Fishing down Canadian aquatic food webs

The mean trophic level (TL) of fish landed in fisheries on the east and wes t coasts of Canada is declining by 0.03-0.10.decade(-1), similar to global trends. This finding is based on data from United Nations Food and Agricult ure Organization and the Canadian Department of Fisheries and Oceans and ot her Canadian sources for the period 1873-1997. Significant rates of decline in mean TL were obtained even when key species - Atlantic cod (Gadus morhu a) on the east coast and Pacific herring (Clupea pallasi) and Pacific hake (Merluccius productus) on the west coast - were omitted from the analysis. Fish taken in inland water fisheries did not exhibit a decline in mean TL. Two models were developed, based on length and age, respectively, for corre cting TL estimates of individual species for the effects of changes in body size due to changes in fishing mortality. Both produced corrections that w ere small relative to changes in mean TL that resulted from changes in spec ies composition of the catch over time. Overall, these results suggest that the mean TL of fish landed can be used as an index of sustainability in mu ltispecies fisheries and that its reliability will depend on the quality of the data and length of the time series available for analysis.