Analysis of the spatial distributions of mature cod (gadus morhua) and haddock (melanogrammus aeglefinus) abundance in the North Sea (1980-1999) using generalised additive models

Patterns of mature cod and haddock abundance in the North Sea were estimated from International Council for the Exploration of the Seas (ICES) International Bottom Trawl Survey (IBTS) data acquired from 1980 to 1999 in the first quarter of each year. Relationships between abundances and environmental properties (water depth, bottom temperature and bottom salinity) were analysed through the use of generalised additive models (GAMs). Cod were tolerant of a wide range of environments found throughout the North Sea, but predominantly occurred in, either, shallower (depths less than 50 m), colder and less saline waters of the southern North Sea, or, deeper (depths greater than 100 m), warmer and more saline waters of the northern North Sea. In contrast, the bulk of the haddock population was confined to the northern North Sea. Greatest haddock abundances were found between 75 and 125 m, and abundances were positively related to temperature and salinity. While there was no decadal shift in haddock spatial distribution, the 1990s were associated with a displacement of cod towards deeper waters in the northern North Sea.