Response of Wintering Steller’s Eiders to Herring Spawn
Author(s): Ramũnas Žydelis and Daniel Esler
Source: Waterbirds, 28(3):344-350.
Published By: The Waterbird Society
DOI: http://dx.doi.org/10.1675/1524-4695(2005)028[0344:ROWSET]2.0.CO;2
URL: http://www.bioone.org/doi/
full/10.1675/1524-4695%282005%29028%5B0344%3AROWSET%5D2.0.CO
%3B2
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Response of Wintering Steller’s Eiders to Herring Spawn
RAMUNAS
Zˇ YDELIS1,2
1
2
3
AND
DANIEL ESLER3
Institute of Ecology of Vilnius University, Akademijos g. 2, LT-08412 Vilnius, Lithuania
Present address: Duke University Marine Lab, 135 Duke Marine Lab Road, Beaufort, NC 28516, USA
Internet: rzydelis@sfu.ca
Centre for Wildlife Ecology, Simon Fraser University, 5421 Robertson Road, Delta BC, V4K 3N2, Canada
Abstract.—Distributional and dietary responses of wintering Steller’s Eiders (Polysticta stelleri) to spring spawning of Baltic Herring (Clupea harengus) were studied along the Lithuanian coast of the Baltic Sea. Herring spawn is
patchy, but is abundant and energy-rich when present. The objective of this study was to determine whether Steller’s
Eiders modified their foraging sites and food habits to take advantage of spawn, or whether they were inflexible
foragers as suggested by earlier studies. Steller’s Eiders altered their habitat use during herring spawn, moving to
habitats where fish spawning occurred. Also, diet analysis demonstrated that herring eggs became an important
food when available. Although the importance of herring spawn for Steller’s Eiders remains speculative, this study
indicates that spawning sites could be important as a source of nutrients and energy for subsequent migration or
reproduction, and should receive conservation consideration. Received 12 June 2004, accepted 20 December 2004.
Key words.—Steller’s Eider, Polysticta stelleri, herring spawn, habitat use, diet, foraging, Baltic Sea.
Waterbirds 28 (3): 344-350, 2005
Ephemeral, abundant, and predictable
food resources can be important for animals,
which often adapt their annual cycles to
make use of plentiful food (Castro and Myers 1993; Botton et al. 1994; Restani et al.
2000; Brian et al. 2002). Mass fish spawning is
one such phenomenon, offering superabundant food to a variety of predators. Many
birds, including sea ducks, are known to aggregate at fish spawning sites to forage intensively on abundant and energy-rich fish eggs
(Bustnes and Erikstad 1988; Vermeer et al.
1997; Bishop and Green 2001; Sullivan et al.
2002; Rodway et al. 2003). Spring spawning
events by herring (Clupea spp.) are particularly well known to attract foraging birds.
The nutrition and energy derived from
fish spawn could be important for predators
later in their annual cycle. For example, nutritional status of female waterfowl in spring
has important consequences for subsequent
productivity, through effects on breeding
propensity, timing of reproduction, and
clutch size (Alisauskas and Ankney 1992; Esler et al. 2001). Changes in prey availability
during spring have been implicated as a potential factor affecting broad-scale and longterm population declines in Lesser Scaup
(Aythya affinis) (Anteau and Afton 2004), indicating the potential demographic importance of spring foraging conditions.
Sea ducks, particularly Long-tailed Duck
(Clangula hyemalis), wintering in the Baltic
Sea have been observed foraging on fish
eggs (Leipe 1985; Stempniewicz 1995). However, distributional and dietary shifts associated with fish spawning events have never
been quantified for sea ducks in that region.
