Alaska Fishery Research Bulletin Issues, Vol.10 No. 1 - Summer 2003

The Feasibility of Reducing the Variance of Fish Relative Abundance Estimates by Integrating CPUE Data from Two Demersal Trawl Surveys in the Gulf of Alaska

Paul G. von Szalay - Vol. 10(1):1-13. 2003.

Catch per unit effort data of walleye pollock Theragra chalcogramma, Pacific cod Gadus macrocephalus, arrowtooth flounder Atheresthes stomias, and flathead sole Hippoglossoides elassodon from the National Marine Fisheries Service and Alaska Department of Fish and Game 1999 and 2001 Gulf of Alaska bottom trawl surveys were integrated to evaluate the feasibility of reducing the variance of the National Marine Fisheries Service biomass estimates for these species. Because of differences in the spatial design and areas covered by the 2 surveys, 2 new strata, recognizing bays as a separate stratification element and made up of portions of some of the original National Marine Fisheries Service strata, were introduced. Variance estimates based on the combined surveys were similar (<10% difference) to those based exclusively on the National Marine Fisheries Service survey data for all species and both years except for walleye pollock in 2001 (84% higher for the integrated surveys). Biomass estimates were also similar (<10% difference) between the integrated and nonintegrated data for all species and both years except for walleye pollock in both 1999 and 2001 (22% and 82% higher estimates for the integrated surveys in 1999 and 2001, respectively). A potential reason for the unexpected increase in the biomass and variance estimates for walleye pollock is a significant time-dependence of the fishing power correction factor due to vertical migration of these fish. The appropriateness of integrating the data from these surveys is discussed.

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Sex, Age, and Growth of Black Rockfish Sebastes melanops from a newly exploited population in the Gulf of Alaska, 1993–1999

Carrie L. Worton and Gregg E. Rosenkranz - Vol. 10(1):14-27. 2003.

A newly exploited population of black rockfish Sebastes melanops were sampled from commercial landings in the Gulf of Alaska from 1993–1999 to identify issues in future management of this resource. Fish were measured, sexed, and sagittal otoliths were removed and aged. Males dominated the catches, comprising 71% of the samples. The majority of the catch was between 38 and 53 cm fork length, with a distinct mode at 39 cm fork length in 1997 in both the Kodiak and Alaska Peninsula Areas. Length-at-age for each year revealed differences in growth between sexes, with females larger than males. Growth differences between areas were observed but were significant for only males. The oldest fish aged was 48 years old, with the majority of the catch between 8 and 20 years old. Variations in year class strength were large and apparent in all areas. Strong recruitment events in 1979 and 1990 were tracked throughout the entire range of collection years and throughout the Gulf of Alaska. Research priorities for black rockfish in the Gulf of Alaska should focus on determining the relationship between sex-specific depth distributions, changes in sex compositions with respect to reproductive timing, age of maturity, and the affects of local and large scale environmental conditions on recruitment. Maintenance of a time series of biological data such as age and growth and monitoring specific fishing practices may offer indicators of long-term fishing effects on successful recruitment.

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Coherence of Observed Adult Sockeye Salmon Abundance Within and Among Spawning Habitats in the Kvichak River Watershed

Ian J. Stewart, Ray Hilborn, and Thomas P. Quinn - Vol. 10(1):28-41. 2003.

Despite coherent trends in aggregate abundance of Pacific salmon Oncorhynchus species at ocean-basin scales, individual populations often fluctuate out of phase with one another. However, spatial covariation in shared environmental conditions may lead to correlation among proximate populations in survival, recruitment, and subsequent adult abundance. We found strong covariation in 45 years of aerial abundance estimates of adult sockeye salmon Oncorhynchus nerka in 93 spawning populations in the Kvichak River system, Bristol Bay, Alaska. A significant negative relationship with geographic distance (P<0.01) accounted for just 2.96% of the variability in both correlation of abundance between populations, and a related similarity coefficient. There was no evidence for a threshold of rapid decay in this relationship, despite distances of up to 175 km between populations. Island beaches showed significantly higher covariation (P<0.05) than other types of spawning habitat, even after removing the effect of distance. We conclude that these patterns are likely a result of the commercial fishery, broad environmental patterns experienced by populations throughout the drainage, and differences in population-specific early life history survival rates among habitats. A better understanding of the degree to which spawning sites show covariation in abundance, as well as of patterns present within a single river system, could assist researchers in locating suitable control sites for experiments and may allow for more accurate interpolation of missing historical data.

