Research Document - 2001/84
Estimation of research trawl survey catchability for biomass reconstruction of the eastern Scotian Shelf
By J Harley, S.J., Ransom M., Barrowman, N., Bowen, K., Amiro R.
Abstract
This report summarizes results and methods of estimating the catchability of trawl surveys, from a meta-analytic perspective.
We have attempted to compile all quantitative data in the world on the subject to allow reconstruction of fishing community structure on the eastern Scotian Shelf of Canada.
We consider two types of analysis. ``Bulk'' catchabilities in which the survey swept-area biomass is converted into an estimate of absolute biomass using a single estimate catchability for all individuals. These were generally obtained directly from the literature or from assessment models where length-specific results could not be obtained. The other type of analysis estimates the length-specific catchability from quantitative assessments, i.e., various types of sequential population analysis. The length-specific data were analysed using maximum likelihood and hierarchical Bayesian models.
We collated 80 estimates of bulk and length specific catchability for 23 species from stock assessments conducted in North America, Europe, and New Zealand. Using hierarchical Bayesian methods, we combined estimates of catchability within species (and across similar species) to derive {\em synthetic} estimates of length-specific catchability that can be applied in the calculation of absolute abundance from swept-area biomass estimates from trawl survey time series. After ecological and morphological features of various species found on the Scotian Shelf, we provide recommendations as to the values of catchability to be applied for species for which we could not obtain direct estimates.
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