Evidence for density‐dependent growth in North Sea gadoids

The classical view on density‐dependent growth is that an increase in population density heightens intraspecific competition for food, leading to reduced food intake and depressed growth. This paper summarizes the analysis of English Groundfish Survey, International Young Fish Survey and ICES Roundfish Working Group data for evidence of density‐dependent growth of I‐ and II‐group cod, haddock and whiting in the North Sea. Within Roundfish areas there was a negative relationship between growth and density in some instances, but it was small. However, there were persistent differences in both growth rate and density between Roundfish areas. In all cases, except for I‐group cod, there appeared to be a tendency for fish in low‐density areas to be larger than fish in high‐density areas; this might be due to feeding competition but other explanations are possible. The results urge caution in interpreting commercial catch data for evidence of density‐dependent growth, since changes in fishing effort in particular areas could significantly bias the conclusions. There was evidence of enhanced survival of small fish in good year‐classes; this would depress the mean size at age, thereby mimicking the effects of density‐dependent growth.