In response to the global fisheries crisis, characterized by falling abundances of resource species, falling catches, increasing habitat destruction, and extremely high subsidies, the advocates of whaling have been advancing a new rationale for hunting of marine cetaceans. They claim that marine mammals, particularly the great whales, compete with humans for fish resources; that efforts to protect these whales from extinction have led marine ecosystems to be “out of balance;” and that such balance can only be re-established by large-scale culling.
This argument flies in the face of numerous observations on the widely different ecological impacts of fisheries (which tend at first to concentrate on large fishes wherever these can be caught) and marine mammals (which, if they feed on fish, tend to consume smaller individuals).
Thus, the decline of the mean trophic levels of fisheries catch over the past 50 years (which is largely similar to mean sizes, as big fish eat smaller ones), is a signature of “fishing down marine food webs” and leaves marine mammals exonerated.
For more information (PDF):
Food Web Models and Data for Studying the Interactions between Marine Mammals and Fisheries
Are Whales Eating Too Many Fish, Revisited
Pew is no longer active in this line of work, but for more information, visit the Pew Whale Conservation Project campaign.