiedler, 1984). This
large short‐term/spatial heterogeneity in SST was also
evident in the coastal sea surface height (Fiedler, 1984),
which is a more consistent indicator of El Niño than SST,
being less sensitive to high‐frequency variability.
Biological communities do not respond in the same
way to each El Niño or La Niña. El Niño events impact
the fish community assemblages of the Southern
California Bight as reported in studies back to the 1940s
(Hubbs, 1948; Radovich, 1960). Warm‐water species were
found further north than usual during both the strong
1983 and 1992 El Niño events. But the most spectacular
change was observed following the extreme El Niño of
1997/98, with a huge shift toward subtropical communities off southern California due to advective processes
(Chavez et al., 2002; Checkley & Barth, 2009; Lea &
Rosenblatt, 2000). Shifts in the northern ranges of 29
families of eastern Pacific tropical fishes were reported
into Southern Californian waters (Lea & Rosenblatt, 2000).
ENSO Impact on Marine Fisheries and Ecosystems 437
El Niño
Developing
J
F
M
A
Decaying
Mature
M J
J
A
S
O
N
D
J
F
M
A
M
J
J A
S
O
N
D
O
N
D
Surface
Winds
Heavy
Rainfall
Drought
Tropical
Cyclones
Sea
Level
Sea
Surface
Temperature
El Niño
Developing
J
F
M
A
M
Decaying
Mature
J
J
A
S
O
N
D
J
F
M
A
M
J
J
A
S
Coastal
Flooding
Rain
Flooding
Coastal
Erosion
Wildfire
Coral
Bleaching
Public
Health
Figure 19.3 Summary of forcings (top) and impacts (bottom) during a strong El Niño in Micronesia. (Reprinted
from Rupic et al., 2018)
These included species not recorded for almost a century
in this region, corroborating the physical evidence that
the 1997–1998 event was unusually intense. The authors
speculated that many arrivals would have been as ichthyoplankton (i.e. fish larvae), or juveniles perhaps associated with flotsam, but that larger fishes may have arrived
simply by swimming in suitable water masses.
19.5.1. Anchovy
Fiedler et al. (1986) found no consistent relationship
between El Niño and recruitment of anchovy in the CCE
in a 17‐year record covering three El Niño events.
However, their analysis showed that the extreme 1982–
1983 Eastern Pacific El Niño had a pervasively negative
effect on anchovy, affecting the growth, mortality, size‐at‐
age, fecundity, spawning distribution, and the movements
of the juveniles and adults. Growth was 47% slower on
average for the 1982 and 1983 year‐classes during the El
Niño compared to the 1978–1981 year‐classes. Both
adults and juveniles were smaller by as much as 10–25 mm
(over a mean size range of 10–15 cm) in El Niño years.
Fecundity was lower in 1983 and 1984 compared to
1980–1982, and this was attributed to smaller female size‐
at‐age (Fiedler et al., 1986). Lower fecundity did not
translate directly into proportional decrease in fish
recruitment due to density‐dependence mechanisms that
compensated for lower fecundity in 1983. In addition to
438
EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE
impacts on growth and fecundity, it was observed that
smaller anchovy moved into the nearshore (shallower
than 100 m) Southern California Bight from the south
during the 1982–1983 El Niño, while larger anchovy
moved north and west of Point Conception.
19.5.2. Sardine
In contrast to anchovy, sardine appear to recruit more
successfully in El Niño years. The increase in abundance of
sardines in the northwest of the CCE in 1992 was, for
example, attributed to the 1991–1992 El Niño event
(Hargreaves et al., 1994). Prior to this event, fishing reports
indicated only occasional catches in this region (Emmett
et al., 2005). The change in sardine abundance was associated with changes in the entire small pelagic fish community
(Emmett et al., 2005). The 1992 resurgence may not be
entirely due to El Niño because none of the previous events
back to 1957 produced a similar impact. A possible explanation is that El Niño may not have a detectable effect on
the sardine until a population threshold is exceeded.
