Long-term changes in the trophic level of the Celtic Sea fish community and fish market price distribution.
Abstract
1. The intensive exploitation of fish communities often leads to substantial reductions in the abundance of target species, with ramifications for the structure and stability of the ecosystem as a whole. 2. We explored changes in the mean trophic level of the Celtic Sea (ICES divisions VII f-j) fish community using commercial landings, survey data and estimates of trophic level derived from the analysis of nitrogen stable isotopes. 3. Our analyses showed that there has been a significant decline in the mean trophic level of survey catches from 1982 to 2000 and a decline in the trophic level of landings from 1946 to 1998. 4. The decline in mean trophic level through time resulted from a reduction in the abundance of large piscivorous fishes and an increase in smaller pelagic species which feed at a lower trophic level. 5. Similar patterns of decline in the trophic level of both catches and landings imply that there have been substantial changes in the underlying structure of the Celtic Sea fish community and not simply a change in fishery preferences. 6. We suggest that the reported changes in trophic structure result from reductions in the spawning stock biomass of traditional target species associated with intensive fishing, together with long-term climate variability. 7. The relative distribution of fish market prices has changed significantly over the past 22 years, with high trophic level species experiencing greater price rises than lower trophic level species. 8. Although decreased abundance of high trophic level species will ultimately have negative economic consequences, the reduction in mean trophic level of the fish community as a whole may allow the system to sustain higher fishery yields. 9. Management objectives in this fishery will depend on the relative values that society attaches to economic profit and protein production.