Spanish acoustic surveys: analysis of the fish
pelagic community
M. Iglesias 1, M.B. Santos 2, C. Porteiro 2, M. Bernal 3, F. Ramos 3, D. Oñate 1, A. Giráldez 4, E. Nogueira 5,
N. Díaz 1, P. Tugores 1, G.J. Pierce 2 & J. Miquel 1
1
Centro Oceanográfico de Baleares, 07015 Palma de Mallorca; 2 Centro Oceanográfico de Vigo, 36200 Vigo; 3
Unidad de Cádiz, 11510 Puerto Real; 4 Centro Oceanográfico de Málaga, 29640 Fuengirola; 5 Centro Oceanográfico
de Gijón, 33212 Gijón
Instituto Español de Oceanografía, Spain. E-mail: magdalena.iglesias@ba.ieo.es
Abstract
The Ecosystem Approach requires joined-up thinking in fisheries, building from traditional
framework of target species up to communities, populations, ecosystems, and the interaction
with humans. In Spain, acoustic research surveys have traditionally been used to evaluate and
monitor the population of commercially important pelagic fish species such as sardine (Sardina
pilchardus) and anchovy (Engraulis encrasicolus). These surveys have been carried out annually
both in the Mediterranean and Atlantic coasts since the 1980s collecting acoustic and
hydrographic data and lately also information on distribution and abundance of top predators.
They constitute 3 time series with different characteristics (PELACUS, ECOCADIZ and ECOMED).
Here we present results of the last years from the three series when all main pelagic fish
species started to be routinely evaluated. Acoustic data presented include estimates of
abundance, distribution and mean size for the eleven main pelagic species found in Spanish
waters. We promote the use of acoustic surveys as platforms for the integrated monitoring of
pelagic communities.
Introduction
Commercial fish stocks interact with non-target species, including prey, predators and
competitors, in a complex web of ecological relationships which are tuned at multiple scales by
a range of forcing factors including anthropogenic impacts (fishing, habitat loss, pollution,
invasive species) and environmental processes.
Trawling and acoustic surveys are primarily directed at evaluation of distribution and
abundance of target stocks. The implementation of the Ecosystem Approach requires changes
in our knowledge base, moving from the stock to the ecosystem. An important contribution
can be made using existing survey databases. Surveys can collect data on non-target species
and environmental conditions that could potentially be used to provide indicators of
ecosystem status. Here we provide examples, based on acoustic surveys in Spanish waters,
to move one step towards an ecosystem approach to fisheries, namely to carry out
community level analysis.
Four acoustic surveys are carried out annually in Spain, covering the shelf (20 to 200 m) in
spring (Cantabrian coast), summer (Gulf of Cadiz) and late autumn (Mediterranean and Bay of
Biscay) (Fig. 1): PELACUS spring, which was developed to estimate the spawning stock
biomass (SSB) of sardine in northern waters, ECOMED, which was implemented to obtain a
recruitment index for anchovy in Mediterranean waters, ECOCADIZ which started in 2004 to
monitor SSB of anchovy in the Gulf of Cadiz and PELACUS autumn which started in 2006 to
obtain an index of the anchovy juvenile stock in the Bay of Biscay.
Material and Methods
Acoustic and biological sampling
- R/V Thalassa (northern waters) and Cornide de Saavedra (Gulf of Cadiz and Mediterranean).
- Echo-sounder at 38 kHz, calibrated prior to each survey (Foote et al., 1987).
- Transects perpendicular to the coast and evenly spaced (4 – 8 nm), constant speed (10 knots)
from dusk till dawn.
- Pelagic trawls at daytime (Cantabrian coast and Gulf of Cadiz) or at night (Mediterranean).
Data processing
-
Acoustic data is post-processed using SonarData Echoview©.
Fish density integrated by nm and expressed in m2 x nm-2 (MacLennan et al., 2002).
Energy split into species following their proportions in the trawls.
Length frequency distributions are obtained for all fish species in the trawl.
Length-weight relationships are obtained for the main pelagic species.
Complementary data
-
Sampling of fish communities (trawl stations), hydrography and hydrodynamics
(rosettes and CTD), ichthyoplankton (Continuous Underwater Fish Egg Sampler) and
visual censuses of top predators (marine mammals and seabirds).
Results
There has been a decline in both abundance and distribution area of sardine in the Cantabrian
Sea (Fig. 2) and anchovy in Northern Mediterranean (Fig. 3).
Results from the 2007 surveys, when all main fish species were evaluated acoustically, show
that the most abundant species were: sardine (Sp), mackerel (Sc) and horse mackerel (Tt) in
the northern waters; anchovy (Ee), sardine and chub mackerel (Sc) in the Gulf of Cadiz and
sardine, anchovy (Ee), Mediterranean horse mackerel (Tm) and bogue (Bb) in the
Mediterranean.
The lowest fish biomass per squared nautical mile was estimated for the Mediterranean.
In the Mediterranean, where the longer time series exists, there have been changes in the
relative proportion of target/non-target species in the survey (Fig. 4). The Simpson´s index
indicates that species have become more evenly distributed (Fig. 5).