High‐frequency variability in spawning and recruitment is rather typical of small pelagic stocks. This is the
case for sardines present along the U.S. West Coast (Lo
et al., 2005; Reiss et al., 2008; Weber & McClatchie, 2010),
and this has been frequently investigated (Smith, 1990;
McFarlane et al., 2002; Bakun & Broad, 2003; Emmett
et al., 2005; Agostini et al., 2007; Lo et al., 2010). The
spawning habitat is associated with isotherms of
14°C–15°C (Emmett et al., 2005) and sometimes, but not
always, shifts northward of San Francisco in El Niño
years (Bjorkstedt et al., 2010). Sampling of sardine egg
densities from research cruises between 1997 and 2012
showed high contrast between the 2002 La Niña and the
2003 El Niño (Bjorkstedt et al., 2010). Similarly, spawning habitat area was an order of magnitude larger (Reiss
et al., 2008), and daily egg production was lower
(Bjorkstedt et al., 2010) during the 1999 La Niña compared to the 1998 El Niño. However, recent work shows
that a northward shift of sardine spawning also occurs
with non‐ENSO warm events (Auth et al., 2018). During
the 2006 El Niño and 2007 La Niña, the ocean states
showed significant differences, but densities of sardine
eggs were not as dramatically different compared to the
2002–2003 El Niño to La Niña transition. Rather than
simply driven by change in temperature fields, Weber &
McClatchie (2010) suggest that variability in density of
sardine eggs and larvae associated to ENSO is likely due
to the combination of changes in favorability, predicted
from
temperature,
salinity
and
chlorophyll‐a
concentration, and extension of spawning habitat. These
changes occurred under the influence of offshore transport of upwelled nutrient‐rich water, driven by the main
wind patterns that vary with ENSO.
The sardine stock in the CCE has continuously declined
since the mid‐2000s and has reached its lowest level currently, leading to a closure of the fishery in 2015. It has
not reopened since. Recent results based on statistical
modeling suggest that concomitant decline of adult sardine, anchovy, and hake are occurring concurrently with
tropicalization of the southern CCE due to increased
presence of Pacific equatorial‐influenced water in the
inshore Southern California region (McClatchie et al.,
2018). This collapse has consequences for the ecosystem
as sardines are an important food source for several
marine species, including sea lions, salmon, brown pelicans, dolphins, and whales.
19.5.3. Market Squid
The market squid, Doryteuthis (formerly Loligo) opal­
escens, fishery off southern and central California is
highly variable, and catches have declined to as little as
10% of the catch quota during past El Niño events
(Vojkovich, 1998; Marinovic, et al., 2002; Jackson &
Domeir, 2003). During the recent 2015–2016 El Niño,
California market squid landings dropped by 65%,
leading to a $48 million decline in revenue for California
fishers (NMFS, 2017). Hypotheses proposed to explain
the decline associated with El Niño events include reduced
krill densities (Ish et al., 2004), since krill can make up as
much as 65% of the diet of market squid (Karpov &
Cailliet, 1979), or changes in the growth rate of squid
paralarvae in the month after hatching (Reiss et al., 2004,
2008). The species has a short life span, estimated between 6 and 9 months (Butler et al., 1999; Zeidberg et al.,
2006) to a maximum of 18 months (Spratt, 1979; Jackson,
1998), and the population can increase by orders of magnitude in a few generations during periods of rapid
growth (Reiss et al., 2004, 2008). This seems to be an
advantage for quick recovery after El Niño–related
crashes, especially in Southern California (Ish et al.,
2004). The rate of recovery may differ from one ENSO
event to another, and the mechanisms are complex and
incompletely understood (McInnis & Broenkow, 1978;
Zeidberg et al., 2006; Koslow & Allen, 2011).
19.6. THE NORTHEAST PACIFIC SUBPOLAR GYRE
The ocean circulation of the northeast Pacific region is
dominated by two oceanic gyres, the North Pacific
Subpolar Gyre to the north and the North Pacific
Subtropical Gyre to the south. The two circulation systems are separated by the North Pacific Current flowing
west to east from Japan to Canada. Off the British
Columbia coast, this slow and warm current splits into
the equatorward California Current and the poleward
Alaska Current (Batchelder & Powell, 2002). Here the
ENSO Impact on Marine Fisheries and Ecosystems 439
focus is on ENSO impacts on fisheries in the Alaska
Current and North Pacific Subpolar Gyre, including the
Gulf of Alaska. This biologically productive region supports subsistence and tribal fisheries, as well as lucrative
commercial and recreational fisheries. The Alaska
commercial fishery alone generated more than $4 billion
in sales in 2015, with landings revenue being dominated
by groundfish, salmon, and crab (NMFS 2017).
ENSO, via its effect on large‐scale atmospheric
circulation, is a major driver for interannual variability of
the North Pacific and also influences the decadal variability in this region (Di Lorenzo et al., 2013; Newman
et al., 2016). ENSO impacts can be transmitted from the
equatorial region to the northern region via atmospheric
teleconnections, particularly during winter (Schwing
et al., 2002; Alexander 2002; see also chapter 14).
Anomalous heating at the equator during an El Niño
event generates planetary waves that propagate to high
latitudes and influence the Aleutian Low (see chapter 14).