In this study, we aimed to determine whether
Steller’s Eiders (Polysticta stelleri) wintering
on the Lithuanian coast of the Baltic Sea actively responded to Baltic Herring (Clupea
harengus) spawn, which is produced in early
spring. Steller’s Eider is not abundant in the
Baltic Sea, and is found at a very few restricted wintering areas (Nygård et al. 1995;
Schäffer and Gallo-Orsi 2001). Although fish
eggs were known to occur in the diet of Stellˇ
er’s Eider on the Lithuanian coast (Zydelis
2000), it was not certain whether they actively searched for fish eggs or foraged opportunistically when they became available in
their regular wintering habitat. Bustnes and
Systad (2001) found that Steller’s Eider wintering in Varangerfjord, northern Norway,
did not feed on fish spawn, whereas the
Long-tailed Duck wintering in the same region exploited mass spawning of Capelin
(Mallotus villosus). Determining the response of Steller’s Eider to herring spawn is
a potentially important conservation issue,
as the duck is a species of conservation con-
344
STELLER’S EIDERS AND HERRING SPAWN
cern in the region (Schäffer and Gallo-Orsi
2002) and herring spawn could play an important role in its population dynamics.
If herring spawn is important for Steller’s
Eiders on the Lithuanian coast we predicted
the following two responses:
1. Birds would be expected to change
their habitat use during herring
spawning and aggregate at sites where
herring spawn occurred. This prediction is based on the fact that herring
spawning in Lithuanian coastal waters
occur in specific, limited habitats
compared to the range of habitats
used by Steller’s Eider in the region
during winter.
2. Steller’s Eider would be expected to
change their diet during the herring
spawning period, with herring eggs
constituting a large proportion of the
food when available.
METHODS
Study Area
This study was conducted along the Lithuanian
mainland coast of the Baltic Sea, which is a recognized
wintering site for Steller’s Eider (Nygård et al. 1995;
Schäffer and Gallo-Orsi 2002). The boundaries of the
study area were defined by the most northerly and
southerly extremes at which Steller’s Eiders had been
observed on the Lithuanian coast. Areas beyond these
boundaries were surveyed during the study period, and
Steller’s Eider were never observed, so these areas were
not used in the analyses.
The Lithuanian coastline is characterized by an exposed open sandy shoreline. The seafloor slopes gently
with the 10 m isobath at 1-2 km and 20 m isobath 3-5 km
off shore. Tidal amplitude is only c.0.6 m and depends
ˇ
primarily upon prevailing wind direction (Asmontas
1995). Due to erosive wave action, the sea bottom generally lacks stable benthic communities (e.g., mussel
beds or underwater vegetation) at depths less than four
meters (Oleninas et al. 1996). Below four meters, the
bottom consists of sand, gravel and boulders, and supports a relatively diverse and rich benthic community
(Olenin 1997; Oleninas et al. 1996).
The study area is an important spawning ground for
ˇ 1995; Maksimovas et al. 1996).
Baltic Herring (Repecka
The spawning substrate is a hard bottom with submerged
vegetation (Kääriä et al. 1997), particularly beds of the
ˇ
red macroalgae Furcellaria lumbricalis (Repecka
1995;
Maksimovas et al. 1996). Furcellaria lumbricalis grows at
depths from 4-14 m on boulder substrate, which occurs
in mosaic patches within the study area (Oleninas et al.
1996; Olenin 1997). This habitat also includes a high diversity of macrozoobenthos with mussels (Mytilus edulis)
dominating in terms of biomass (Oleninas et al. 1996).
For the purposes of our analyses, it has been assumed
that locations of Furcellaria lumbricalis algae beds corre-
345
sponded to herring spawning sites, given the close association between the two. Locations of Furcellaria
lumbricalis algae beds were derived from high resolution
GIS maps (M 1:25000, S. Olenin, unpubl. data). Herring
spawn in the study area from April, when water temperˇ 1995; Maksimoature reaches 6°C, until June (Repecka
vas et al. 1996). For analysis, data for eiders in April have
been used to represent the spawning season, because it
is the only period when presence of Steller’s Eider and
spawning activity overlap.
Surveys
Bird surveys were conducted 1-2 times per month
during four consecutive wintering seasons (December
1997-April 2001). Surveys were performed from land
with the use of binoculars (10 × 50) and a telescope (2045×). Positions of birds observed on the water were plotted on maps into contiguous survey sectors, extending 1
km along the shore and 2 km off shore. For each survey,
the entire study area was surveyed by one person on one
or two consecutive days.
