Behaviour of large Atlantic cod at ship wrecks and similar rough bottom structures in the north-eastern part of the central North Sea

No abstracts are to be cited without prior reference to the author.

There is a substantial difference of opinion between fishermen and the scientific community about the development during the latest decade of the biomass of large Atlantic cod in the North Sea. Knowledge about the distribution of cod on a small geographical scale is important to interpret catch rates of commercial vessels as well as of scientific surveys. Still, the dynamics of small scale spatial distribution of cod is poorly understood. Together with other related research activities, a study was therefore conducted during the summer of 2008 at 10 selected ship wrecks known to hold large cod (>70 cm). The aims were to examine the time budget of cod at these wrecks by use of acoustic telemetry and to investigate the effects of a variety of variables on the observed absence/presence patterns. As foraging was considered a major incentive to locomotion of cod outside the spawning season, stomach samples were collected from wrecks during the study period. A total of 121 cod were tagged with acoustic tags during three different tagging sessions. Two listening buoys were deployed at each wreck. After a field period of more than four months (MaySeptember), 63 cod tagged at 9 wrecks were included in the analyses. Several hundred stomach samples were collected. Preliminary results indicate that the diet of large cod at wreck sites is dominated by prey types from smooth bottom and that absence/presence patterns differ among wrecks. The influences of physical variables will be presented as well.