This triggers a change in atmospheric circulation at
higher latitudes: in particular, an intensification, deepening, and eastward movement of the Aleutian Low
pressure system, which results in a change in surface
winds and ocean transport pathways (Schwing et al.,
2002). A canonical eastern North Pacific response to an
El Niño event is characterized by cyclonic (counterclockwise) wind anomalies, an intensification of poleward
winds along the North America coast, stronger downwelling, anomalously negative sea‐level pressure anomalies, warmer ocean temperatures, increased ocean
stratification, and stronger poleward flow along the west
coast of North America (Schwing et al., 2002). The
opposite patterns are true for La Niña.
Remote ocean forcing, whereby changes in the water
column structure at the equator excite trapped waves that
move poleward along the North American coast, can also
generate anomalous North Pacific sea surface temperature (see also chapter 14). However, their effect is largely
relegated to southern coastal regions of the northeast
Pacific (Newman et al., 2016). It is important to point out
that because North Pacific ENSO effects are mediated by
atmospheric teleconnections, which are also affected by
random atmospheric noise, not all ENSO events have the
same strong impacts on North Pacific atmospheric
circulation and ocean dynamics (Newman et al., 2016).
One of the most striking consequences of the strengthening of poleward transport and increased ocean temperatures observed during El Niño events is the dramatic
change in distribution and range expansions of many fish
and invertebrate species. Fishers may need to move away
from their usual fishing grounds as a result of these
dramatic changes in fish availability. For instance, during
the 1982–1983 El Niño, triggerfish were observed in
Alaska, 2800 km north of their previous northern record
(Pearcy & Schoener, 1987). During that same event,
market squid abundance increased in Alaska, but they
disappeared from their usual fishing grounds in southern
California (Pearcy & Schoener, 1987).
Changes in distributions are also evident for planktonic organisms, with more tropical plankton species present in the Alaska Current region during El Niño events
(Mackas & Gailbraith, 2002). Changes in food web structure, brought about by anomalous advection or changes
in ocean mixing and nutrient availability, can affect small
pelagic fish and the predators that feed upon them. El
Niño events have been associated with reduced availability of forage fish and extensive seabird die‐offs from
starvation in the Gulf of Alaska (Bailey et al., 1995;
Morgan, 1999).
Variations in ocean transport associated with El Niño
also impact fish recruitment. Pacific halibut (Hippoglossus
stenolepis) is a winter offshore spawning groundfish. To
ensure their survival, its larvae must make their way from
offshore spawning areas in the Gulf of Alaska to nursery
grounds on the coastal shelf. Doing so requires crossing
the fast‐flowing Alaska coastal stream current (Bailey &
Picquelle, 2002). Submarine canyons traversing the shelf
act as larval transport corridors, but the efficacy of such
transport pathways is modulated by large‐scale atmospheric forcing. Pacific halibut larval abundance in
nursery areas and year‐class recruitment strength are
higher during El Niño events, as the increased coastal
current speed acts to entrain more offshore waters, and
larvae, onto the shelf (Bailey & Piquelle, 2002). Variability
in cross‐shelf transport driven by large‐scale atmospheric
forcing also controls Greenland halibut (Reinhardtius
hippoglossoides) and Pacific halibut recruitment in the
eastern Bering Sea (Duffy‐Anderson et al., 2013; Vestfals
et al., 2014).
Recent work also demonstrates that different types of
El Niño events (EP or CP), by eliciting different teleconnections, have distinct influences on North Pacific
decadal scale oceanic and ecosystem variability (Kilduff
et al., 2015). Both the PDO and the North Pacific Gyre
Oscillation (NPGO), two indices of low frequency oceanic variability in the North Pacific Ocean, are influenced by El Niño (Newman et al., 2016, Di Lorenzo
et al., 2010). EP events, via atmospheric teleconnections,
force variability in the Aleutian Low pressure system and
hence affect the PDO (Alexander et al., 2002; Newman
et al., 2016). By contrast, CP events impact the North
Pacific Oscillation and the NPGO (Di Lorenzo et al.,
2010). A positive PDO pattern is characterized by anomalous warming all along the coast of North America
(Mantua et al., 1997; Figure 19.4). Variability in the
PDO index was associated with fluctuations in salmon
catch, with Alaska stocks positively, and California,
Oregon, and Washington stocks negatively correlated
440
EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE
with the PDO (Mantua et al., 1997). The contrasting
response of salmon survival rates to increased ocean
temperature is a result of a differential response of food
web productivity to increased ocean warming in the two
systems (Malik et al., 2015, Kilduff et al., 2015). By contrast, CP teleconnections influence low‐frequency variability of the NPGO (Di Lorenzo et al., 2010). The
NPGO, unlike the PDO, is associated with anomalously
cold coastal waters in the Gulf of Alaska (Kilduff et al.,
2015; Figure 19.4). With the increase of CP events since
the 1980s, the NPGO has intensified and now appears to
be a better index of salmon survival than the PDO
(Kilduff et al., 2015; Figure 19.4). This intensification of
the NPGO is also associated with increased coherence of
survival rates among different salmon stocks (Kilduff
et al., 2015; Figure 19.4).
