Correcting for variation in catchability: maturity-related catchability variation in Icelandic cod

Abstract

A simple model relating catchability to maturity is presented, which allows for computation of annual age-specific correction factors for groundfish survey indices. The model is based on the hypothesis that mature fish are less catchable than immature fish during the spawning season and are therefore less available to groundfish surveys carried out during that period. The effect modelled is an interaction effect - its operation requires both a change in catchability at maturity and temporal changes in maturity at age in the stock. In recent years, assessments of the Icelandic cod stock have relied on VPAs calibrated (tuned) with bottom trawl c.p.u.e. data. The c.p.u.e. data are from the commercial trawler fleet and from the Marine Research Institute's annual groundfish survey conducted in March, covering the entire distribution area of the stock around Iceland. At the time of the survey, most of the year's spawners have migrated towards the spawning grounds. Independent data from commercial catches indicate that the proportions of fish mature at age in the Icelandic cod stock have increased substantially in recent years. Furthermore, comparison of proportions mature in the catches with survey indices for mature and immature fish indicate that immature fish are substantially more catchable than mature fish of the same age. The effects of changes in proportion mature on estimated stock numbers are discussed with reference to limitations in the available data.