The Alaska arrowtooth flounder stock assessment: how can we make it better?

Dr. Ingrid Spies1
February 16, 2016 9:00 (PST): FSH 203

The Alaska arrowtooth flounder stock assessment: how can we make it better?

Coauthors: Steve Barbeaux and Miriam Doyle

A single stock assessment model written in AD Model Builder is used for the Gulf of Alaska and Bering Sea/Aleutian Island stocks of arrowtooth flounder. The age-structured model begins in 1961 in the GOA and 1976 in the BSAI. The modeled population includes individuals from age 1 to age 21, with age 21 defined as a “plus” group (all individuals age 21 and older). The age composition of the species shows fewer males relative to females as fish increase in age, which suggests higher natural mortality (M) for males. To account for this process, natural mortality is fixed at 0.2 for females and 0.35 for males in the model. The model is fit to time series of catch, survey indices of abundance (3 surveys in the BSAI: Bering Sea slope, Bering Sea shelf, and Aleutian Islands), and estimates of age and length composition from fishery and survey. Model parameters are estimated by maximizing the log likelihood of the data, and there are likelihood components for: survey biomass, lengths from the fishery and the survey, survey ages, catch, recruitment, and female and male fishery selectivity.

There are several aspects of arrowtooth distribution and life history that have not been considered in the stock assessment, and this presentation is designed to stimulate a discussion on how to improve the assessment. Models suggest that the population sizes have increased 8-fold in the BSAI and four-fold in the GOA over the past few decades. The model does not incorporate carrying capacity, but there are some signs that population growth is decreasing. In addition, the maximum age and sex ratio of arrowtooth flounder appears different by region, which may contradict the assumption of different natural mortality rates for males and females.

Posted in Fisheries Think Tank.

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