Conflicting Conservation Objectives Between the Marine Mammal Protection Act and Recovery of Endangered Species

Limits on whaling and reduced seal harvests have led to recovery of many once depleted marine mammal populations, making them conservation success stories. In North America, for example, the biomass of many pinniped species has increased 10-fold in the last 40 years following their protection by the Marine Mammal Protection Act (1972). As biomass of these predators has increased, so has their consumption of prey species. Fisheries assessment models have not included these long term changes, and tend to assume predator-induced natural mortality is constant. To illustrate the effects of ignoring these long term trends, we demonstrate a simple food web model in the Northeast Pacific Ocean, including direct effects of generalist pinnipeds on threatened and endangered salmon, as well as indirect effects on other marine mammal species. We use several data sources to estimate consumption for harbor seals and sea lions.  Even if salmon are relatively rare in pinniped diets, increases in biomass may have increased the overall consumption of salmonids beyond what assessments have previously assumed.  Increased salmon mortality due to pinnipeds may reduce prey availability to fish-eating killer whales, potentially interfering with their recovery. Using this model, we demonstrate the potential for conflict between multiple conservation objectives.