Humpback Whale Predation and the Case for Top-Down Control of Local Herring Populations in the Gulf of Alaska

C over the production of highly fecund species such as Pacific herring (Clupea pallasii) has been attributed historically to bottom-up effects such as ocean conditions or food availability. In contrast, top-down control exists when predation limits population production. Recent observations of humpback whales (Megaptera novaeangliae) foraging on depressed herring populations suggests top-down control of herring may be underappreciated. Pacific herring populations are depressed in several locations in Alaska, and humpback whale populations are increasing. A 2004-06 census estimated the North Pacific humpback whale population at 18,000 to 20,000 and concluded that the population of whales wintering in Hawaiian waters (one half of the North Pacific population) is doubling approximately every 15 years. Humpback whales could potentially be a significant source of mortality on herring populations because they are large homeotherms that often consume herring, and they display a remarkable fidelity to their feeding grounds. If whales repeatedly return to locations to forage with increasing numbers, then production of their prey may become constrained. Anecdotal evidence of humpback whales foraging in locations where herring populations are depressed led to the hypothesis that humpback whale predation impedes the recovery of depressed herring populations, even when the commercial herring fisheries have been closed for decades. It is important to understand the effect of humpback whales on herring because both species are conspicuous elements in the Gulf of Alaska ecosystem. Herring are ubiquitously distributed and play a key role in the ecosystem by making the energy bound in primary consumers available to apex predators. Humpback whales are voracious predators. A humpback whale weighs around 30 metric tons (t) and requires the equivalent of about 1,100 herring per day to meet its average daily metabolic cost. However, while it is evident that whales depend on herring to some degree, the impact of this dependence on herring is unknown. If we are to evolve towards ecosystembased management, we will need to begin quantifying the benefits whales gain from herring and the costs of whale predation to herring. Understanding the relationship between herring and humpback whales Humpback Whale Predation and the Case for Top-Down Control of Local Herring Populations in the Gulf of Alaska