Diet Analysis
Birds accidentally drowned in fishing nets were collected for diet analysis. Carcasses were frozen within
hours of collection. Contents from the esophagus and
gizzard were analyzed separately. The degree of digestion was assessed according to the following scale: 1—
food objects intact, visually not affected by digestion, 2—
food at initial stage of digestion, soft prey still easy identifiable, lots of identifiable tissues, 3—food objects heavily affected by digestion with some remains of tissues, 4—
food objects heavily affected by digestion, no tissues left.
Only contents showing digestion stages 1 and 2 were
used in diet analyses. Diet composition was described
according to wet weight of prey, including mollusk
shells. Prey objects were identified to species when possible, counted, measured, and weighed to the nearest
0.01 g after removing excess water by placing items on
filter paper.
Bird diet composition was summarized as mean proportions of the wet weight of each prey taxon per individual bird (Krapu and Reinecke 1992). Prey frequency
of occurrence was calculated as the percentage of birds
containing a certain food item. Sand, pebbles, and amber pieces were excluded from analyses.
Statistical Analysis
Assuming that Furcellaria lumbricalis algae beds repˇ 1995; Makresent herring spawning grounds (Repecka
simovas et al. 1996; Kääriä et al. 1997), Steller’s Eider
habitat use was analyzed by applying a multiple linear
regression to contrast relationships between bird abundance and Furcellaria lumbricalis algae beds in April and
during the rest of wintering season. In this analysis, the
response variable was the average number of Steller’s Eiders for a given coastal sector in a given season. Explanatory variables included (1) the percent of Furcellaria
lumbricalis coverage for a given coastal sector, (2) a categorical variable for season (winter (December through
March) and spawn (April)); and (3) an interaction term
between the two main effects (algae cover and season).
This model structure allowed simultaneous consideration of the linear relationship between Steller’s Eider
abundance and herring spawn habitat in the two sea-
346
WATERBIRDS
sons. Because the winter period was set as the reference
value for season, the parameter estimates were interpreted as follows. The model intercept and the parameter estimate for the main effect of percentage of algae
describe the linear relationship during winter. For the
herring spawn season (April), the intercept of the linear
relationship is calculated as the model intercept plus
the main effect of season, and the slope of the relationship is the main effect of percent algae plus the parameter estimate for the percent algae by season interaction
term. Simple correlations were also conducted between
average Steller’s Eider abundance and the percent of algae per coastal sector on a monthly basis, to achieve a
finer temporal resolution for considering these relationships. The statistical package SAS (SAS Institute
1994) was used to conduct analyses. Data are presented
as means ± standard error.
RESULTS
Distributional Response
Based on multiple regression analysis, it
was found that the relationship between Steller’s Eider abundance and Furcellaria lumbricalis coverage differed considerably between
winter and the herring spawn season. The
model and parameter estimates (± SE) were:
#STEI = 29.72 (± 10.83) – 32.69 (± 15.32)
SEASON + 26.85 (± 31.87) %ALGAE +
95.43 (± 45.07) SEASON*%ALGAE;
r2 = 0.27, P < 0.01
The model parameter estimate for percent algae was not different from 0, indicating that during winter Steller’s Eiders were
distributed fairly evenly with respect to Furcellaria lumbricalis coverage. In contrast, during
the herring spawn period in April, there was
a distributional shift of eiders to areas with
extensive algae beds (Fig. 1). This is reflected
in the model parameter estimates, which indicated that average number of Steller’s Eider was approximately zero at 0% Furcellaria
lumbricalis coverage and increased linearly
(and significantly; P < 0.05) to an estimated
119 birds at 100% Furcellaria lumbricalis coverage. These results are corroborated by those
from monthly simple correlations; the relationship between average Steller’s Eider
numbers and percent algae coverage was relatively weak during December through
March (r22 = -0.01, r22 = -0.11, r22 = 0.36, r22 =
0.18, respectively, n.s.) but was much stronger during April (r22 = 0.58, P < 0.01).
