Initial results from the 2004 ichthyoplankton survey of the North Sea

Abstract

In 2004 an international consortium comprising England, Scotland, Netherlands, Germany, Denmark and Norway conducted an ichthyoplankton survey covering the whole of the North Sea in order to comprehensively survey cod and plaice spawning areas. At each station, ‘cod-like’ eggs were pre-sorted at sea from the plankton sample and preserved in ethanol for subsequent analysis using species-specific genetic probes. The remainder of the plankton sample was fixed in formalin and ichthyoplankton subsequently sorted and identified using traditional visual methods. The results showed stage I plaice spawning to be located in the traditional areas reported from the literature although with evidence of a more northward extension up the eastern edge of the Dogger Bank compared to data from the 1930s. The distribution of stage I cod eggs also conformed to historical patterns being most abundant around the southern and eastern edge of the Dogger Bank, in the German Bight, off the Moray Firth and to the east of the Shetland Isles. ‘Cod-like’ eggs were also found in southern Bight but this area was not as well sub-sampled for genetic analyses. Most of the ‘cod-like’ eggs on stations that were sub-sampled adequately in this region were shown to be whiting or other species. Data was also produced on the distribution of haddock eggs which were found over a wider region of the north-western North Sea than shown in historical maps. Whiting eggs were found south of the Dogger Bank and to the east of the Shetland Isles but were absent from the central North Sea. These results are discussed in relation to historical patterns of spawning and recent changes in the abundance of the North Sea plaice, cod and haddock stocks. Keywords: cod, plaice, spawning, ichthyoplankton, North Sea