Implications of compensatory and additive mortality to the management of selected sportfish populations

The compensatory mortality hypothesis postulates that a population's total mortality remains unchanged at low to intermediate exploitation rates because natural mortality decreases to compensate for reduced density, whereas the additive mortality hypothesis postulates that any increase in exploitation mortality results in an increase in total mortality. Fishing and natural mortality rates have generally been assumed to be additive rather than compensatory. We reviewed mortality estimates for some prominent sportfish populations to identify evidence for compensatory or additive mortality. For largemouth bass Micropterus salmoides , total annual mortality increased with annual exploitation suggesting additive mortality. For crappies Pomoxis nigromaculatus and P. annularis , annual exploitation did not seem to affect annual mortality at low to moderate annual exploitation, but annual mortality increased with annual exploitation as it increased beyond 40%, conforming to the compensatory mortality hypothesis. Northern pike Esox lucius mortality estimates revealed no relation between annual mortality and annual exploitation, and highly variable annual mortality for a given annual exploitation. Evidence from the literature suggests that mortality of northern pike may be compensatory for fish smaller than 40 cm total length, but additive for larger fish. Because compensatory natural mortality reduces the managers’ability to control annual mortality, we suggest that further consideration of the compensatory mortality hypothesis be given to species that have shown variable mortality responses to reductions in annual exploitation.