Discussion
Acoustic surveys constitute ideal platforms for the integrated monitoring of pelagic
communities and ecosystems. Data on abundance, distribution and behavior of pelagic species
can be collected systematically and geo-referenced, at the same temporal and spatial scales as
environmental data.
Historically, surveys monitored changes in abundance of commercially important species. By
collecting data on other components of the community we can monitor changes at community
level.
Fishing removes important amounts of small pelagic fish in Spanish waters and this selective
removal of species and in many cases size-classes can cause changes in pelagic community
structure and functioning (Pauly et al., 1998).
As we have shown, some of these changes have already taken place in the abundance and
distribution of some species both in northern and Mediterranean Spanish waters, and the
influence of fishing and environmental changes should be assessed.
In the Mediterranean, an oligotrophic environment, pelagic species are becoming more evenly
distributed and there is a substantial reduction in total biomass.
Parallel changes can be observed in fisheries landings. Total fish landings have decreased but
number of exploited species is on the increase.
An increase in evenness implies a higher diversity, higher diversity in an exploited ecosystem
can be explained by the Intermediate Disturbance Hypothesis (Huston, 1994) which states
that disturbance (e.g. exploitation) prevents competitive exclusion and therefore systems at
intermediate levels of disturbance possess highest diversity.
Future work
- Historical analysis of long time series of acoustic data to obtain estimates of non-target
species.
- Study the temporal and spatial dynamics of non-target species.
- Explore alternative biodiversity indexes and other ecosystem metrics to monitor changes at
ecosystem level in Spanish waters.
- Application of hydrodynamics - Nutrient Phytoplankton Zooplankton - fish community
coupled models to disentangle the effects of climate variability and fisheries on the
community.
Acknowledgements
We would like to thank all the crew and participants in the surveys.
References
- Foote, K.G., Knudsen, H.P., Vestnes, G., MacLennan, D.N. and Simmonds, E.J. 1987.
Calibration of acoustic instruments for fish density estimation: a practical guide. ICES Coop.
Res. Rep. 144, 57 pp.
- Huston, M. 1994. Biological diversity. Cambridge University Press, Cambridge, UK.
- MacLennan, D.N., Fernándes, P.G. and Dalen, J. 2002. A consistent approach to definitions
and symbols in fisheries acoustics. ICES J. Mar. Sci. 59, 365-369.
- Pauly, D., Christensen, V., Dalsgaard, J., Froese, R., Torres Jr., F. 1998. Fishing down marine
food webs. Science (Wash.) 279, 860-863.
PELACUS
autumn
FRA NCE
Ca n ta b ri c S e a
PELACUS
spring
Tr am u n ta n a
A tla n tic
O ce a n
G u lf o f
C a d iz
ed
ite
rr
an
ea
n
M
E
C
O
M
E
P O R TU G A L
Se
a
D
SP A IN
A lb o ra n S e a
ECOCADIZ
MO RO C CO
Figure 1. Map of Spain showing the areas covered by each survey series
ARG E L IA
2007
2006
2005
2004
2003
#
#
Cedeira
Ortigueira# Burela#
Viveiro
#
Foz
Ferrol
#
#
Ribadeo
#
Luarca
#
Cudillero
#
GIJÓN
#
#
Ribadesella
A CORUÑA #
Betanzos
Malpica
#
Villaviciosa
Pontedeume
#
#
#
#
#
#
Llanes
#
SANTANDER
#
Santillana
Santoña
San Vicente
de la Barquera del Mar
Bermeo
#
#
#
1 - 100
101 - 350
351 - 650
651 - 1200
1201 - 2218
Irún
#
Zumaia
BILBAO
SAN
SEBASTIÁN
#
Muxía
Cabo Fisterra
#
#
Noia
Muros
#
Villagarcía
de Arousa
#
#
#
PONTEVEDR A
Vigo
#
Baiona
A Guardia
#
Figure 2. Spatial distribution of energy allocated to sardine during PELACUS surveys (2003-07)
Rosas
#
Blanes
#
BAR CELONA
#
TARR AGON A
#
San Carlos de la Rápita
#
Vinaroz
Torreblanc a
CASTELLÓN
#
#
#
El Puerto
#
VALEN CIA
2 - 10
11 - 60
61 - 120
121 - 350
351 - 544
#
Suec a
#
Gandía
#
2003
2004
2005
2006
2007
Figure 3. Spatial distribution of energy allocated to anchovy during ECOMED surveys (2003-07)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
2003
2004
Sp
Ee
2005
Ss
Sj
Tt
2006
Tm
Tp
Bb
2007
Sa
Figure 4. Relative proportion in abundance of the nine main pelagic species evaluated during the
ECOMED surveys (2003-07)
0.90
12000
0.80
10000
0.70
0.60
8000
0.50
6000
Simpson´s index
Total abundance
0.40
0.30
4000
0.20
2000
0.10
0.00
0
2003
2004
2005
2006
2007
Figure 5. Total abundance of the nine main pelagic species evaluated during the ECOMED surveys (200307) and Simpson´s index of species evenness