© 2004 International Council for the Exploration of the Sea
Changes in the North Sea fish community: evidence of indirect effects of fishing?
a Netherlands Institute for Fishery Investigations PO Box 68, 1970 AB, The Netherlands
b Danish Institute for Fishery Research, Charlottenlund Slot DK-2920 Charlottenlund, Denmark
c The Old Rectory Staithe Road, Burgh St. Peter, Beccles, Suffolk NR34 0BT, England, UK
d DFO Science Advisory Secretariat, Department of Fisheries and Oceans 200 Kent Street, Ottawa, Ontario, Canada K1A 0E6
*Correspondence to N. Daan. e-mail: niels{at}wur.nl; hg{at}dfu.min.dk; PopeJG{at}aol.com; RICEJ{at}DFO-MPO.GC.CA.
We investigate changes in the North Sea fish community with particular reference to possible indirect effects of fishing, mediated through the ecosystem. In the past, long-term changes in the slope of size spectra of research vessel catches have been related to changes in fishing effort, but such changes may simply reflect the cumulative, direct effects of fishing through selective removal of large individuals. If there is resilience in a fish community towards fishing, we may expect increases in specific components, for instance as a consequence of an associated reduction in predation and/or competition. We show on the basis of three long-term trawl surveys that abundance of small fish (all species) as well as abundance of demersal species with a low maximum length (Lmax) have steadily and significantly increased in absolute numbers over large parts of the North Sea during the last 30 years. Taking average fishing mortality of assessed commercial species as an index of exploitation rate of the fish community, it appears that fishing effort reached its maximum in the mid-1980s and has declined slightly since. If the observed changes in the community are caused by indirect effects of fishing, there must be a considerable delay in response time, because the observed changes generally proceed up to recent years, although both size and Lmax spectra suggest some levelling off, or even recovery in one of the surveys. Indeed, significant correlations between all community metrics and exploitation rate were obtained only if time lags 
6 years were introduced.
Keywords: community metrics, direct and indirect effects of fishing, Lmax spectra, size spectra
Received 4 July 2003; accepted 26 August 2004.
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