(a)
19.7. THE NORTHWEST PACIFIC
The northwest Pacific (NWP) is one of the most productive fishing areas in the Pacific Ocean. This region
supports many commercial fisheries (e.g., Yasuda et al.,
2003), including small pelagic species such as Japanese
sardine (Sardinops melanostictus), Japanese anchovy
(Engraulis japonicus), Pacific saury (Cololabis saira),
chub mackerel (Scomber japonicus), and Japanese
common squids (Todarades pacificus), as well as highly
migratory species, including albacore tuna (Thunnus
alalunga), skipjack tuna (Katsuwonous pelamis), Pacific
bluefin tuna (Thunnus orientalis), and the neon flying
squid (Ommastrephes bartramii).
The major oceanographic features in the NWP are
(Figure 19.5) the North Equatorial Current (NEC), the
(c)
SPATIAL NPGO & PDO SSTA PATTERNS
PC1 OF SALMON SURVIVAL
60N
COHO PC1
NPGO
40N
1
0
20N
–1
0
R = 0.71
20S
(d)
(b)
60N
CHINOOK PC1
NPGO
40N
1
0
20N
–1
0
R = 0.75
20S
1980
200W
–0.6
–0.4
–0.2
150W
0
R
0.4
1990
1995
2000
2005
Year
100W
0.2
1985
0.6
Figure 19.4 Adapted from Kilduff et al. (2015). Spatial correlation maps of the (a) annual North Pacific Gyre
Oscillation (NPGO) and (b) annual PDO with wintertime (January–March) sea surface temperature anomalies.
Colored rectangles and ovals highlight differences between the NPGO and PDO spatial signals. Correlation of
dominant mode of variability (first principal component) of (c) coho salmon (red line) and (d) chinook salmon
(blue line) survival rates with the NPGO (black).
ENSO Impact on Marine Fisheries and Ecosystems 441
warm Kuroshio Current formed off the southern part of
Japan (Tsujino et al., 2006), and the Kuroshio Extension
(KE), which lies at approximately 35°N (Qiu & Chen,
2005). The cold Oyashio current flowing along the
northern coast of Japan joins the Kuroshio in a transition
zone between 35°N and 40°N, creating a productive area
characterized by high surface chlorophyll‐a concentration
and defined as the Transition Zone Chlorophyll Front
(TZCF; Polovina et al., 2001, 2017; Figure 19.5). The KE
delineates the northern boundary of the subtropical
mode water (Holbrook & Maharaj, 2008; Oka, 2009). It
has been shown to shift between extended and contracted
regimes with less energetic conditions at interannual time
scales, likely in relation to the PDO (Qiu & Chen, 2005).
Studies on climate variability and fisheries oceanography of the NWP have focused more on the PDO and
multidecadal regimes in species biomass and total catches
than on ENSO interannual variability (Mantua et al.,
1997; Yatsu et al., 2013; Wang et al., 2012). Decadal fluctuations have been detected in total zooplankton biomass
and significantly correlated to wintertime PDO (Chiba
et al., 2006). Decadal variations potentially driven by the
PDO phases have also been found in distributions of
small pelagic fishes such as sardine, anchovy, and saury
(Noto & Yasuda, 1999; Tian et al., 2004). More recent
research (Ichii et al., 2018) indicated that saury recruitment variability is related to the winter SST in
the Kuroshio or the spring chlorophyll‐a concentration in
the Kuroshio‐Oyashio transition area. Therefore, the
observed multiyear extended and contracted regimes of
the KE can easily produce different decadal phases of
high and low recruitment. However, the mechanisms
explaining the relationships with climate indices as the
PDO index and fluctuation in abundance of marine
species or shift in ecosystem regimes are still far from
being fully understood. For instance, theoretical modeling (e.g., Di Lorenzo & Ohman, 2013) suggests that the
cumulative integrations of white‐noise (high‐frequency)
atmospheric forcing can generate red‐noise (low‐­
frequency) responses in oceanographic variables (as for
the PDO) and thus generate marine population responses
that are characterized by different regimes and strong
transitions.
