Are social aggregation and temporary immigration driving high rates of increase in some Southern Hemisphere humpback whale populations?

Abstract

Humpback whales (Megaptera novaeangliae) in the Southern Hemisphere were heavily exploited by commercial whaling. Today, their recovery is variable: Humpbacks remain surprisingly scarce in some formerly populous areas (e.g., New Zealand, Fiji), while in other regions (such as eastern Australia), they appear to be rebounding at or even above the maximum plausible rate of annual increase. Here, we propose that this phenomenon cannot be explained solely in demographic terms. Through simulation, we test the hypothesis that reported high rates of increase represent a combination of true intrinsic growth rates and temporary immigration, driven by a strong tendency to aggregate for mating. We introduce the idea that overexploitation diminished density at major breeding grounds such that these were no longer viable; then, during subsequent population recovery, a critical mass was attained in certain areas which drew in whales that formerly bred elsewhere. The simulations show that, to maintain high increase rates, the contribution to that rate by temporary immigration from a second, “source” population would have to represent a larger and larger proportion of the source stock and would require relatively high (but quite plausible) intrinsic rates of increase for each population. In the modeling scenarios, the demand for immigrants would eventually exceed the supply and exhaust the source population, but the simulations demonstrated that high increase rates can be sustained over periods of more than 20 years. This hypothesis, if correct, would not only explain excessively high rates of increase in current “hotspots” such as eastern Australia, but also imply that formerly important areas (e.g., Fiji) host few whales today not necessarily because of a failure to recover, but because the species’ mating system leads the whales concerned to migrate to higher-density breeding grounds elsewhere. Overall, we caution that assessments of depleted animal populations that do not consider the social behavior of a species are missing a potentially vital component of the picture.