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A method of estimating spawning biomass from egg surveys with an application to walleye pollock, Theragra chalcogramma, in Shelikof Strait, Alaska
  • Published Date:
    1993
Filetype[PDF - 3.17 MB]


Details:
  • Corporate Authors:
    Alaska Fisheries Science Center (U.S.)
  • Description:
    The annual egg production method of estimating spawning biomass was applied to the population of walleye pollock that spawn in Shelikof Stait, Alaska. 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 bell-shaped curve relating egg production to date within the season. This was estimated by regressing total abundance 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 thet combines the annual egg production curve with an exponential mortality model. This model allows for the different production rates of eggs of different ages from the same survey and was fit to the one survey from each year that occurred during peak spawning. Annual egg production appears to have declined since the highest level in 1981. Annual fecundity per gram female varied over years but was linear with female weight. Sex ratio was assumed to be 0.5 in numbers of fish, but was scaled to average male AND femele weights. The resulting spawning biomass estimates follow the same downward and female weights. The resulting spawning biomass estimates follow the same downward trend as the annual egg production estimates end follow the same tend as biomass estimates for walleye pollock based on other methods and surveys.

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