The influence of ENSO has been detected among the
large, highly migratory species inhabiting the NWP. Based
on Japanese longline fisheries data, the North Pacific albacore migration patterns seem more widely dispersed in El
Niño years (Kimura et al., 1997). Also, given that all tuna
(a)
(b)
110°
50°
120°
130°
140°
150°
160°
Oyashio
40°
KBF
AN
JAP
170°
110°E
120°E
130°E
140°E
150°E
160°E
50°N
40°N
TZCF
WCR
Kuroshio Extension
30°N
30°
Kuroshio
Kuroshio recirculation
20°N
20°
NEC
10°N
10°
ME
MC
0°
–10°
NECC
HE
NGCC
0°
10°S
Figure 19.5 (a) Major oceanographic feature in the northwest Pacific Ocean. NEC: North Equatorial Current;
NECC: North Equatorial Counter Current; MC: Mindanao Current; ME: Mindanao Eddy; HE: Halmahera Eddy;
NGCC: New Guinea Counter Current; KBF: Kuroshio Bifurcation; WCR: Warm Core Ring. (b) Seasonal climatological Chl‐a distribution from NASA Ocean Color Web (https://oceandata.sci.gsfc.nasa.gov/MODIS‐Aqua).
TZCF: Transition Zone Chlorophyll Front defined as 0.2 mg/m3 (Polovina et al., 2001).
170°E
442
EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE
species spawn in tropical warm waters under the influence
of ENSO, it can be assumed that their spawning habitats
and the subsequent fish recruitment are all impacted by
ENSO variability, as has been demonstrated for skipjack
tuna (cf section above). The result can be a delayed fluctuation in abundance of juvenile and adult tuna moving to
the NWP. This also seems to be the case for the Japanese
eels (Anguilla japonica) that spawn in the subtropical NWP.
Larvae and juvenile are transported by currents, especially
the NEC, which diverges before reaching the coast of
the Philippines (Figure 19.5). The NEC bifurcation to the
north is at the origin of the Kuroshio. The intensity of the
flow and position of the bifurcation change with ENSO
and therefore impact larval transport and the recruitment
of juveniles in the NWP (Hsiung et al., 2018).
The impact of ENSO has been shown to be also favorable for spawning and nursery grounds of the neon flying
squid (Ommastrephes bartramii), which supports a major
fishery in the North Pacific. The stock was exploited by
an international driftnet fishery between 1978 and 1992,
with total annual catches reaching more than 350,000 t
from the 1980s. The driftnet fishery has been prohibited
and the squid is now targeted by jigging vessels from
Japan, China, South Korea, and Taiwan at a much lower
level (Bower & Ichii, 2005). Neon flying squid is a large
oceanic squid distributed in the Pacific between 20°N and
50°N. The distribution of paralarvae suggests that they
hatch where the SST ranges from 21°C to 25° C (Bower &
Ichii, 2005). Population dynamics are linked with the
basin‐wide oceanic circulation and with life history stages
that are very responsive to the changes in oceanographic
regimes (Ichii et al., 2009, 2011). The species undergo
seasonal migration between spawning (subtropical
region) and foraging grounds (transition and subarctic
regions), and like most cephalopods, neon flying squids
are opportunistic species that prey on zooplankton, other
squids, and myctophid fishes (Watanabe et al., 2002).
Using habitat models, Alabia et al. (2016) found that the
potential squid spawning and nursery habitats were largely
influenced by ENSO‐forced environmental changes during
the period of reproduction. These changes result in a substantial reduction/enhancement of available habitats in the
summers after CP El Niño/La Niña, where the latter leads
to an expansion of favorable spawning and nursery
grounds. However, the autumn–winter periods of weaker
and short‐lived EP El Niño showed elevated potential habitats for these species due to warmer than average sea surface temperature and better feeding conditions.
19.8. THE SOUTHWEST PACIFIC
The southwest Pacific Ocean is defined here by a
northern boundary at 5°S coinciding with the westward‐
flowing South Equatorial Current, and a southern
boundary that is the subtropical convergence with the
Southern Ocean to the south of New Zealand. This region
is also influenced by currents in the western limb of the
South Pacific subtropical gyre. The East Australian
Current is the major western boundary current of the
gyre, flowing from the southern Coral Sea and along the
coast of northern New South Wales before separating at
the Tasman Front and flowing east to New Zealand, or
south toward Tasmania as a series of eddies (Suthers
et al., 2011). On the east coast of Australia, ENSO has a
relatively weak but nevertheless significant influence on
the southward‐flowing East Australian Current (Holbrook
et al., 2011; Suthers et al., 2011). On the large scale, historical temperature records indicate that an ENSO response
can be identified over most of the upper southwest Pacific
Ocean, with the strongest signal in the tropics but with
significant signals also evident in the subtropical gyre and
south Tasman Sea (Holbrook & Bindoff, 1997).
