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Resource Ecology and Ecosystem Modeling Program

Fish Stomach Collection and Lab Analysis

Laboratory analysis was performed on 2,028 groundfish stomachs from the eastern Bering Sea and 3,187 stomachs from the Aleutian Islands region. Observers returned 1,257 stomachs from the Bering Sea during this quarter. A total of 21,772 new data records were added to the groundfish food habits database.

By Troy Buckley and Geoff Lang

Analyzing Groundfish Stomach Contents at Sea

This year, the Resource Ecology and Ecosystem Modeling (REEM) Program is experimenting with performing detailed analysis of groundfish stomach contents at sea instead of the usual practice of returning stomach samples back to the lab for analysis. Personnel participating in the bottom trawl surveys of the Aleutian Islands and upper continental slope of the eastern Bering Sea are equipped with motion compensating scales (that measure to the nearest 0.5 g) and other equipment to make this at-sea analysis feasible. The goal is to produce data that retain the most important detailed aspects of laboratory analysis, such as identification of commercially important prey to the species level, while reducing the costs associated with buckets, chemicals, bags, storage, shipping and laboratory analysis. To accomplish this, the minimum identification standards have been adjusted to require less detail, especially for small invertebrates; prey that aren’t identifiable at-sea to the minimum standard are being preserved and returned to the lab. A cursory examination of the data collected during the first leg of each survey indicates that sample sizes obtained are 20% lower than the number of samples collected using traditional methods in previous years. After the completion of the field season, the sample sizes and taxonomic detail obtained will be compared with those from the more traditional collection and analysis methods, and a decision will be made whether to continue at-sea analysis. In any event, detailed stomach content analysis in the lab will still be necessary when addressing more complex ecological questions that require analyzing small invertebrate prey to more detailed taxonomic levels.

By Troy Buckley

Surplus Production in Marine Ecosystems

The question of the amount of “surplus” production available in marine ecosystems is a critical one for fisheries management. On one hand, catching a fish makes it unavailable to predators. In a tightly connected ecosystem, there may be little overall energy surplus. On the other hand, the removal of large fish leads to younger and possibly more energy-efficient fish populations. While humans cannot control the interactions in an ecosystem, modeling these shifts of energy flow is an important step to understanding the overall yields that might be expected from ecosystems, especially if maintaining the health of top predators such as marine mammals is a management priority.

“Reconciling ecosystem-level surplus production predictions between Ecosim and bioenergetics models” by Kerim Aydin reviews the energetic assumptions most commonly used in ecosystem models and explores the energetic implications of model predictions while suggesting model improvements for quantifying the energetic shifts that might be expected to occur through fishing. Specifically, the paper suggests that the biomass dynamics model Ecosim has a tendency to underestimate the dietary requirements of large, slow-growing, older fish in unfished ecosystems, and may, therefore, overestimate the biomasses of top predator populations that were supportable by ecosystems before fishing began. Some relatively simple bias correction formulas for Ecosim, based on the expanding the model to take life-history strategies into account using von Bertalanffy growth parameters, are derived, tested, and recommended.

The paper was recently accepted for publication in the African Journal of Marine Science.

By Kerim Aydin

Seabird-Fisheries Interaction Research

Two field activities were implemented in the third quarter of 2004 to study seabird mortality and seabird pelagic distribution. The first was a special project for North Pacific groundfish observers. In their normal duties on commercial trawl vessels, observers record any seabirds recovered while sampling the catch. Observers have also recorded, in anecdotal notes, that seabird mortalities occur from interactions with the trawl main cables or trawl sonar cables. These interactions and mortalities are not recorded in a systematic way, so we cannot make estimates of this additional source of mortality. To address this issue, a pilot project had been completed by four observers in January through March 2004 to capture this information while not disrupting normal sampling duties. Based on comments from these observers and other reviewers, project instructions were revised in June and the work was expanded such that most observers on trawl vessels will collect this information in the latter half of 2004.

The second project, implemented in May, will increase our understanding of seabird distribution and habitat use in pelagic waters of the Bering Sea and Aleutian Islands. This project was an expansion of the stationary seabird surveys developed by the Washington Sea Grant Program for use in longline surveys carried out by the International Pacific Halibut Commission, Alaska Department of Fish and Game, and NOAA Fisheries. That project is in its third year, and the 2002 data report is available from Washington Sea Grant. The surveys have yielded valuable information on seabird distribution and relative abundance. To increase the time and area coverage of these surveys, the protocols were revised for use on trawl vessels and implemented on Alaska Fisheries Science Center summer research charters. Five vessels will complete three legs each, covering the Bering Sea shelf, Bering Sea slope, and Aleutian Islands. At each station, if weather permits, a seabird survey will be completed and numbers of birds by species or species groups within a specific distance from the vessel recorded. These data will add greatly to our increasing understanding of seabird distribution, abundance, and habitat use in these waters.

By Shannon Fitzgerald


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