HARBOR PORPOISE AND FISHERIES - AN UNCERTAINTY ANALYSIS OF INCIDENTALMORTALITY

The harbor porpoise (Phocoena phocoena) in the western North Atlantic is subject to mortality due to entanglement in gillnets. Such incident al mortality threatens a population if it is too large relative to the potential population growth rate. Critical values for incidental mort ality have been established by the International Whaling Commission an d the U.S. Marine Mammal Protection Act. As in many situations in cons ervation biology, use of these critical values depends on demographic calculations that are based on uncertain data. It is important to repo rt not only estimates of demographic parameters, but also the uncertai nty in those estimates. Hen, we use a Monte Carlo approach to evaluate uncertainty in population size, incidental mortality, and population growth rate of harbor porpoise. To describe survival, we used model li fe tables derived from other mammals with similar life histories. By r andomly sampling the space of model life tables and the distributions of estimated fertility and age at first reproduction, we produced a pr obability distribution that characterizes the uncertainty in the poten tial population growth rate. The median estimate for the potential ann ual rate of increase lambda is approximately 1.10. Combining this info rmation with the uncertainty of incidental mortality and of population size, we estimate the probability that the rate of incidental mortali ty exceeds the critical values established by the various management a gencies; this probability ranges from 0.46 to 0.94. We conclude that r ecent incidental mortality rates are a threat to harbor porpoise popul ations. The methods developed here are applicable to other situations in which demographic analyses must be based on uncertain data.