Size Selectivity in Marine Mammal Diets as a Guide to Evolutionarily Enlightened Fisheries Management

Abstract

Commercial fisheries have a long history of nonsustainable harvests—not through deliberate mismanagement but through an inability to simultaneously account for the complexity of relevant factors, including the ecological and coevolutionary interactions within ecosystems. We argue that these factors (e.g., natural selection and coevolution) are among the structuring processes behind the patterns of predation exhibited by species such as marine mammals. As such, these patterns provide an indication of harvest practices that are sustainable in the long term—that is, normal for the systems and circumstances involved. Thus, patterns in selectivity exhibited by predators in their consumption of prey species meet the needs for evolutionarily enlightened guidance in management, long seen as missing in current practice. Our analysis of data characterizing the diets of 63 species of marine mammals indicates that the majority of prey consumed are 30 cm or less in length. This pattern is common to all marine mammal taxa and is apparently independent of the body size of predator species and largely independent of the body size range of prey species, many of which exceed 100 cm in maximum length. The size selectivity of commercial fisheries differs from that of marine mammals for all but the smallest prey species and is positively correlated with size. When possible, commercial fisheries tend to target individuals greater than 30 cm. The selective pressure this exerts on large-bodied prey species is abnormal in comparison with that of the natural predator–prey systems that have evolved to sustain consumption of individual prey items of 30 cm or less. To minimize the abnormal effects of selective pressure imposed by fishing, we argue that the targeted size composition of catches should be modified to more closely match the patterns exhibited by marine mammals as another example of the application of systemic management, a holistic alternative to conventional management.