Research Document - 1999/022

1998 Evaluation of the natural mortality rate used in the assessment of British Columbia herring stocks

By J.F. Schweigert and R. Tanasichuk

Abstract

The estimate of the natural mortality rate assumed in conducting assessments of population abundance of Pacific herring in British Columbia was reviewed and compared to empirical estimates derived from information on numbers of fish in the population based on spawn deposition surveys. Retrospective analyses of stock trajectories based on assuming a range of values of natural mortality were compared within and between stocks. Likelihood profiles indicated that natural mortality rate parameters were well determined in the catch-age analysis although the estimates remained more variable than might be anticipated from other biological information. There was no evidence that estimates of natural mortality obtained from either the empirical approach or from the catch-age analysis were unrealistic or flawed in any obvious manner. It was concluded that the current approach of estimating natural mortality annually as part of the assessment exercise was reasonable and should be continued but further effort is required to understand the interactions among model parameters such as vulnerability and natural mortality.

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