19.8.1. Impacts on Pelagic Species
The tuna species found in the southwest Pacific are
part of the same stocks as the central and western tropical
Pacific regions (section 19.2). The variability of albacore
tuna (Thunnus alalunga) longline catch per unit effort
(CPUE) in New Caledonia’s exclusive economic zone
(EEZ) has been explained by seasonal and interannual
influence of ENSO, with highest CPUEs recorded from
1986 to 1998, which corresponds to a period with frequent El Niño events (Briand et al., 2011). The analysis
was extended to Samoa and French Polynesia using the
SOI index (http://www.bom.gov.au/climate/glossary/soi.
shtml) as an explanatory variable. While it confirmed that
higher albacore CPUE occurs with El Niños in New
Caledonia, lower CPUEs were found in Samoa and
French Polynesia, with the reverse situation encountered
during La Niña events (Figure 19.6).
This east‐west dipole effect is confirmed in other EEZs
with a transition region approximately located around
Fiji (~180°E). The immediate effect of ENSO on albacore CPUE suggests that catchability may be the main
factor to explain this CPUE variability. During El Niño,
the vertical habitat of albacore in New Caledonian waters
may be compressed to the surface due to shallowing of
the vertical thermal structure, illustrated by the change in
the 20°C isotherm in the west (Figure 19.7). This increases
catchability for the surface fishery. In contrast, regions
such as French Polynesia experience a deepening of that
habitat during El Niño events, reducing albacore catchability (Jurado‐Molina et al., 2011). The impact of ENSO
on the South Pacific albacore abundance is less clear,
although a link between recruitment and La Niña phases
has been proposed from population dynamics model simulations (Lehodey et al. 2006).
ENSO Impact on Marine Fisheries and Ecosystems 443
Much below
average
Much above
average
Average
Below average
Above average
NC
S
FP
El Niño
Neutral
La Niña
Figure 19.6 Results of the general linear models (GLMs) fitting catch per unit effort (CPUE) in NC (New Caledonia),
S (Samoa), and FP (French Polynesia) versus SOI stratified into five classes. SOI values were stratified into five
classes representative of Strongest Niño (<–15), moderate Niño (>–15 and <–5) neutral (–5 to 5) and weak Niña
(>5 and <15) and strongest Nina (>15). Three GLMs were fitted for EEZs. The CPUE response was also stratified
into five classes.
Other large pelagic species also respond to ENSO signals in the southwest Pacific. Off northeast Australia, for
example, there is evidence for a greater abundance of black
marlin (Makaira indica) during El Niño years (Williams
et al., 1994), with Hill et al. (2016) showing that suitable
habitat extended up to ~300 km further south during La
Nina events. Further south in the East Australian Current,
there is little evidence of ENSO phases influencing the distribution or abundance of pelagic species (Holbrook et al.,
2009). In Australia’s largest pelagic fishery, the east coast
longline fishery, studies seeking ENSO links to distribution and abundance of the target tuna and billfish have not
revealed strong signals. This is due in part to weak temperature anomalies in the region in either ENSO phase.
19.8.2. Impacts on Coastal Benthic and Demersal
Species
There is currently little evidence for direct ENSO impacts
on benthic or demersal species in the southwest Pacific. An
0
–0
5S
2
.1
–0.1
8
8
–0.1
–0.12
–0.24
S
0.12
FP
15S
0.06
NC
20S
0.00
0.00
25S
140E
0.06
.06
6
–0.
–0
–0.0
12
0.00
–0.12
0.
06
Latitude
0.18
10S
160E
180
140W
160W
Longitude
.30
–0.24
–0.18
–0.12
–0.06
0.00
0.06
0.12
0.18
0.24
Figure 19.7 First EOF of 20°C isotherm depth (explaining 20% of the total variance) from the 2004–2017 ARGO
product in colors and contours. (ftp://kakapo.ucsd.edu/pub/gilson/argo_climatology/RG_ArgoClim_Temperature_2017.
nc.gz) The three exclusive economic zones hashed are for New Caledonia (NC), Samoa (S), and French Polynesia (FP)
as labeled on the map.
