Publication Abstract

Vertical reality: utilising knowledge of cod behaviour to interpret survey results

D. Righton, J.D. Metcalfe and G.P. Arnold

Fish behaviour has a profound effect on how the data from sampling gears are interpreted. In particular, the vertical movements of fishes determine their accessibility to trawl and acoustic gears. In addition, vertical movements have a significant effect upon the target strength of roundfish that have closed swimbladders. We attached electronic data storage tags on cod in the North Sea and Irish Sea to investigate the natural behaviour of free-ranging fish and show that the vertical movement patterns of cod change through the year, and differ between regions. In addition, we use DST data to calculate depth adaptation rates and demonstrate that the rates of ascent and descent of cod can only be explained by the maintenance of negative buoyancy at mean residence depth. The synthesis of our results indicates how an improved understanding of fish behaviour can be used to estimate the proportion of time that fish are accessible to different sampling gears, and how variations in vertical movement patterns affect estimates of target strength.

Reference:

D. Righton, J.D. Metcalfe and G.P. Arnold, 2001. Vertical reality: utilising knowledge of cod behaviour to interpret survey results. ICES CM 2001/Q:20. 15pp.