The large, long-lived fishes at or near the top of aquatic (especially marine) food webs, when exploited by multispecies fisheries, tend to decline faster than smaller, short-lived fishes with lower trophic levels. This results in the size and mean trophic level of exploited fish assemblages gradually declining, as does the mean trophic level of catches from an ecosystem exploited in this manner.
This phenomenon, now known as ‘Fishing Down Marine Food Webs’ (Pauly et al., 1998), has been documented through detailed analyses of fisheries catch data from a wide range of ecosystems all over the world (see The Science of Fishing Down). The widespread occurrence of ‘fishing down’ is the reason why, in 2004, the Convention of Biological Diversity (CBD) chose the mean trophic level of fisheries catches as an index of the biodiversity of large fishes (defined as fish with trophic levels > 3.5), called the Marine Trophic Index (see Pauly and Watson, 2005).
This website is designed to encourage exchanges about ‘fishing down’, notably about its intensity (in trophic level units per decade) and the various masking factors, which in certain circumstances, can make its detection difficult.
Aquatic Ecosystems Research Laboratory University of British Columbia 2202 Main Mall