Vertical migration and spatial distribution of pelagic 0-group gadoids (cod, haddock, whiting and Norway pout) prior to and during settlement in the North Sea

Abstract

       This paper investigates diel vertical migration and spatial distribution in 0-group gadoids in the central and northern North Sea in late spring and early summer, covering the pelagic phase of development and the transition to a demersal habit. The species investigated include cod (Gadus morhua L.), haddock (Melanogrammus aeglefinus L.), whiting (Merlangius merlangus L.) and Norway pout (Trisopterus esmarkii NILSSON).

      In the northern North Sea in June there was considerable overlap in the spatial distribution of the pelagic 0-group gadoids. The catch rates of the various species were often positively correlat­ed. The presence of high concentrations of fish in the same area is likely to increase intra- and inter­specific competition.

      During the pre-settlement stage in the northern North Sea in June, pelagic 0-groups of around 3 cm in length exhibited diel vertical migration patterns, the extent and the timing of which varied. In 1991, catches of fish near the surface peaked in the evening, whilst in 1992 the peak was in the middle of the day, as was the peak catch rate of haddock west of Denmark. However, west of Denmark, near-surface catches of pelagic 0-group cod and whiting peaked at night. It is likely that the concentration of fish near the surface is a feeding response, with the 0-groups shadowing the vertical migration patterns of the copepods on which they feed.

      During settlement to take up a demersal life style in July 1994 in the central North Sea, 0-group cod of 4-6 cm in length were demersal by day and pelagic at night. Cod of over 8 cm in length were wholly demersal, having completed the transition to a demersal life style.

      The provision of possible management measures, such as the implementation of temporary closed areas, to protect pre-recruit 0-group gadoids on settlement sites is discussed.