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The Fishery Interaction Team:  Investigating the Potential
Impacts of Commercial Fishing On the Foraging Success
of Endangered Steller Sea Lions  (cont.)

Effect of Ocean Conditions on the Cross-shelf Distribution of Walleye Pollock and Capelin

map of walleye pollock and capelin study area
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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