Variability in frontal zone formation and distribution of gadoid fish larvae at the shelf break in the northeastern North Sea

Hydrography and larval fish distribution in the northeastern North Sea were studied during a research programme carried out during the period from 199 1 to 1994. The aim was to examine the connection between frontal zone forma tion and nursery characteristics of gadoid larvae at the shelf break. Empha sis was placed on the year-to-year variation in frontal characteristics and distributional patterns of larvae. An area of about 67 000 km(2) covering the northeastern North Sea, the Skagerrak and the Kattegat was surveyed by grid or transect sampling. At each sampling station the hydrography was stu died by CTD casts, and the abundance of fish larvae was measured by depth i ntegrating tows of a 2 m ring net. Five species of gadoid larvae and small juveniles were found in the area: cod Gadus morhua, whiting Merlangius merl angus, haddock Melanogrammus aeglefinus, Norway pout Trisopterus esmarki an d saithe Pollachius virens. Larval abundance differed markedly between spec ies and years. The abundance of all species was the highest in 1992 and dec lined during the following 2 years. In 1994, cod and whiting were the only gadoid species observed. Peak abundance of all gadoids was found in the vic inity of the frontal zone; however, the relationship between larval distrib ution and hydrography differed among species. Correspondence between spatia l and interannual Variation in characteristics of frontal zones and larval distributions suggests that frontal zone variability is a central element i n the early Life of gadoid larvae from the area.