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, α= 0.05), given the frequency and precision of recent monitoring effort. We recommend alternatives to improve performance.