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NOAA Technical Memorandum NMFS-AFSC-285

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Extension of genetic stock composition analysis to the Chinook salmon bycatch in the Gulf of Alaska walleye pollock (Gadus chalcogrammus) trawl fisheries, 2012

Abstract

The genetic stock composition of Chinook salmon (Oncorhynchus tshawytscha) samples from the 2012 U.S. Gulf of Alaska (GOA) trawl fishery for walleye pollock (Gadus chalcogrammus) was extended to provide an overall stock composition for the fishery bycatch and stock-specific harvests.  Genetic samples were collected opportunistically in 2012 from Chinook salmon taken as the bycatch in this fishery.

These samples had previously been genotyped for 43 single nucleotide polymorphism (SNP) DNA markers and the results were estimated using the available coastwide baseline of SNP markers for Chinook salmon.  However, the opportunistic nature of the sampling raised concerns about applying the proportional composition of the samples to the entire bycatch because of unknown, but potentially significant, biases.  Here we investigate the most appropriate means by which this extension can be made.  While sample sizes varied widely and did not achieve the 10% target used to sample bycatch in the Bering Sea, using a stratified estimator weighted by stratum-specific bycatch, it was possible to estimate the stock composition of the total catch with acceptable accuracy and precision.

Based on the reanalysis of 948 Chinook salmon bycatch samples, the proportions of reporting groups did not change significantly from previously published values: British Columbia (50%), U.S. West Coast (28%), and coastal Southeast Alaska (19%) made up the largest reporting groups.


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