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Effect of Ocean Conditions on the Cross-shelf Distribution of Walleye Pollock and Capelin
Figure 1. Study area for walleye pollock and capelin study,
with transects and locations of moorings (solid circles).
In 2000, scientists from the REFM Division and the Center’s Resource Assessment and
Conservation Engineering (RACE) Division initiated a 4-year investigation of the
effects of fishing on sea lion prey abundance and distribution in a commercial
fishing ground located on the eastside of Kodiak Island (Figure 1 above). In 2001,
investigators from PMEL joined the project to provide enhanced biophysical
sampling to characterize the marine habitat and to assist in the investigation
of the potential role of biophysical factors in regulation of the spatial
distribution and abundance of walleye pollock and capelin.
The Kodiak Island region was of particular interest because Steller sea lion
populations in this region have exhibited marked declines since the 1970s, and
populations continue to exhibit a low level of decline. Steller sea lion
diet studies conducted by the National Marine Mammal Laboratory revealed that in
the summer, walleye pollock represented 64% of the diet, and smelt including
capelin represented 7%. These fish species represented the principal
pelagic fish biomass in the study region in 2000 and 2001.
The sampling design utilized control (unfished) and treatment (fished) areas. When
control and treatment sites are reasonably similar, the control allows the
analyst to differentiate responses due to the treatment from factors due to
natural variability. Barnabas and Chiniak Troughs (Figure 1 above) share
many biological and physical characteristics, making them good candidates for
the control and treatment sampling sites. Feasibility studies conducted in
2000 in the absence of commercial fishing revealed that the distribution and
abundance of fish in each trough was stable during the study period (two passes
in each trough). In 2001, the survey design was expanded to include
periods before and during a commercial fishing season. Regulations were
established to close Chiniak Trough to fishing. Two passes were made in
both troughs before the start of the fishing season, and one full pass of each
trough was made after fishing commenced. Survey results revealed that most
of the pollock biomass was located in the inner portion of Barnabas Trough. An
additional partial pass through the inner portion of Barnabas Trough
was conducted to verify pollock distributions during fishing.
Fish distributions in each trough were assessed using echo integration-trawl (EIT)
survey techniques during daylight hours aboard the NOAA research ship Miller
Freeman. The survey methods are similar to those used during
other routine acoustic surveys. Trawl hauls were conducted when
significant acoustic sign was encountered to determine the species composition
and to collect other biological information of the acoustic layer.
Acoustic backscatter was attributed to four groups of animals in 2000 and 2001. In
2000, trawl samples confirmed that these sign types were a mixture of capelin
and age-0 pollock, adult pollock, and age-1 pollock. In 2001 the sign
types were capelin, adult pollock, and age-1 pollock.
A variety of oceanographic data were collected to describe the influence of biophysical
factors on the distribution and abundance of sea lion prey. Five moorings
were deployed in May 2001 (Figure 1 above). The moorings were designed to measure
temperature, salinity, fluoresence, and currents. Temperature and salinity
measurements were also collected while the ship was under way and by water
column profiles taken at selected sites. In spring and summer 2001,
satellite track drifters were deployed to measure the spatial characteristics of
the Alaska Coastal Current. Finally, satellite images collected on
13, 21, and 22 August 2001 provided information on the spatial distribution of
chlorophyll - a in the study region.
Subsurface temperature measurements and surface temperature measurements revealed the
presence of a shelf break front in Chiniak Trough and a midtrough front in
Barnabas Trough ( Figures 2a, 2b ). Cool slope water intrudes over the shelf at
depth in Barnabas Trough. The satellite photos taken in 2001 demonstrate
that primary production was concentrated throughout Chiniak Trough, with highest
concentrations near shore and along depth gradients. In Barnabas Trough,
primary production was restricted to regions landward of the thermal front.
Spatial distributions of capelin appear to be associated with subsurface intrusions of
cool slope water in Barnabas Trough ( Figures
3a, 3b ) and regions of sharp gradients in
topography in Chiniak Trough. During 2000 and 2001, schools
containing capelin were observed in offshore waters in Barnabas Trough where
subsurface temperatures were relatively cool ( Figures 4a,
4b ). This pattern
of association with cool ocean conditions has been noted in Atlantic capelin
populations. In Chiniak Trough, capelin-age-0 mix (in 2000) and capelin
(in 2001) were primarily concentrated near Cape Chiniak along Transects 5
and 6 with pockets of high concentrations of fish landward of Transect 6 (
Figures 4a, 4b ). This region exhibits sharp topographic gradients with a deep region
near Cape Chiniak.
Comparison of subsurface temperature patterns in Barnabas Trough with juvenile and adult
pollock distributions revealed that adult pollock and age-1 pollock were
concentrated in regions of warmer ocean conditions where production is likely to
be concentrated ( Figures 5a to 5d).
There are several possible explanations for the distribution pattern exhibited by walleye
pollock. It is possible that pollock exhibit an affinity for warmer waters
because they have become acclimated to local environmental conditions. However,
it is unlikely that temperatures in the “offshore area” exceed the tolerance
level for adult and age-1 pollock because pollock are distributed throughout the
Aleutian Islands region and Bering Sea, where ocean temperatures are much
cooler. An alternative explanation for the pollock distribution patterns
observed in this study is that pollock exhibit a broad temperature tolerance but
select habitats where productivity is high. A third explanation for the
pollock distribution patterns observed in this study is that circulation
patterns in Barnabas Trough enhance cross-shelf advection of oceanic zooplankton
into the head of the trough. In 2001, satellite track drifters
and temperature records taken at the five moorings revealed that the net flow in
the troughs is shoreward on the east side and offshore on the west side,
indicating that zooplankton would be advected into the trough from the outer shelf.
This is the first report of findings from the first 2 years of research. As our
time series accumulates, the data collected from the region can be used to
evaluate more detailed ecological questions such as: Can knowledge of the
factors influencing prey distribution be used to develop reliable simulations of
the energy expended by a foraging sea lion to obtain sustainable amounts of
prey? The answer to this question will allow investigators to predict the
threshold number of prey schools or the threshold prey density required for
successful foraging of Steller sea lions.
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Quarterly April-June 2002 sidebar
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