The study of whale ecology did not begin until most populations had been depleted by commercial whaling. Some species still teeter on the edge of extinction, whereas others have shown encouraging signs of recovery. The Eastern Pacific population of gray whales (Eschrichtius robustus) was hunted to low levels in the 19th century but was protected from commercial harvest by the International Whaling Commission in 1947. The population then grew to almost 27,000 by 2016 (1), which is near the upper range of estimates of pre-whaling abundance (2). Most population models assume that after this recovery, gray whales would reach a relatively stable equilibrium. On page 207 of this issue, Stewart et al. (3) challenge this assumption by documenting boom-andbust cycles in abundance, driven by surprisingly large and rapid changes in their food supply over the past three decades.
Gray whales make one of the longest annual migrations of any mammal, from breeding grounds in Baja California, Mexico, to feeding grounds in the Bering and Chukchi seas, between Alaska and Russia. These whales feed by filtering crustacean prey using plates of baleen suspended from their upper jaws. Their near-shore migratory route has allowed researchers to collect a rich trove of data on abundance, reproduction, mortality, and body condition over the past 50 years; Stewart et al. used these datasets to construct a demographic model of the population. In their model outputs, carrying capacity—the number of whales that can be supported by the ecosystem—demonstrated a large degree of annual variation, caused by changes in prey availability and sea ice cover on the feeding grounds. In turn, these changes were driven by variation in the sub-Arctic climate. Large reductions in annual carrying capacity caused substantial mortality events in 1988, 1999, and 2019, which led to large decreases in gray whale abundance.When population dynamic models were first applied to whales in the past century, few scientists could have imagined the magnitude and speed of changes in ocean environments caused by a rapidly warming climate. Whales were assumed to be largely buffered from environmental variation by their enormous size, mobility, and slow life histories. But from a variety of recent studies, it is clear that climate change is affecting the ecology of these animals in ways that can exacerbate existing conservation problems or create new ones (4).
The most immediate effects of a changing climate on whale populations are modifications to their patterns of distribution, which are typically driven by prey availability. These are also the most straightforward effects to document. For example, over the past decade, many North Atlantic right whales (Eubalaena glacialis) shifted their primary summer feeding grounds from the Gulf of Maine to the Gulf of St. Lawrence, resulting in an increase in mortality from ship strikes and entanglement in fishing gear in Canadian waters (5). Similarly, the marine heat wave that occurred from 2014 to 2016 off the US West Coast resulted in a shoreward shift in the distribution of humpback whales (Megaptera novaeangliae) and an increase in the number of entanglements in the Dungeness crab fishery (6).Other populations of whales are benefiting from climate change, at least in the short term. For example, humpback whales feeding on krill along the western Antarctic Peninsula are taking advantage of longer feeding seasons that are due to reductions in sea ice, resulting in excellent body condition (7) and high rates of fecundity (8). But what will happen when the sea ice, on which the krill ultimately depend, disappears?
Stewart et al. describe two factors that may be responsible for the swings they observed in the Eastern Pacific gray whale population. The whales feed on amphipods, a low-trophic-level crustacean prey that is affected directly by environmental fluctuations. In addition, gray whales use a capital breeding strategy, in which they feed intensively during the summer and draw on stored energy reserves to fuel their long migrations and the costs of reproduction during the remainder of the year. Interannual variation in the duration of their feeding season, caused by the timing of sea ice formation and breakup, can thus affect their ability to store enough energy during the critical summer feeding period.How applicable are the lessons from gray whales for other species of baleen whales? Some species, such as humpback whales, have relatively broad diets that include both crustaceans and fish and are adept at prey-switching when environmental conditions change (9). This behavior may allow humpback whales to be more buffered than gray whales from environmental variation. By contrast, blue whales (Balaenoptera musculus) feed almost entirely on krill and track their prey closely, even when its distribution changes (10). If Stewart et al. are correct, then species with a narrow dietary niche composed of low-trophic-level prey species should be expected to exhibit considerable variation in their demography as environmental conditions change. As more baleen whales recover from overhunting in the coming decades, these hypotheses can be tested.Today, Eastern Pacific gray whales experience very limited levels of human-caused mortality. Stewart et al. included mortality from entanglements and ship strikes in their model and concluded that these factors could not have caused the changes in abundance that they observed. But what if this population is subjected to higher levels of anthropogenic mortality in the future? The paradigm used to manage whales and other marine mammals in US waters relies on biological reference points to establish limits on the number of whales that can be removed from each population by human activities. These reference points, known as potential biological removal levels, were developed by using simulation models that assume that populations will remain at relatively constant carrying capacity (11). The boom-and-bust cycles observed by Stewart et al. do not fit this paradigm, and it is unclear how human-caused mortality would be managed on top of such a large degree of climate-induced variation in abundance.During the past century, commercial whaling removed almost 3 million large whales from the world’s oceans (12). The era of unregulated hunting of whales is largely behind us, but the findings of Stewart et al. remind us that the recovery of these populations may not be as straightforward as expected in the era of a rapidly changing climate.
Source : Science