Density dependence only affects increase rates in baleen whale populations at high abundance levels.
Abstract
Most baleen whale populations are increasing after the end of industrial whaling, but their recovery patterns challenge long-standing assumptions about density dependence. It has long been assumed that population growth rates will decline with recovery, until reaching equilibrium ('carrying capacity', K). Indeed, the International Whaling Commission assumes that growth rates will slow long before K is reached, with maximum productivity at 0.6K. This 0.6K population level is used as an international benchmark that forms the basis of whaling regulations and decisions about whether baleen whale populations are declared depleted. We fit models to four long-term data sets for baleen whales with multiple abundance estimates that span the range from low to high abundance, finding strong evidence that increase rates remain at near-maximal levels across a wide range of abundance levels, and only decline as the population nears K. As a result, maximum productivity occurs at 0.69-0.87 of K across these populations, which predicts more rapid recovery for baleen whale populations than currently assumed. The overall mean of these values (0.8K) would be a more sensible default choice than the 0.6K currently assumed. Synthesis and applications. Estimated recovery rates imply that management thresholds currently used are lower than actual maximum productivity and that populations can increase rapidly even at high abundance. However, if population models continue to assume that maximum productivity is at 0.6K, they will estimate abundance relative to K to be lower than it is, providing conservative assessment results. Our results should stimulate further discussion about the role of maximum sustainable yield as a fundamental concept in fisheries and wildlife management.