Lessons from monitoring trends in abundance of marine mammals

We assessed scientists' ability to detect declines of marine mammal stocks based on recent levels of survey effort, when the actual decline is precipitous. We defined a precipitous decline as a 50% decrease in abundance in 15 yr, at which point a stock could be legally classified as depleted under the U.S. Marine Mammal Protection Act. We assessed stocks for three categories of cetaceans: large whales (n = 23, most of which are listed as endangered), beaked whales (n = 11, potentially vulnerable to anthropogenic noise), and small whales/dolphins/porpoises (n = 69, bycatch in fisheries and important abundant predators), for two categories of pinnipeds with substantially different survey precision: counted on land (n = 13) and surveyed on ice (n = 5), and for a category containing polar bear and sea otter stocks (n = 6). The percentage of precipitous declines that would not be detected as declines was 72% for large whales, 90% for beaked whales, and 78% for dolphins/porpoises, 5% for pinnipeds on land, 100% for pinnipeds on ice, and 55% for polar bears/sea otters (based on a one-tailed t-test, alpha = 0.05), given the frequency and precision of recent monitoring effort. We recommend alternatives to improve performance.