© 2005 International Council for the Exploration of the Sea
Disentangling the effects of capture efficiency and population abundance on catch data using random effects models
a Department EMH, IFREMER Rue de l'Ile d'Yeu, BP 21105, 44311 Nantes Cedex 3, France
b IMR, PO Box 1870 Nordnes, 5817 Bergen, Norway
*Correspondence to V. M. Trenkel: tel: +33 240 374053; fax: +33 240 374075. e-mail: verena.trenkel{at}ifremer.fr.
We propose a random effects model for disentangling population abundance and capture efficiency effects on bottom-trawl catches. The spatial distribution of individual fish is assumed random leading to a Poisson distribution for the number of individuals in the trawl path (no schooling). Capture efficiency, i.e. the proportion of individuals in the trawl path being retained by the gear, is modelled as a random variable. We propose model extensions that include the effects of body size on haul efficiency. We applied the models to several species from the Celtic Sea groundfish community based on small-scale repetitive hauls. The resulting abundance estimates allowed us to study population abundance ratios; the estimated capture efficiencies were comparable between species and showed that generally gear efficiency increases for larger species with the exception of haddock (Melanogrammus aeglefinus), which had low estimated gear efficiency despite its large body size. Model identifiability was studied using simulations and an independent trawl data set from the same area.
Keywords: abundance, catchability, Celtic Sea, Poisson distribution, trawl survey
Received 10 September 2004; accepted 18 May 2005.
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