Hydrobiological variability on the northwest European continental shelf during the 1990s and its relation to changes in fish stocks

Multidisciplinary survey data have been obtained from the first quarter ICES International Bottom Trawl Surveys from 1986 to 1999. The data include estimates of the number of young demersal fish by ICES Square (based on numbers caught per hour of research vessel fishing), along with near-bed temperature, salinity, and water depth. The database forms a near-synoptic picture of the winter distribution of young fish in the North Sea, particularly in relation to hydrographic conditions. Two species have been selected for examination, cod and haddock. Their average distribution over the observation period with respect to temperature indicated that the young haddock had a more restricted temperature range, centred on approximately 8°C and were not found in waters with a temperature of less than 5°C. Young cod were found in a wider range of temperatures, from less than 1°C to 9°C, but with a maximum at approximately 6°C. This is partly explained by their different geographical distributions, with young haddock confined to the deeper northern North Sea, and young cod to the shallower southern areas. The data have also been used to examine the interannual variability of both temperature and young fish numbers. The influence of the Atlantic inflow is clear when looking at data from the northern North Sea, as interannual variability of temperature was less than in the shallower southern areas. In the south, temperatures were closely associated with the variability of the North Atlantic Oscillation index (NAO). The 1990s was a decade of warming in the North Sea. However, 1996 was a particularly cold year, which may have influenced the large catch of young cod in the following winter.

Article from Marine Science Symposia Vol. 219 - "Hydrobiological variability in the ICES Area, 1990-1999", symposium held in Edinburgh, 8-10 August 2001. To access  the remaining articles please click on the keyword "MSS Vol. 219".