Research Document - 1999/015
Review of hydroacoustic methodology and Pacific hake biomass estimates for the Strait of Georgia, 1981 to 1998
By R. Keiser, M.W. Saunders and K. Cooke
Abstract
The purpose of this paper is 1) to make available to assessment scientists a review of the methods used in six acoustic surveys that were completed between 1981 to 1998 to estimate Pacific hake (Merluccius productus) abundance in the Strait of Georgia and 2) to provide a time series of hake biomass standardised to a single target strength value. This includes a description of the evolution of the acoustic system, data collection, analysis, presentation techniques and sources of uncertainty unique to each survey. A method for calculating biomass based on a target strength length relation is presented and biomass estimates from all surveys are given. Calibration, target strength and survey data are statistically examined to obtain a representative confidence interval that can be used as a guide for all surveys. Additional sources of uncertainty should be recognised when interpreting Pacific hake abundance from this time series. These include: 1) the 1993 survey is an overestimate due to inclusion of plankton back-scattering; 2) based on examination of sex ratio and survey timing, all surveys are conservative estimates of biomass; and 3) comparable results from all surveys are obtained by providing biomass estimates based on a single mean TS and on our new length-based TS model. We include recommendations for improving future surveys.
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