Modelling the length-dependent offshore distribution of redfish, Centroberyx affinis

Redfish, Centroberyx affinis, are distributed on the continental shelf and slope along the south-eastern coast of Australia from northern New South Wales (NSW) to eastern Tasmania, the main fishery being on the southern coast of NSW. During 1993-1994, the NSW Fisheries research vessel 'Kapala' conducted an independent stratified survey of abundances and length distributions of fish on the continental shelf of the NSW coast. Three depth strata were surveyed: inshore (< 60 m), mid-shelf (90-125 m), and outer-shelf (125-165 m). Redfish showed a strong length-dependent offshore distribution, with small fish occurring more frequently in the shallow inshore waters and large fish in the deeper mid-shelf and outer-shelf waters. Two logistic-type functions were used to model this length-dependent offshore distribution. There are two parameters to be estimated in each of these two logistic functions, one being the length at which 50% of fish remain inshore (L50) and the other being a parameter that determines the shape of the logistic curve (m). The estimation of the parameters was tested with a simulated fishery under different assumptions for annual recruitment, fishing mortality, variation in length at age, sampling errors, and sampling intensity. In this simulation study, the estimation of L50 was robust while the estimation of m was sensitive to error variations, The estimated L50 of redfish varied from 12.4 cm to 17.4 cm over sampling years and seasons, indicating a temporal variation in redfish length-dependent offshore distribution. The estimated L50 provides fisheries managers with a quantitative estimate of the length of redfish migrating onto the commercial fishing grounds. The proposed model provides an approach to incorporating a size-dependent offshore distribution of fish into models of fish population dynamics and stock assessment.