RESOURCE ECOLOGY & FISHERIES MANAGEMENT (REFM) DIVISION (cont.)

Status of Stocks & Multispecies Assessment Program

Fishery Interaction Team Presentation at Ocean Research Conference

Susanne McDermott and Libby Logerwell, members of the Fishery Interaction Team subtask, gave oral presentations at the 2004 Ocean Research Conference held in Honolulu, Hawaii, during 15-20 February. The conference was sponsored by the American Society of Limnology and Oceanography and The Ocean Society, and their presentations were given in a session on “Oceanography and Ecology of the Aleutian Archipelago.”

Libby Logerwell gave a presentation entitled, “Geographic patterns in the demersal ichthyofauna of the Aleutian Islands shelf ” coauthored by Kerim Aydin, Steve Barbeaux, Eric Brown, Liz Conners, Sandra Lowe, Jay Orr, Ivonne Ortiz, Rebecca Reuter, and Paul Spencer. Geographic patterns in the Aleutian Islands ichthyofauna from the individual to the community level of organization were presented. At the highest level of organization (the community), patterns and trends in species occurrence and community composition were shown. At the population level, some species showed depth and longitudinal trends in distribution and abundance while others showed patchy distributions. At the individual level, geographic patterns in demersal fish diet and growth were observed. These geographic patterns were consistent with patterns in the region’s physical and biological properties described by other presenters in the session. For instance, many characteristics of demersal fish changed at Samalga Pass. Other presenters described changes in climate variability, physical oceanography, zooplankton species composition, seabird diet, and marine mammal distribution at Samalga Pass. The authors’ results also suggest that there may be other transition zones in the western Aleutian Islands that have not yet been identified oceanographically, such as Adak Strait, Amchitka Pass, and Buldir Island. In addition to step-changes at the passes, there were longitudinal trends in demersal fish characteristics, such as growth, that point to continuous environmental variation along the length of the Aleutian Islands chain. Finally, some species were more patchily distributed than others, and high catches of these patchily distributed species may indicate areas of increased production due to mixing and/or upwelling.

Susanne McDermott presented “Estimating movement and abundance of Atka mackerel (Pleurogrammus monopterygius) with tag release data,” coauthored by Lowell Fritz and Vivian Haist. This presentation described results from a mark-recapture experiment conducted in Seguam Pass, inside and outside a trawl exclusion zone. The purpose of the experiment was to estimate local Atka mackerel abundance around a Steller sea lion rookery. In 1999, 1,375 tagged fish were released. A biomass of 76,679 metric tons (t) was estimated outside the trawl exclusion zone. In 2000, 8,773 tagged fish were released. An integrated tagging model estimated the biomass to be 117,900 t inside and 82,057 t outside the trawl exclusion zone. Probability of fish moving from outside to inside the zone was small (0.0056) and from inside to outside was potentially large (0.81), but was associated with high uncertainty (the 95% confidence interval ranged from 0 to 1.00). Preliminary results for the tag and release events in 2002 showed that there was a potential influx of young fish that happened after fish were tagged and showed up during the recovery event. An immigration factor was calculated by adjusting length frequency distributions of tagged fish and fish examined for tags. This immigration factor was then used to adjust the numbers of fish examined for tags and resulted in adjusted population abundance estimates. Population sizes were within the range, but slightly higher than the 2000 results in Seguam Pass with similar movement rates. Population sizes for Tanaga Pass were estimated at 100,000 t inside and 67,000 t outside the trawl exclusion zones. Movement rates were 0.11 from the inside to the outside and 0.6 from the outside to the inside, but again were associated with high uncertainty. The results indicated that tagging as a means of abundance and movement estimation seems to work well for Atka mackerel. The results also showed that Atka mackerel do not appear to move outside of their localized aggregations (<20 km). This lack of movement might result in local adaptations to differences in ocean conditions along the Aleutian Islands chain, such as those documented by other presenters in the session.

The other presentations in the session described research on prehistoric human populations, climate variability, physical oceanography, primary productivity, zooplankton distribution, seabird foraging ecology, marine mammal distribution (including endangered Steller sea lions), and pollock egg and larval characteristics. Much of the work presented in this session will be published in a special issue of Fisheries Oceanography focusing on the Aleutian Islands region.

By Libby Logerwell


A New Assessment Technique for Shortraker and Rougheye Rockfish

A new stock assessment model for Bering Sea and Aleutian Islands (BSAI) rougheye and shortraker rockfish was used in 2003. In previous stock assessments, biomass of rougheye and shortraker rockfish was estimated by simple averaging of recent survey biomass estimates. The new assessment procedure uses a Kalman filter to fit a surplus production model to the survey and catch data. The Kalman filter provides a statistically rigorous methodology that considers process errors (underlying natural variability), observation errors (errors in our estimates of the true state of nature, such as the biomass of fish), and covariances between observations. For BSAI shortraker and rougheye rockfish, catches have been reported for the two-species complex, although an estimate of catch by species can be made by extrapolating species-specific catches from fishery observer data to the total aggregated catch. However, sampling variability in the observer catch proportions would be expected to affect the variance of the resulting catch estimates, and consideration of statistical errors in the observer data adds considerably to the assumed observation errors in catch for BSAI rougheye and shortraker rockfish. Additionally, the new assessment model allows for estimation of parameters with biological and management importance, such as population growth rate. The new assessment methodology was presented to the North Pacific Fishery Management Council’s BSAI Plan Team in September 2003 and was applied to BSAI shortraker and rougheye rockfish in November 2003.

By Paul Spencer

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AFSC Quarterly Research Reports Jan-March 2004