0.30
444
EL NIÑO SOUTHERN OSCILLATION IN A CHANGING CLIMATE
exception is along Australia’s north coast, where enhanced
catches of banana prawns (Panaeus merguiensis) in the
Gulf of Carpentaria occur during high rainfall/river flow
during La Niña years (Vance et al., 1985).
In the northwest portion of the southwest Pacific,
greater likelihood of unusually warm waters during El
Niño years leads to coral bleaching in the Great Barrier
Reef, with dramatic bleaching reported in 2015–2016
(Hughes et al., 2017). Bleaching and cyclone damage led
to loss of habitat, which subsequently affects the fisheries
of the Great Barrier Reef, particularly the coral trout
fishery (Hodgkinson et al. 2014). Along the Queensland
coast, Meynecke and Yee (2011) showed region‐ and
species‐specific environmental relationships for a number
of commercially important fish species. The SOI was a
significant explanatory variable in many cases; however,
the overall effect was relatively minor, and overall explanatory power for the full catch‐environment models was
weak to moderate (most model R2 < 0.7).
In South Australia and Victoria and midlatitudes of
New Zealand, settlement of southern rock lobster (Jasus
edwardsii) is higher during El Niño years (Hinojosa et al.,
2017). La Niña years were associated with higher
settlement in Tasmania and southern New Zealand. In
both cases, the relationships were relatively weak. The larval stage for this species spends up to 18 months in the
pelagic environment of the Tasman Sea, and exposure to
different environmental conditions is likely to lead to these
disparate relationships between relatively close regions.
19.9. DISCUSSION
Classically linked to the Peruvian anchoveta fishery, we
now know that ENSO has an impact on a large number
of ecosystems and marine resources of the Pacific Ocean
and other basins as well. While we have described the
impact of ENSO on many major ecosystems and fisheries, other case studies are discussed in the literature,
including for example effects on seabirds, jellyfish, or
cetaceans. In addition, through atmospheric and oceanic
teleconnections (Yeh et al., 2018; see also chapters 14 and
15), ENSO influences all the other oceanic basins,
including the Antarctic, where ENSO has been strongly
linked to change in sea ice concentration (Kwok et al.,
2016), which has a central role in the dynamics of the
Antarctica oceanic ecosystem. In the Indian Ocean,
single or conjugate effects of ENSO and IOD have been
shown to impact tropical tuna distributions and recruitment in the eastern Indian Ocean basin (cf section 19.3).
Similar mechanisms effect reef ecosystems and tuna fisheries in the western Indian Ocean (Moustahfid et al.,
2018). ENSO has also a strong impact on the intensity of
the southward‐flowing Leeuwin Current along Australia’s
west coast in the Indian Ocean, being transmitted by
ENSO‐generated planetary waves that propagate from
the western Pacific Ocean through the Indonesian
Throughflow. Therefore, the ENSO signal in the Leeuwin
Current is further transmitted along the south coast of
Australia (Holbrook et al., 2009). These physical changes
result in impacts on regional fisheries. Along Australia’s
west coast, for example, La Niña events have been found
to enhance the transport of western rock lobster
(Panulirus cygnus) larvae, while El Niño events enhance
scallop recruitment. The strength of the Leeuwin Current,
linked to the equatorial Pacific circulation through the
Indonesian Throughflow, also influences recruitment of
pilchard, whitebait, Australian salmon, and herring along
Australia’s south coast (see Holbrook et al., 2009).
Teleconnections between the Pacific and the tropical
Atlantic Ocean are responsible for the variability of
upwelling intensity off the West African coast that supports a productive ecosystem and multiple fisheries (Roy &
Reason, 2001), as well as the growth of coral and associated changes of coral reef fauna off the Brazilian coast
(Kelmo et al., 2004, 2014; Evangelista et al., 2007). The
research effort to understand mechanisms leading to the
development of Atlantic ENSO is growing but still
relatively small compared to what has been deployed in the
Pacific Ocean. There is no doubt that with the rapid
development of knowledge, especially on the timing of the
ENSO–tropical Atlantic connection, more biologists will
start to investigate in detail the influence of this variability
on fisheries and marine species and ecosystem dynamics.
It is well demonstrated that despite several classical
phases of development, each ENSO event is unique in
terms of its intensity and impact, and the sequence of
cold, neutral, and warm phases (Timmermann et al.,
2018). The interaction with decadal signal and the effect
of long‐term climate change add to ENSO complexity. It
is still difficult to attribute recent observed changes in the
typology of El Niño events to anthropogenic‐induced climate change. Lee & McPhaden (2010) have reported
increasing amplitudes of El Niño events in the Niño‐4
(central equatorial) region, and the last El Niño event
(2015–2016) generated an unprecedented warm temperature anomaly in the central equatorial region. Its extreme
intensity has been attributed in part to unusually warm
conditions in 2014 and to long‐term background warming
(Santoso et al., 2017; Newman & Wittenberg, 2018;
Brainard et al., 2018). Given the relatively limited period
of modern Earth observation, it is possible that such an
extreme event still belongs to the range of natural variability that occurred for ENSO in the last few centuries.
