ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on September 20, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(9):1800-1819; doi:10.1093/icesjms/fsm145
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Taking account of catchability in groundfish survey trawls: implications for estimating demersal fish biomass
1 Fisheries Research Services, Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen AB11 9DB, UK
2 Wageningen–IMARES, PO Box 68, 1970 AB IJmuiden, The Netherlands
Correspondence to H. M. Fraser: tel: +44 1224 295439; fax: +44 1224 295511; e-mail: h.fraser{at}marlab.ac.uk
Fraser, H. M., Greenstreet, S. P. R., and Piet, G. J. 2007. Taking account of catchability in groundfish survey trawls: implications for estimating demersal fish biomass. – ICES Journal of Marine Science, 64.Groundfish surveys are a key component of current scientific data monitoring and data-collection activities undertaken in support of fisheries management. Recent requirements to develop and implement an ecosystem approach to management are placing increasing and varied demands on such datasets. Successfully incorporating ecosystem and environmental objectives within fisheries management will, for example, require greater understanding of foodweb trophodynamics, which in turn requires detailed information on the abundance and distribution of fish predators and prey species on spatial scales hitherto rarely considered. However, no trawl gear catches all the fish in its path, so density estimates provided by such trawl samples do not reflect true densities of fish. Catchability in a trawl gear is affected by many factors and varies both between species and between different sized conspecifics, and therefore has the capacity to confound our understanding of predator–prey interactions and of the relative abundance of different species and size classes of fish at any point in time or space. To overcome such problems, estimates of the catchability of each size class of each species sampled in a given survey are required for trawl sample densities to be raised to the actual densities of fish present at each location sampled. Here, we present a method for estimating catchability coefficients for 1-cm size classes of fish species sampled by the Grande Overture Vertical trawl during the third quarter ICES International Bottom Trawl Survey. The catchability coefficients obtained are applied to survey data collected between 1998 and 2004 to examine annual variation in actual abundance of the demersal fish assemblage in the North Sea.
Keywords: catchability, demersal fish, GOV, IBTS
Received 14 March 2007; accepted 27 July 2007; advance access publication 20 September 2007.
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