INCREASE RATES IN SEVERELY DEPLETED STOCKS OF BALEEN WHALES

The status of all known stocks of baleen whales that were severely depleted (to an estimated less than 10% of their original abundance) is reviewed. Of 44 such stocks, 18 are classified as not feasible to monitor. Of the remaining 26 stocks, 12 have been or are being monitored, and significant rates of increase have been demonstrated for 10 of them. At least 10 of the 16 monitorable stocks, for which a significant rate of increase has not been demonstrated, are believed to be increasing. The reasons for a lack of monitoring in most of these 16 stocks do not seem to be related to population size, and may reflect practical difficulties in obtaining representatives samples due to temporal or spatial segregation of the animals in relation to the study areas. In total, at least 77% of monitorable stocks are either believed or demonstrated to be increasing. Rates of increase measured in one bowhead, four right, one gray, one blue, and three humpback whale stocks range from 0.031 to 0.144. Estimates of stock depletion associated with these increase rates are only available for six stocks; the more depleted stocks show higher rates of increase than the less depleted. Under the “strong convexity” hypothesis for the relationship between sustainable yield rate and population size, the maximum sustainable yield rate (MSYR) is more than half the per capita increase rate at very low population sizes; this translates into MSYR values of 0.026 to 0.049 (mean 0.039) in seven of these severely depleted stocks. There remaining two stocks are believed to be at or slightly above 50% of their initial size, and thus in the vicinity of the maximum sustainable yield level for baleen whales as conventionally adopted by the International Whaling Commission: their increase rates (adjusted for hunting mortality) are 0.034 and 0.045. These data tend to support MSYR values (for these stocks) at the higher end of the range 0.01–0.04 (of total population size) as sometimes used in baleen whale assessments. © 1993 Conseil International pour l’Exploration de la Mer.