Diet Response
Thirty-four Steller’s Eiders were recovered from fishing nets, of which 30 contained undigested food. Seventeen of these
Figure 1. Maps illustrating distribution of Steller’s Eider within coastal sectors in winter (December-March) and herring spawning (April) periods, and location of Furcellaria lumbricalis algae beds (following S. Olenin, unpubl. data),
which represent herring spawning grounds at the Lithuanian coast of the Baltic Sea.
STELLER’S EIDERS AND HERRING SPAWN
birds were obtained during January-March,
i.e. during the non-spawning period, and 13
individuals were collected in April, when
spawning occurred.
Fourteen food types were identified during the non-spawning period and nine during herring spawning (Table 1). In terms of
wet mass, Steller’s Eider diet composition
during the non-spawning period was dominated by Gammarus crustaceans and the Mussel (58% and 37% respectively). In April,
herring eggs were an important diet item,
constituting on average 35% (±11.2) of the
total food intake (Table 1).
In terms of frequency of occurrence,
some other food objects were found frequently, although these did not contribute
appreciably to wet weight of the diet. In addition to gammarids and mussels, the gastropod Theodoxus fluviatilis and barnacle Balanus improvisus were commonly found in Stell-
347
er’s Eiders during January-March. During
the herring spawning period, birds also frequently ingested Theodoxus fluviatilis and red
algae Furcellaria lumbricalis fragments were
found in 62% of bird stomachs (Table 1).
Furcellaria lumbricalis fragments in bird diet
were detected more frequently in April than
during winter months (62% and 24% respectively; χ12 = 4.43, P < 0.05), presumably reflecting their feeding habitat.
Herring eggs were observed in nearly
half (46%) of the Steller’s Eiders collected in
April (Table 1). Precise data about timing
and extent of herring spawning were not
known for each survey sector; however, presumably there was spatial and temporal variation in spawning activity that would influence local availability of herring spawn.
Therefore, fish eggs likely were not available
at every spawn site during all of April, but we
consider this a reasonable approximation of
Table 1. Steller’s Eider diet composition during winter (January-March; N = 17) and herring spawn (April; N = 13)
periods at the Lithuanian coast of the Baltic Sea.
January-March
Taxa
Mean % of
wet weight (±SE)
April
Frequency of
occurrence, %
Mean % of
wet weight (±SE)
Frequency of
occurrence, %
Algae
Furcellaria lumbricalis
Unidentified algae
traces
traces
24
6
2.8 (1.1)
traces
62
8
Polychaeta
Nereis diversicolor
Unidentified polychaete
traces
traces
6
6
—
—
—
—
Bivalvia
Mya arenaria
Macoma baltica
Mytilus edulis
0.6 (0.5)
—
37.5 (9.2)
35
—
100
—
0.8 (0.8)
31.3 (9.9)
—
8
69
Gastropoda
Hydrobia sp.
Polisyphonia sp.
Theodoxus fluviatilis
traces
—
3.0 (1.1)
12
—
82
—
traces
9.8 (6.0)
—
8
69
Crustacea
Mysis sp.
Gammarus sp.
Corophium sp.
Idothea sp.
Jaera albifrons
Balanus improvisus
Unidentified crustaceans
traces
58.4 (9.3)
traces
traces
traces
0.6 (0.2)
traces
6
100
35
6
12
53
6
—
17.7 (10.3)
—
—
—
0.3 (0.3)
2.8 (1.4)
—
46
—
—
—
8
31
—
—
34.7 (11.2)
46
Pisces
Clupea harengus eggs
348
WATERBIRDS
herring spawn activity. The birds that had
fish eggs in their stomachs relied highly on
this food item, which constituted 75%
(±6.8%) of the diet in terms of wet weight.
DISCUSSION
Wintering Steller’s Eiders actively responded to herring spawn, in terms of changes in both their habitat use and their diets.
Steller’s Eider aggregated in habitats where
herring spawns in April, in contrast to their
more dispersed distribution during winter,
when Furcellaria lumbricalis algae beds also
were used, but with no preference over other
habitat types. Also, herring eggs made up an
important proportion of the diet in April.