Nevertheless, the superimposition of ENSO on a general
warming trend will certainly lead to a higher frequency of
such extreme events.
The biological consequences of the extreme 2015–2016
El Niño event were dramatic on the ecosystems of small,
ENSO Impact on Marine Fisheries and Ecosystems 445
remote Pacific islands in this central region, especially in
Jarvis Island (0°22′S, 160°01′W), on the equator south of
Hawaii. Unlike in previous strong El Niño events, the
2015–2016 event was not followed by a strong La Niña
phase, depriving this region of a strong subsequent
recovery of the equatorial upwelling and high productivity associated with it. Consequently, the longest and
most widespread coral bleaching event was recorded in
Jarvis Island, with massive mortality, i.e. 95% of Jarvis
corals were killed (see also chapter 18 on coral reef habitats). Although it was not the first catastrophic bleaching
event on Jarvis, it was unprecedented in magnitude
(Barkley et al., 2018). In the meantime, the biomass of
planktivore and reef fishes significantly declined, as did
the seabird abundance (Brainard et al., 2018).
These recent observations pose the decisive question
regarding the evolution of ENSO under the influence of
climate change, and how it will modify the impacts on
ecosystem diversity as described in this review. A better
knowledge and modeling of this evolution would make it
possible to predict its impacts and thus to prevent and
limit the most harmful economical and societal ones. The
latest projections of ENSO under the IPCC business‐as‐
usual emission scenarios suggest more frequent extreme
EP El Niño events (Cai et al., 2014, 2018), as well as
extreme La Nina events (Cai et al., 2015) associated with
the mean‐state changes under greenhouse warming (see
chapter 13). Projection uncertainties, however, remain due
to model biases (Chen et al., 2017). Therefore, to consider
this uncertainty, models of ecosystem or key population
species should be coupled to a diversity of climate models,
allowing us to explore the diversity of responses to future
ENSO patterns forecasted by these projections.
The interest for shorter‐term forecasts of ecosystem
and marine resources at seasonal and interannual time
scales is also rapidly growing (Salinger et al. 2016; Payne
et al., 2017; Tommasi et al., 2017) because they can offer
a large range of applications for management issues. The
last two decades have seen substantial progress in the
development of operational ocean models that simulate
the state of the ocean in real time, at high resolution, and
with assimilation of data from multiple platforms (e.g.
satellite, moorings, drifters, and argo floats). It is now
possible to envisage ocean forecasts ranging from a few
months to several years. These ocean forecasts can be
used to drive statistical or ecosystem models to assist in
the management of living marine resources. With short‐
term predictions, the model skills are rapidly evaluated in
the following months of the forecast, allowing a faster
loop of development and progress.
A simple statistical relationship between fish recruitment and ENSO as shown for skipjack (Figure 19.2) can
be used in conjunction with ENSO forecasts to provide a
useful monitoring index of the stock, taking advantage
of the predictability linked to the recruitment index propagating over time in the whole population. Similarly, forecasts of species habitat maps could be provided to assist
in marine spatial planning and fisheries monitoring.
However, unless they are based only on physical variables, such forecasts would likely require the development
of coupled ocean‐biogeochemical forecast systems to
provide productivity indices, such as surface chlorophyll
concentration or total primary production, which are
often used as explanatory variables in species habitat
modeling.
Several of these systems are now implemented in various ocean forecasting centers (Gehlen et al., 2015). A
study on the predictability of primary production in the
tropics (Séférian et al., 2014) suggests a predictive skill of
3 years, which is higher than that of sea surface temperature (1 year). This higher predictability is attributed to the
poleward advection of nutrient anomalies (nitrate and
iron), which sustain fluctuations in phytoplankton productivity over several years.
Therefore, multiyear forecasting of marine ecosystems
and fish dynamics models that have the potential to
support strategic and investment scale decisions (Salinger
et al., 2016) can be envisaged. This represents considerable potential to improve sustainability of marine
resource management and industry decisions, improving
resilience to ENSO and other climate variabilities.
ACKNOWLEDGMENTS
The authors are extremely grateful to the anonymous
reviewers for constructive suggestions to improve this
manuscript.
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