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NOAA Technical Memorandum NMFS-AFSC-159

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Comparison of size selectivity between marine mammals and commercial fisheries with recommendations for restructuring management policies


Conventional fisheries management schemes often maintain harvest practices quite different from those of natural predators, explicitly or implicitly stressing the importance of avoiding strategies similar to those of other species. While this may minimize some of the perceived direct interactions between fisheries and marine mammals, such management often exerts strong abnormal influences, both on the target species and the ecosystems of which they are a part. It is impossible to exhaustively account for the indirect effects of such practices.

In contrast, in Systemic Management, it is argued that the patterns of predation exhibited by marine mammals, structured by natural selection over thousands of generations, are evidence of sustainable resource use in the very long term, thus accounting for such complexity. As such, these patterns can be used to guide the ways in which humans extract resources from the environment.
In this study, we compiled prey size data from the food habits literature for 63 marine mammal species as a guide for sustainable harvest practices. Our results show that the overwhelming tendency of marine mammals is to target prey smaller than 30 cm in length. This pattern of selectivity applies regardless of the maximum attainable size of either the predator or the prey species. In contrast, commercial fisheries tend to select individuals larger than 30 cm when possible. Thus, the size selectivity of the commercial catch of small-bodied prey species fits within the norm of the patterns exhibited by marine mammals, but the commercial catch of large-bodied prey species does not.

We conclude that, in order to minimize the negative effects of abnormal selective pressures that commercial fisheries exert directly on prey stocks and indirectly on the ecosystems of which they are a part, the targeted size composition of larger prey species should be reduced to more closely mimic the patterns exhibited by marine mammals.

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