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Change in the Performance of a Bering Sea Survey Trawl Due to Varied Trawl Speed

Kenneth L. Weinberg - Vol. 10(1):42-49. 2003.

An experiment was conducted by the Alaska Fisheries Science Center to study the effect of trawl speed through the water on the footrope bottom-tending performance, and other measures of trawl geometry, of the 83/112 Eastern flatfish trawl used annually in surveys of commercially important eastern Bering Sea crab and groundfish resources. A series of tows was made in one eastern Bering Sea location at vessel speeds (speed over ground) ranging from 2.0 to 5.0 knots at 0.5 knot intervals while simultaneously measuring the speed of the trawl moving through the water. The distance of the center of the footrope above the bottom was measured using an electronic bottom contact sensor. The flatfish trawl gear remained relatively stable as evidenced by only minor increases in measured footrope distances off the bottom with increasing trawl speed. Predicted heights were 1.1 cm at 3 knots, 2.5 cm at 4 knots, and 4.9 cm at 5 knots speed through the water. Footrope distances off the bottom were relatively minor at trawl speeds through waters typically encountered during a Bering Sea survey and likely would have a nominal effect on the capture rates of most commercially targeted semi-benthic fish species. However, variable footrope contact caused by variable trawl speed through the water could potentially introduce unwanted variability in estimates of catch per unit of effort for bottom dwelling fauna such as sculpins, skates, flatfish, red king crab Paralithodes camtschaticus, snow crab Chionoecetes opilio, and Tanner crab C. bairdi. Survey scientists might better control the variability in catch per unit of effort estimates caused by varying trawl efficiency by lending attention to local current conditions. One improvement to surveys may well be standardizing towing speed to trawl speed through the water.

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Identifying the Spatial Distribution of Stocks of Migrating Adult Sockeye Salmon Using Age Composition Data

Lucy Flynn, Ray Hilborn, and André E. Punt - Vol. 10(1):50-60. 2003.

We present a model for identification of the spatial distributions of component stocks of sockeye salmon Oncorhynchus nerka in the Port Moller test fishery, which are migrating to Bristol Bay, Alaska, using the differential age composition of those stocks. We model the spatial distribution of each stock as a normal density with parameters of mean and standard deviation of distance along the sampling transect. The model predicts the number and age composition of sockeye salmon at different sampling stations, and compares them to the observed abundance and age composition at these sampling stations. Some level of stock separation is apparent at Port Moller, but the ability to discriminate a component stock depends on both its uniqueness of age composition and its relative magnitude. We believe these methods may provide additional information that can be integrated with other sources currently used for inseason projections of sockeye salmon runs in Bristol Bay.

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Interactions Between Commercial Fishing and Walleye Pollock

Christopher D. Wilson, Anne B. Hollowed, Michiyo Shima, Paul Walline, and Sarah Stienessen - Vol. 10(1):61-77. 2003.

Results from the first two years of a multiyear fishery interaction study near Kodiak Island in the Gulf of Alaska are presented. Findings from acoustic surveys, which were conducted in August 2000 and 2001, provide important information that begins to address the question of whether the abundance and spatial patterns of various species, including walleye pollock Theragra chalcogramma are impacted by commercial fishing activities over short spatio-temporal scales. The biomass and distribution of walleye pollock were stable over periods of days to weeks although during the second year an unusual, extremely dense, small-scale walleye pollock aggregation was detected during one of several survey passes. Several morphological descriptors of the walleye pollock echosign layers were evaluated to better understand whether differences at the scale of the fish aggregations occurred in response to fishing. Variography was also used to quantify walleye pollock spatial patterns. Results from the second year, when the commercial fishery took place within the study area, do not suggest a significant link between fishing activities and changes in estimates of juvenile and adult walleye pollock geographical distribution, biomass, or vertical distribution. It will be important, however, to evaluate whether these trends persist during subsequent years.

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