Fishing through marine foodwebs

Tim Essington, Anne Beaudreau and John Wiedenmann have a interesting new paper in PNAS Fishing through marine food webs.

The paper elaborates on Pauly and others influential 1998 paper Fishing Down Marine Food Webs that showed that the mean trophic level of global fisheries statistics declined from 1950 to 1994 (from an average of 3.3 to 3.1).

Essington et al analyzed regional fisheries data from 1950 to 2001. They also found a decline in trophic level in 30 of 48 large marine ecosystems, and that the average decline was .42 trophic levels (almost twice as large as the decline found by Pauly et al). However, they did more than replicate Pauly et al’s work at a regional level, they also tested two alternative models of fishing down foodwebs – sequential collapse (the removal of top levels) vs. sequential addition (adding lower level fisheries). They evaluated these models by examining the temporal dynamics of upper-trophic-level fishery catches when fishing down the food web was occurring:

Under the sequential collapse replacement mode, a decline in the mean trophic level should be accompanied by reduced catches of high-trophic-level species as these species become economically extinct. Under the sequential addition mode, however, we expect catches of upper-trophic-level species to be maintained or even increase.

In the 30 large marine ecosystems that exhibited a decline in trophic level , they found 15 that matched the sequential addition model, 6 that showed no pattern, and 9 that showed sequential collapse. They differences between the two models are illustrated in Figure 1 from the paper, shown below.

Fishing down food websFig. 1. Illustrative examples of the sequential collapse replacement (A) and sequential addition (B) mode of fishing down the food web. Total yearly catch for each 0.1 trophic-level increment is indicated by the color bar on the right (104 kg yr 1). The mean trophic level (white line) was smoothed by using a locally weighted regression smoother. (A) The Scotian Shelf ecosystem exhibited a sharp decline in mean trophic level from 1990 to 2001 owing to the collapse of the cod fishery followed by a decline in the herring fishery and then the growth of the northern prawn fishery. (B) The mean trophic level of the Patagonian Shelf declined from 1980 to 2001, during which time catches for upper-trophic-level species (Argentinean hake) grew substantially while new fisheries for shortfin squid developed.

Essington and his coauthors point out that fisheries science, at least in the published literature, has assumed that fishing down food webs follows the sequential collapse model, and this model has different policy implications to the sequential addition model.

Perhaps the most important policy consideration of the sequential addition mode is that, in most ecosystems of the world, several trophic levels are now exploited simultaneously. These diverse fisheries impose conflicting demands on marine ecosystems that are not generally well represented in single-species management plans that do not consider the effects of these alternative fisheries on each other. As the structure of fisheries and the management environment evolve, the scientific community faces a new challenge of conducting broad-scale ecological research to support the development of more holistic, ecologically based approaches to fisheries management.

Another description of the research is provided in a U Washington press release.

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