CALCULATING LIMITS TO THE ALLOWABLE HUMAN-CAUSED MORTALITY OF CETACEANS AND PINNIPEDS

A simulation method was developed for identifying populations with lev els of human-caused mortality that could lead to depletion, caking int o account the uncertainty of available information. ii mortality limit (termed the Potential Biological Removal, PBR, under the U.S. Marine Mammal Protection Act) was calculated as the product of a minimum popu lation estimate (N-MIN), one-half of the maximum net productivity rate (R-MAX), and a recovery factor (F-R). Mortality limits were evaluated based on whether at least 95% of the simulated populations met two cr iteria: (1) that populations starting at the maximum net productivity level (MNPL) stayed there or above after 20 yr, and (2) that populatio ns starting at 30% of carrying-capacity (K) recovered to at least MNPL after 100 yr. Simulations of populations that experienced mortality e qual to the PBR indicated that using approximately the 20th percentile (che lower 60% log-normal confidence limit) of the abundance estimate for N-MIN met the criteria for both cetaceans (assuming R-MAX = 0.04) and pinnipeds (assuming R-MAX = 0.12). Additional simulations that in cluded plausible levels of bias in the available information indicated that using a value of 0.5 for F-R would meet both criteria during the se ''bias trials.'' It is concluded that any marine mammal population with an estimate of human-caused mortality that is greater than its PB R has a level of mortality that could lead to the depletion of the pop ulation. The simulation methods were also used to show how mortality l imits could be calculated to meet conservation goals other than the U. S. goal elf maintaining populations above MNPL.