OCEANOGRAPHIC INFLUENCES ON THE DISTRIBUTION OF BALTIC COD, GADUS-MORHUA, DURING SPAWNING IN THE BORNHOLM BASIN OF THE BALTIC SEA

The Baltic Sea is a stratified, semi-enclosed sea typified by a low-sa linity surface layer and a deep saline layer of varying volume, salini ty, temperature and oxygen concentration. The relationships between th ese oceanographic factors and the distribution of Baltic cod are prese nted, utilizing results from a survey carried out during the 1995 spaw ning period in the Bornholm Basin, at present the main spawning area o f this stock. Cod distribution, abundance and population structure wer e estimated from hydroacoustic and trawl data and related to hydrograp hic parameters as well as to bottom depth. In the central basin, cod w ere aggregated in an intermediate layer about 15 m thick. This area of peak abundance was defined at its upper limit by the halocline and at the lower limit by oxygen content. The majority of individuals caught in the basin centre were in spawning or pre-spawning condition with a high proportion of males to females. On the basin slopes, aggregation s of cod were found near the bottom. These individuals were mainly imm ature and maturing stages with an increasing proportion of females to males with size. Salinity and oxygen conditions were found to be the m ajor factors influencing the vertical and horizontal distribution of a dult cod. Abundance of immature cod was also positively related to dec reasing bottom depths. The effect of temperature was minor. The observ ed size- and sex-dependent spawning aggregation patterns, in associati on with habitat volume and stock size, may influence cod catchability and thereby the assessment and exploitation patterns of this stock.