Icelandic Groundfish Survey Data Used to Improve Precision in Stock Assessments

The main objective of this study was to improve the precision of stock assessments of demersal fish in Icelandic waters, with particular emphasis on cod. Sampling was carried out on approximately 600 stations in the Iceland shelf area in March 1985 and again in March 1986. Stations were distributed in the survey area through a semi-randomly stratified process. A standardized bottom trawl was designed for the sampling and the data collected included length measurements, otolith samples and sex determination, as well as information on the environment and fishing gear. Results are presented for cod, haddock, saithe, redfish, catfish and long rough dab. The distributions of the different species indicate common nursery grounds in the northern part of the survey area. The youngest age-groups were generally not fully represented in the survey. Pronounced diurnal catch variations were observed for catfish and redfish. Age disaggregated stock indices of the gadoid species are well correlated with virtual population analysis (VPA) values. However, the total stock indices differ in their proportion of the VPA stock size. In comparison with previous surveys, the precision of stock indices has been increased markedly through this survey for all species except saithe. This is basically the result of an increased number of stations. Skipper-selected stations (non-random) generally gave higher aggregated indices than random ones, although statistically significant differences are exceptional. On a length disaggregated basis, however, a highly significant difference was observed for all species.