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Doctoral Dissertation

Susanne McDermott successfully defended her doctoral dissertation “Improving abundance estimation of a patchily distributed fish, Atka mackerel (Pleurogrammus monopterygius)” on 13 March 2003 at the School of Aquatic and Fishery Sciences, University of Washington. Two separate survey approaches were developed to estimate Atka mackerel local abundance. A mark recapture experiment was conducted in Seguam Pass, Alaska, inside Area 1 and outside Area 2 of the trawl exclusion zone. In 1999, 1,375 tagged fish were released. A biomass of 76,679 t  was estimated in Area 2 using the Peterson method. In 2000, 8,773 tagged fish were released. An integrated tagging model was developed to estimate biomass and movement rates. Biomass was estimated to be 117,900 t (standard deviation (SD) 45,267) in Area 1 and 82,057 t (SD 13,311) in Area 2. Movement rate after 107 days from Area 2 to Area 1 was 0.0056 (SD 0.0072) and from Area 1 to 2 was 0.81 and associated with high uncertainty (SD 0.3). The feasibility of a demersal egg count survey was also examined. Estimated fecundity for a 40-cm female was 30,256 eggs, spawning a total of 4.6 batches per year, after adjusting for atresia. A new approach to population estimation, the Annual Batch Production method, estimates abundance from a survey of batches per area within the spawning grounds using underwater cameras and scuba divers. A simulation was conducted to examine the number of batches present in nests throughout the spawning season for 11 different reproductive scenarios. Optimal timing for a batch count survey was determined to be 1 month after beginning of spawning, with a window of 34 to 78 days. Defining the spawning area was identified as the biggest challenge followed by the development of sampling platforms and tools to withstand the harsh conditions in the Aleutian Islands.

By Libby Logerwell.

FIT Presentations at Marine Science Symposium

Members of the Fishery Interaction Team (FIT) from the Center’s REFM and RACE Divisions presented results of their work at the symposium, “Marine Science in the Northeast Pacific: Science for Resource Dependent Communities,” held in Anchorage, Alaska in mid-January 2003. The following papers and posters were presented during the sessions on Steller Sea Lion Investigations.

