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Status of Stocks & Multispecies Assessment Program

Western Groundfish Conference Presentations

SSMA Program staff attended the Western Groundfish Conference held 30 January to 3 February in Newport, Oregon, and presented talks and/or posters. Abstracts from select presentations (below) are provided with SSMA staff names presented in italics.

Looking for Localized Depletion in Bering Sea Pacific Cod

M. Elizabeth Conners, Peter Munro, Sandi Neidetcher, and Elizabeth A. Logerwell
The Fisheries Interaction Team at the AFSC has conducted a field experiment on Pacific cod in the heavily-fished region of the southeast Bering Sea. Alaskan groundfish fisheries have been restricted in recent years due to concern over possible competition with the endangered Steller sea lion. The goal of the experiment was to determine if intensive trawl fishing for cod creates a localized depletion in fish abundance that could adversely affect prey availability for sea lions.

A regulatory notrawl zone intersecting historical trawl grounds provided opportunity for a treatment/control type experiment. The experiment uses a before-after, treatment-control type design to compare the seasonal rate of change in cod abundance within the Cape Sarichef notrawl zone to the rate of change in the adjacent heavily trawled area. Use of fixed gear (pots) for the study allowed much higher sample size than would have been feasible with trawls, and much lower variance in the catch data. These features made it possible to perform a statistically accurate, powerful, and robust test for the presence of a localized depletion.

While the cod catch rates and observed seasonal changes were variable over the 3 years of the study, the result of the comparison between trawled and untrawled areas was remarkably consistent. In each of the 3 years, the nonparametric statistical test overwhelmingly indicated no difference between sites in the trawled and untrawled areas (P-values of 0.86 to 0.98). Power of the 2003 test was low due to small sample size, but power calculations indicate that the experiments in 2004 and 2005 would have been able to detect a reduction in the average catch of the trawled zone in the range of 20%-30%. Maps of the observed catches and seasonal percentage change showed no consistent spatial pattern.

The concept of localized depletion is strongly dependent on assumed spatial and temporal scale. This experiment looked for an effect based on assumptions that fishing effects would be evident within 5 nmi of the removal and persist for at least several weeks. The observed results indicate that actual fishing effects occur at different spatial and temporal scales. The results of concurrent tagging and biological studies suggest that cod stocks in the study area were highly mobile over time scales shorter than 2 weeks. In order for a fishery to produce persistent localized depletion, the target pool of fish must be static over an extended time period. Our work near Unimak Pass leads us to believe that this assumption is not valid for Pacific cod, at least in this region.

Aspects of the Life History of the Alaska Skate, Bathyraja parmifera

Beth Matta and Don Gunderson
The Alaska skate (Bathyraja parmifera) is a large-bodied species that accounts for over 90% of the skate biomass on the eastern Bering Sea shelf. Bathyraja parmifera is commonly caught as bycatch in Bering Sea trawl and longline fisheries, yet information regarding its life history is limited. Studies focusing on age, growth, and reproductive biology are essential to stock assessment and management of this species. Since 2003, over 600 specimens have been collected during summer groundfish surveys conducted by NMFS and seasonally by the NMFS Observer Program.

Banding patterns in thin sections of vertebral centra and in whole caudal thorns are being compared for age determination. Maturity, estimates of batch fecundity, and seasonal reproductive timing are also being studied. Males exhibited a preliminary maximum observed age of 17 years and females an age of 21 years. The preliminary median age and size at maturity was 10 years and 91 cm for males and 11 years and 94 cm for females. This species also appears to be reproductively active throughout the year, supported by gonadosomatic index (GSI), uterine egg case, and ovarian egg data. Research is ongoing, and results will be considered in future stock assessments for B. parmifera.

The Management Strategy Evaluation Approach and the Gulf of Alaska Walleye Pollock Fishery

Z. T. A'mar, Punt, A. E., and Dorn, M. W.
Management strategy evaluation (MSE) is the process of using simulation testing to examine the robustness of proposed management strategies to error and uncertainty.

Implementing a MSE involves three main parts: 1) a model (the "operating model") to represent the true underlying dynamics of the resource and to generate future data; 2) an estimation model to assess the state of the stock relative to agreed target and limit reference points at each time step based on the simulated data; and 3) a catch control rule to determine management actions (e.g., the Total Allowable Catch) given the results of the estimation model.

The latter two steps constitute the management strategy. The parameters of the management strategy can be selected to achieve desired (but conflicting) management goals and objectives. The results of an MSE are evaluated based on performance measures designed to quantify the effectiveness of the management strategy toward stated management goals. A MSE has been developed for the Gulf of Alaska walleye pollock fishery. A number of factors including pollock stock structure, climate effects, and ecosystem changes constitute hypotheses in the operating model that are being tested.

The Effect of Maternal Age of Spawning on Estimation of Fmsy for Alaskan Pacific Ocean Perch

Paul Spencer, Dana Hanselman, and Martin Dorn
Recent laboratory research suggests that rockfish larval survival rates increase with the age of the spawner, thus potentially necessitating more conservative harvest policies that explicitly consider the age structure of the spawning stock biomass. In this study, we use simple deterministic population dynamics equations to examine how reduced spawning effectiveness of younger spawners may affect commonly used reference points such as Fmsy and Fxx%, the fishing rates corresponding to the maximum sustained yield and conservation of xx% of the reproductive output per recruit relative to an unfished population, respectively.

Reduced effectiveness of younger spawners results in reduced reproductive output conserved for a given level of fishing mortality, but also increased resiliency in stock-recruitment curves due to an equivalent number of recruits being associated with a reduced measure of reproductive output. For Bering Sea/Aleutian Islands and Gulf of Alaska Pacific ocean perch, these two effects nearly equally counteracted each other, producing stable Fmsy estimates for three different measures of reproductive output. However, estimates of Fxx% rates that consider uncertainty in the stock-recruitment parameters and in the degree to which maternal age affects reproductive success are more conservative than those based on analyses using spawning stock biomass as reproductive output.

These results indicate that perception of stock resiliency is not necessarily independent of the life-history parameters describing production of reproductive output. The degree of adjustment of Fxx% rates required will depend upon further analyses with realistic environmental variability and careful consideration of the range and weights assigned to the stock-recruitment and life-history parameters evaluated.

Histological Validation of a Visual Maturity Key for Pacific Cod

Sandra Neidetcher and Jim Stark
Investigations of spawning processes of Pacific cod, Gadus macrocephalus, require knowledge of the gonad developmental stage of individual fish. Visual staging based on the external and internal appearance of gonads can be a quick and easily applicable tool to assess maturity stages. Though these methods allow for increased sample sizes, they are subjective and sometimes inaccurate, especially for gonads in transitional stages of development.

We describe the visual gonad key employed in cod research conducted by the Fisheries Interaction Team of the Alaska Fisheries Science Center, NMFS. We present data collected as part of studies of cod spawning processes near Unimak Pass in the southeastern Bering Sea in the winters of 2002 through 2005. Histological specimens were collected for gonads that had also been coded according to the visual key. The histological specimens were processed to identify the presence of the most advanced structures and were then used to validate the development stages indicated by the visual key.

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