Along with intake of herring eggs, the amount
of ingested gammarids decreased proportionally, while Mytilus edulis was taken at similar
rate during both periods (Table 1). The nutritional value of fish eggs is nearly twice that of
crustaceans (Rumohr et al. 1987; Hislop et al.
1991). Also, the intake of dense, sessile fish
eggs is presumably much easier than pursuit
of mobile gammarids. With availability of herring spawn, Steller’s Eiders likely could increase their energy intake while reducing foraging time. This could allow for behavior other than foraging to become more frequent
than they are during the rest of the wintering
season, when feeding activities take most of
the daily activity budget (Fox and Mitchell
1997; Žydelis 1997; Laubhan and Metzner
1999). Courtship and pair formation could be
essential behavior prior departing to the
breeding grounds that might be enhanced by
decreased foraging time associated with herring spawn. Rodway (2003) suggested that social interaction could be one of the benefits or
even causes for Harlequin Ducks (Histrionicus
histrionicus) to aggregate at fish spawning sites.
In contrast to the results of this study,
Bustnes and Systad (2001) did not observe
Steller’s Eider feeding on fish eggs in
Varangerfjord, northern Norway, although
this food source was available and used by
other sea ducks. They concluded that Steller’s Eider has little flexibility in its foraging
ecology and suggested that in Varangerfjord,
Capelin spawn in areas that could be too
deep for Steller’s Eider to forage. Along the
Lithuanian coast, foraging flocks of Steller’s
Eider have not been recorded further than
2 km off shore, where water depths exceed
10 m, while large aggregations of Longtailed Ducks were observed in the same area
foraging on herring eggs in zones of Furcellaria lumbricalis algae beds 3-4 km off the
ˇ pers. obs.).Therefore, it is likeshore (R. Z.,
ly that Steller’s Eiders foraging on herring
eggs exploited only the part of Furcallaria
lumbricalis algae beds in shallow waters at the
Lithuanian coast. Observations of Steller’s Eider habitat use at the Lithuanian coast support the suggestion by Bustnes and Systad
(2001) that foraging Steller’s Eiders are confined to shallow waters. However, this eider
actively switched from its traditional winter diet to a more profitable food resource when it
became available within its wintering range.
The longer-term ecological significance
for Steller’s Eider of an easy accessible and
highly nutritious food source, such as herring eggs, is not clear. Herring spawn is likely
to contribute to deposition of pre-migratory
fat, which might be used as fuel for long-distance migration. Pre-migratory hyperphagia
and fattening is known in a number of waterbird species. Shorebirds use body reserves as
energy source for migration (Lindström and
Piersma 1993), while larger waterfowl such as
the Common Eider (Somateria mollissima)
and geese use fat accumulated on wintering
quarters not only for migration to nesting
grounds, but also for breeding performance
(Milne 1976; Raveling 1979; McLandress and
Raveling 1981; Parker and Holm 1990; Ebbinge and Spaans 1995). The role of herring
spawn in providing nutrition and energy for
subsequent migration and reproduction for
Steller’s Eider is a topic deserving attention.
The findings have implications for conservation of this rare sea duck. Aggregations
of birds, such as those of Steller’s Eiders on
herring spawn sites, are particularly vulnerable to stochastic natural events and human
activity impacts (Ford et al. 1982). Even a
small oil spill could threaten this local aggregation of Steller’s Eider, which is restricted to
the narrow coastal stretch represented by
the study area; the nearest other wintering
STELLER’S EIDERS AND HERRING SPAWN
site is c. 350 km north along the Estonian
coast (Schäffer and Gallo-Orsi 2002). Structural and functional stability of marine nearshore ecosystem is also important for Steller’s Eiders in winter, which use specific and
restricted habitats, and are probably dependent on cyclic events such as herring spawn.
ACKNOWLEDGMENTS
BirdLife International, the Dutch Ministry of Agriculture, Nature Management and Fisheries through the
PIN/MATRA Funds of the Ministry of Foreign affairs, and
SOF (BirdLife in Sweden) funded part of this study in winter season 2000/2001. We are grateful to W. Sean Boyd for
reviewing the manuscript and making helpful comments.
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