  • Elizabeth Conners, Peter Munro and Sandra Neidetcher: “AFSC Pacific cod studies at Unimak Pass.” This presentation concerned the Fisheries Interaction Team (FIT) field study around the intensive winter fishery for Pacific cod in waters north of Unimak Pass in the eastern Aleutian Islands. The experiment uses pot catch of cod as a measure of local abundance, both in the intensively fished area and in an adjacent control area within the Cape Sarichef no-trawl zone. The goal is to compare the rate of seasonal change in abundance between sites in the treatment and control zones and quantitatively test for presence or absence of a fishing effect. This study has been in feasibility work since 2001, and winter 2003 will be the first full trial of the experiment. This presentation showed the results of feasibility studies to assess the overall variability and spatial correlation structure of pot catch data and presents briefly the final experimental design.
  • Anne Hollowed, Chris Wilson, Phyllis Stabeno, and Sigrid Salo: “Effect of ocean conditions on the cross-shelf distribution of walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus).The presentation provided results of acoustic trawl surveys conducted in 2001 as part of a multiyear (2000-2002), multidisciplinary experiment off the eastern coast of Kodiak Island in the Gulf of Alaska (see Wilson, et al. presentation, below). The authors found that the spatial distribution of walleye pollock and capelin differed and appeared to be related to differences in habitat preferences. Capelin appeared to be associated with cool slope water intrusions and topographic gradients, while walleye pollock appeared to select habitat based on the availability of prey.
  • James lanelli and Martin Dorn: “Using fishing vessels to collect acoustic data for scientific purposes: preliminary results from midwater trawlers in the eastern Bering Sea walleye pollock fishery (poster).” This poster described a project to log acoustic backscatter data on midwater trawlers fishing for walleye pollock in the eastern Bering Sea. The objective of the project is to evaluate fishing impacts on endangered Steller sea lions at fine spatial and temporal scales. Work to date has focused on evaluating the spatial coverage of the data and examining the general characteristics of cruise tracks and uncalibrated backscatter data. Preliminary results show good correspondence with survey data during the same period. However, technical problems related to data processing need to be resolved.
  • Libby Logerwell and Chris Wilson: “Discrimination of Steller sea lion prey fish using frequency-dependent backscatter.” This work evaluates the effect of commercial fishing activity on the prey availability to Steller sea lions in the Gulf of Alaska (see Wilson, et al. presentation below). The project employs acoustic methodology to assess the distribution and abundance of sea lion prey, so one challenge is identifying the species composition of mid-water scatterers, predominantly juvenile pollock and capelin. The difference between mean volume back-scattering strength at 120 and 38 kHz ()MVBS) has been used by researchers to acoustically discriminate between macrozooplankton species and between macrozoo-plankton and fish or small zooplankton. The presentation was on an analysis to determine whether )MVBS can be used to discriminate between aggregations of juvenile pollock and capelin. Analysis of acoustic data at 38 kHz and 120 kHz from the east coast of Kodiak Island was presented. The authors conclude that acoustic differencing at the scale of the fish aggregation and at high integration thresholds can be an effective technique to distinguish between juvenile pollock and capelin.
  • Libby Logerwell and Ruth Christiansen: “Energy density of Steller sea lion prey in western Alaska: Species, regional, and seasonal differences (poster).” The energy density of prey fish is a necessary component of foraging models that show how changes in prey abundance or distribution (natural or fishery-related) might impact the feeding success of Steller sea lions. Although values of fish energy density can be found in the literature, data are not available for many species that sea lions eat. This is particularly true for specific geographic regions or seasons. The goal of the FIT project described in the poster was to fill these gaps by collecting fish that are common in sea lion diets, but for which energy density data are unavailable during the seasons and in the regions that sea lions eat them. The data were presented in tables so that scientists could readily incorporate the necessary energy density values into their foraging models. In addition to providing modelers with data, the poster shows that energy density varies not only with fish species but with region, season, and fish reproductive status. The authors caution against building a foraging model with prey energy density values that are not specific to the time and place the model represents.
  • Libby Logerwell and Susanne McDermott: “Interactions between commercial fishing and Atka mackerel: a pilot study to resolve the experimental design.” The goal of the research project was to characterize the effects of commercial fishing on local abundance and movement of Atka mackerel. The Fishery Interaction Team has been conducting tag release-recovery studies of Atka mackerel in the Aleutian Islands since 1999 (see McDermott and Logerwell presentation, below). Results have provided valuable insight into the effectiveness of trawl exclusion zones at preventing prey shortages for Steller sea lions foraging on Atka mackerel. Tag recoveries in previous years have occurred during and after the commercial fishery, so it is not possible to assess the effect of the fishery on the resulting estimates of local abundance and movement. The authors conducted a pilot study in summer and fall 2002 in the area of Seguam Pass to assess the feasibility of collecting tag recovery data both before and after the commercial fishery in areas open and in areas closed to the fishery. They examined these data to determine whether the number of tag releases and recoveries would be sufficient to detect a fisheries effect. They also evaluated whether the trawl exclusion zone is a suitable control for variability in local abundance and movement due to causes other than fishing.
  • Susanne McDermott and Libby Logerwell: “Estimates of Atka mackerel movement and abundance based on tagging data: Are trawl exclusion zones effective?” The FIT research project uses mark recapture methods to estimate local abundance and small-scale movement of Atka mackerel relative to trawl exclusion zones in the Aleutian Islands region. The motivation for the project was to determine whether trawl exclusion zones are effective at preventing prey shortages for Steller sea lions. To do so, information is needed about 1) local abundance of fish, and 2) movement rates of fish into and out of the zones. Model results to date indicate that there is little movement of Atka mackerel from inside to outside the Seguam Pass trawl exclusion zone. This suggests that the trawl exclusion zone is effective at preventing fisheries outside from impacting fish abundance inside.
  • Kimberly Rand and Susanne McDermott: “Seasonal changes in Atka mackerel sex ratios in Seguam and Tanaga Passes during 2002 (poster)”. The Fisheries Interaction Team has been conducting tag release-recovery studies on Atka mackerel in the Aleutian Islands since 1999. To incorporate data into the best possible tagging model, it is also important to understand whether there are differences in abundance or movement by sex. This poster described data on Atka mackerel length and sex frequency from three research charters conducted by the AFSC during the 2002 field season. Using these data in a GIS map, seasonal changes in the sex ratio were observed in both Seguam and Tanaga Passes. The authors also examined differences between sexes relative to depth. In observing the differences in movement between the sexes, we suggest that it is important to construct a sex-specific tagging model.
  • Chris Wilson, Anne Hollowed, Michiyo Shima, Paul Walline, and Sarah Stienessen: “Fishery interaction study: Interactions between commercial fishing and walleye pollock.” The talk summarized the results from the first 2 years of a multiyear fishery interaction study near Kodiak Island. Acoustic surveys were conducted during August 2000 and 2001. The goal of the surveys is to provide information to assess whether the abundance and spatial patterns of walleye pollock are impacted by commercial fishing activities over short spatio-temporal scales. Results presented do not suggest a significant link between fishing activities and changes in estimates of juvenile and adult pollock geographical distribution, biomass, and vertical distribution. The authors note that it will be important, however, to evaluate whether these trends persist during subsequent years.

