Effect of bottom type on catch rates of North Sea cod (Gadus morhua) in surveys with commercial fishing vessels

Seven surveys with commercial fishing vessels were conducted during a collaborative fishermen-scientist project on the distribution of cod in the north-eastern North Sea between June 2006 and June 2008. A flyshooter, a trawler and a gillnetter participated in this study. In general, catch rates were substantially higher on gravel or stone bottom and at ship wrecks than on sand bottom. The difference in the catch rates between the two bottom categories at paired stations within a short distance was highly significant for all the three fishing methods. Similarly, average CPUE for most surveys was several times higher on rough than on smooth bottom. These differences were highly significant for early autumn surveys conducted with the flyshooter and trawler and all gillnet surveys, the summer surveys for the flyshooter and the gillnetter, but not for the winter surveys with the trawler and the flyshooter. The latter suggest that bottom type preference may change with season, e.g. with respect to spawning migrations in winter and in relation with changes in the availability of food during spring and summer.