Research Document - 2009/021

Assessment and Management advice for Pacific hake in U.S. and Canadian waters in 2009

By S. Martell

Abstract

This is an alternative stock assessment model for Pacific hake (Merluccius productus) in Canadian and U.S. Waters for 2009. The assessment model is parameterized in terms of management variables maximum sustainable yield (C*) and the fishing mortality rate (F*) that achieves C*, and provides catch advice in the form of a decision table based on the risk of exceeding fishing mortality rate and spawning biomass targets. The stock assessment model was fit to historical time series information on relative abundance (from a fisheries independent acoustic survey) and age-composition information from fisheries independent and dependent sources. The likelihood components in the statistical fitting criterion were each weighted using the maximum likelihood estimates of the variance conditional on a prior distribution for the proportion of the total variance associated with observation errors. Parameters were estimated using both maximum likelihood and Bayesian approaches. The decision table is based on a Bayesian interpretation of the data. Sensitivity analysis was conducted using alternative prior distributions for key model parameters; catch advice decreased by as much as 28% if a higher proportion of observation error was assumed. Catch advice was very sensitive to alternative prior distributions for the instantaneous natural mortality rate. There was also some retrospective bias in estimates of current spawning stock biomass, fishing mortality rate and the relative strength of the 1999 year class, but virtually no bias in the estimates of unfished spawning stock biomass, survey catchability coecients and productivity parameters. As each subsequent year of data is removed in the retrospective analysis, the strength of the 1999 year class and spawning stock biomass was reduced, while estimates of fishing mortality increased (i.e., err on the conservative side).

Removals for the 2008 coast wide fishery were estimated at 322,017 metric tons, which is roughly 88.2% of the permissible catch in 2008. The fishing season was protracted in 2008; fishing operations in Canada terminated at the end of December 2008. Median estimates of spawning stock depletion at the start of the 2009 is estimated at 34.7% of unfished states with a 90% credible interval of 12.3–69.4%. Median estimates of instantaneous fishing mortality in 2008 was 0.735 (nearly double the estimate of F*) with a 90% credible interval of 0.303–3.265. Catch advice for the 2009 fishery based on median parameter values and the Pacific Fisheries Management Councils 40/10 harvest rule is 174,000 metric tons, and the 25^th and 75^th percentiles are 72,000 and 308,000 metric tons respectively. Catch advice based on the 50^th percentile of exceeding F* was also 174,000 metric tons, and the 25^th and 75^th percentiles were 91,000 and 256,000 metric tons.

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