By Libby Logerwell.

Cod Pot Studies Summary

Design and feasibility studies for the FIT Pacific cod pot studies and local abundance experiment were completed in 2001-2002, and the project was presented to the North Pacific Fishery Management Council (NPFMC) in October. In order to conduct the experiment without gear conflicts, NMFS asked the Council for a 2-week special closure of the Cape Sarichef study area in March of 2003 -2006. While the request drew some concern from industry, a compromise solution was worked out and the modified closure was approved by the NPFMC.

Field operations in fall 2002 and winter 2003 were a mixture of success and frustration.  Final gear feasibility trials in September - October 2002 ran smoothly.  Newly constructed experimental pots and modular trigger assemblies performed well and fished consistently in comparison with standard industry pots.  However, the 28 December - 12 January cruise was delayed by mechanical and equipment problems. The weather in the study area started out badly and rapidly deteriorated. After only 3 days of fishing, conditions were so rough that continued operations were deemed unsafe, and fishing was suspended. The ship and crew then spent several days in Akutan Harbor sitting out gale winds up to 100 knots. When the storms finally blew over, problems with one of the boat’s two main engines forced a return to Dutch Harbor which effectively ended the cruise.

A second cruise leg was conducted 4-17 February , after the close of the snow crab (Chionoecetes opilio) season. The main purpose of this cruise was to tag and release Pacific cod in the vicinity of the study area in order to learn more about seasonal migration patterns and local movement of cod during the spawning season. The cruise also went equipped to collect local abundance data, in the hope that some of the data collection missed by the January cruise could be collected. However, pollock trawling was running around-the-clock in part of the experimental region outside the Cape Sarichef no-trawl zone, so fishing experimental pots in the area was not possible. Samples were collected at experimental stations within the no-trawl zone. Fish were collected and tagged at three locations: within the Cape Sarichef no-trawl zone, west of the study area within the Akun no-trawl zone, and eastward along the 100-m contour toward Amak Island. Over 2,000 cod with hot-pink spaghetti tags were released. Tag shedding and mortality studies were also conducted.

At the time of this writing, field crews are at sea on the March leg of the local abundance experiment. Due to bad weather conditions at the start, both scientific and ship crews were stranded in Anchorage for nearly a week before the cruise could get under way. So far, however, mechanical difficulties have been minor, and an initial data set for the entire study area has been obtained. The cruise provides a unique platform for collecting tissue samples for genetic studies of cod stock structure; the objective is to collect tissue from approximately 1,000 mature cod over the course of the cruise.

Despite the difficulties encountered, we expect the 2003 field season to provide valuable information on cod movements and the behavior of pot catch data as an experimental variable. With a limited sample size, quantitative tests for the presence or absence of a fishery effect may be inconclusive. The data collected, however, should be sufficient to validate the performance and power of the experimental design.

By Elizabeth Conners.


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