Appetite for Destruction?
New Research Puts Killer Whale Predation in Context

There is little doubt that killer whales worldwide have earned their position atop the marine food chain. In the North Pacific Ocean alone, killer whales hunt either fish or marine mammals with a prowess worthy of legend. Some researchers have even suggested that killer whale predation could be responsible for the dramatic decline in western Alaska’s seal and sea lion populations in the post-industrial whaling era.

The proposed Sequential Megafaunal Collapse (SMC) hypothesis suggests that commercial whaling robbed western Alaska’s transient (mammal-eating) killer whales of their usual diet of large whales, forcing them to prey on smaller mammals such as seals and sea lions. Like all good science, the SMC hypothesis is subject to peer review and was recently critiqued by Drs. Andrew Trites, Volker Deecke and Edward Gregr (of the Marine Mammal Research Unit at the University of British Columbia) and Dr. John Ford and Peter Olesiuk (of Fisheries and Oceans Canada’s Pacific Biological Station).

A right whale on the flensing platform of the former whaling station at Akutan in the Aleutian Islands,
Alaska (Historical Photography Collection, University of Washington Libraries, Seattle).

Are hungry whales to blame for western Alaska’s sea lion woes? To find out, the researchers began by reconstructing the “compelling and eloquently simple” SMC hypothesis.

“In theory, the abundance of seals, sea lions and sea otters in British Columbia should have also declined, or remained at low numbers, if killer whales that once depended on eating large whales were forced to switch to alternative prey,” the authors write. “Instead, the positive response of otters, seals and sea lions in the 1980s and 1990s shows that they were not limited by killer whale predation.”

The relative success of British Columbia’s sea lions, harbor seals and sea otters might be largely due to provincial protection from culling and hunting that began in the mid-1970s. But if killer whale predation was on the increase at that time, as the SMC hypothesis suggests, these depleted populations should not have recovered, much less flourished. Furthermore, this sequential decline of marine mammal populations – also called a trophic cascade – was not recorded in the southern hemisphere following the end of commercial whaling. The situation in western Alaska, it seems, is unique.

Dietary Dilemma
Furthering their defence of killer whales, Trites and colleagues offer a deluge of dietary data showing that gray whale calves and minke whales are the only large whale species known to be regularly preyed upon by North Pacific killer whales—and minke whales were never commercially hunted. Even gray whales, which were once targeted by whalers, have since recovered to historic levels while seals and sea lions have declined. These population trends show no historical shortage of mid-sized whale prey.

Moreover, it is unlikely that killer whales would even attempt to feed on larger whales, much less be impacted by their dwindling numbers. The effort and danger involved in subduing larger, migratory whales such as fin and sei whales may be unprofitable, especially if the carcass sinks too quickly to eat. It would seem more sensible for killer whales to target smaller, easier-to-kill species (such as seals, sea lions, dolphins and porpoises) that are available year-round.

“Pinnipeds [seals and sea lions] and small cetaceans [whales, dolphins and porpoises] appear to represent a profitable food source for transient killer whales in British Columbia and southeastern Alaska,” the authors write. “This is supported by the observations that killer whales focus on pinnipeds and small cetaceans in areas where large whales are locally abundant, suggesting that the preferred and profitable sources of food for transients are the smaller species of marine mammals.”

Contrary to the SMC theory, Trites and colleagues submit that these smaller species formed the historic core of the transient killer whale diet in western Alaska, and that their dramatic decline is likely due to mechanisms that are larger and more insidious than marauding killer whales.

Tip of the Iceberg
“The declines of seals and sea lions in Alaska represent the tip of an iceberg of documented changes in the Gulf of Alaska and Bering Sea ecosystems that began around 1977,” the authors write. At that time, a basin-wide regime shift in ocean climate began to impact all marine life in the North Pacific, causing long-term changes in prey availability that are still evident today. For western Alaska’s sea lions, this translated to a shift in diet from energy-rich prey to energy-poor prey that left them weak, hungry, unable to breed successfully and vulnerable to disease and killer whales. Steep declines in these pinniped populations were evident shortly after the regime shift which, due to climatic and oceanographic variables, severely depleted western Alaskan populations but boosted those in southeastern Alaska and British Columbia.

Unlike the SMC hypothesis, this ocean climate hypothesis does not discount other leading hypotheses that propose to explain the decline of Steller sea lions, such as nutritional stress, fishing, disease and even killer whale predation. Instead, the authors believe it provides a holistic framework to align and associate the other hypotheses.

While acknowledging that more research is necessary to fully understand the subtleties of killer whale prey selection, the authors present a clear “not guilty” verdict for killer whales supported by substantial dietary and climatic research. The rigors of peer-reviewed science occasionally send researchers back to the drawing board, but in the process our collective understanding of the oceans moves forward.

5 November 2007

Publication:
Killer whales, whaling and sequential megafaunal collapse in the North Pacific: a comparative analysis of the dynamics of marine mammals in Alaska and British Columbia following commercial whaling. Trites, A. W., V. B. Deecke, E. J. Gregr, J. K. B. Ford, and P. F. Olesiuk. 2007. Marine Mammal Science 23:751-765. abstract The hypothesis that commercial whaling caused a sequential megafaunal collapse in the North Pacific Ocean by forcing killer whales to eat progressively smaller species of marine mammals is not supported by what is known about the biology of large whales, the ecology of killer whales and the patterns of ecosystem change that took place in Alaska, British Columbia, and elsewhere in the world following whaling. A comparative analysis shows that populations of seals, sea lions and sea otters increased in British Columbia following commercial whaling, unlike the declines noted in the Gulf of Alaska and Aleutian Islands. The declines of seals and sea lions that began in western Alaska around 1977 were mirrored by increases in numbers of these species in British Columbia. A more likely explanation is the seal and sea lion declines and other ecosystem changes in Alaska stems from a major oceanic regime shift that occurred in 1977. Killer whales are unquestionably a significant predator of seals, sea lions and sea otters but not because of commercial whaling.