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

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 that the seal and sea lion declines and other ecosystem changes in Alaska stem 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.

Retrospective characterization of ontogenetic shifts in killer whale diets via δ13C and δ15N analysis of teeth

“…Killer whales have been the focus of much recent debate concerning their role in the historic declines of marine mammals in the northeast Pacific Ocean , Trites et al 2007, Wade et al 2007). The cause(s) of these declines is/are still unknown, despite substantial progress in understanding the current ecology of this system.…”

Harbour seal Phoca vitulina movement patterns in the high-Arctic archipelago of Svalbard, Norway

“…Frost et al 1999, Small et al 2008, Womble et al 2010. The reasons for the observed declines are not clear in all cases, but several hypotheses have been proposed that suggest climate change-related linkages such as shifts in prey distribution, increased predation pressure, vulnerability to pathogens or competitive stress (Trites et al 2007, Matthiopoulos et al 2014. The declines in harbour seal numbers in the Aleutian Islands and the Gulf of Alaska over the last 30 yr have been linked to a large-scale oceanographic regime shift that has likely been induced by climate change (Small et al 2008, Womble et al 2010.…”

Evidence of bottom-up control of diet driven by top-down processes in a declining harbor seal Phoca vitulina richardsi population

“…According to this hypothesis, the sequential declines of great whales under the whaling pressure of the late 1800s were followed by increased predation by killer whales Orcinus orca on Steller sea lions Eumetopias jubatus, harbor seals Phoca vitulina and, more recently, sea otters Enhydra lutris (Estes et al 2006). This explanation of harbor seal population crashes, along with other marine mammals, has been highly controversial (Trites et al 2007a, Wade et al 2007). The alternative hypothesis for the decline of marine mammal populations involves bottom-up mediation through regime shifts and over-harvest of fisheries (Trites et al 1997, Francis et al 1998, Rosen & Trites 2000, Trites & Donnelly 2003, Trites et al 2007b).…”