Potential bias in estimates of abundance and distribution of North Sea cod (Gadus morhua) due to strong winds prevailing prior or during a survey

The impact of strong winds on catches of cod (Gadus morhua) was studied using different fishing methods during small-scale surveys with commercial fishing vessels in the north-eastern central North Sea. Catch per unit effort of a flyshooter and a trawler were considerably lower in the shallower coastal water than in the deeper parts of the study area after a three week period with strong winds and rough weather conditions during the survey. At the same time, catches taken with a gillnetter showed an opposite pattern with the highest catch rates occurring at depths shallower than 50 m relative close to the coast. In another situation in which the weather conditions prior and during the survey were more moderate, the flyshooter and the trawler recorded high catch rates in the shallow coastal waters as well. Generalized Linear Model analyses revealed that wind speed prior to and during the survey had significant effects on the catch rates in particular for the trawler. These results supports fishermen's opinion that strong winds may cause an underestimation of biomass of cod in shallow waters and a bias in the resulting spatial distribution derived from bottom trawl surveys.

► We study the effect of wind on catch rates of cod using different fishing methods during surveys with commercial fishing vessels. ► Wind speed prior and during the surveys were considered. ► The effect of wind differed between fishing method and depth. ► Negative effects were recorded for bottom trawl catches in depth shallow than 50 m for strong winds prior and during the survey. ► Adverse wind conditions may result in inconsistent abundance indices and biased spatial distributions from bottom trawl surveys.