A Preliminary Spawning Biomass Estimate of Walleye Pollock, Theragra Chalcogramma, in the Shelikof Strait, Alaska, Based on the Annual Egg Production Method
Abstract:The spawning biomass of the population of walleye pollock that spawn in Shelikof Strait, Alaska, was estimated using the annual egg production method. Ichthyoplankton surveys are routinely conducted by the National Marine Fisheries Service in Shelikof Strait during peak spawning time of walleye pollock. Survey data from 1981 and from 1985 to 1988 were used in this analysis. The annual production of eggs was estimated in two steps. First, the average spawning season was described by a unimodal curve relating egg production to date within the season. This was estimated by regressing daily production of young eggs from each cruise against the mean date of the cruise. Only those years with multiple cruises during the spawning season were used to estimate parameters of the annual egg production curve. Peak spawning was found to occur between 15 March and 2 May. Next, egg mortality and annual egg production were estimated by integrating a model over the spawning season that combines the annual egg production curve with an exponential mortality model. Annual egg production appears to have declined since the highest level in 1981. Annual fecundity per gram female varied over years. Sex ratio was assumed to be 0.5 in numbers of fish, but it was scaled to average male and female weights. The resulting spawning biomass estimates follow the same downward trend as the annual egg production estimates and follow the same trend as biomass estimates for walleye pollock based on other methods and surveys.
Document Type: Research Article
Publication date: September 1, 1993
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