Shark predation and tooth wear in a population of northeastern Pacific killer whales

Abstract

The cosmopolitan killer whale Orcinus orca feeds on a wide variety of prey types over its global range, but in at least some regions, genetically distinct and ecologically specialised lineages of killer whales coexist sympatrically. In coastal waters of the northeastern Pacific, 2 such lineages have been well described: the so-called 'residents' prey on teleost fish, especially salmonids and the other ('transients') on marine mammals. A third lineage in this region ('offshores') appears from chemical tracers to be ecologically distinct from residents and transients, but its diet is very poorly known. Here we describe 2 encounters with offshore killer whales during which multiple predation events involving sharks were observed. Using DNA analysis of tissue samples collected from these predation events, we identified the prey species as Pacific sleeper shark Somniosus pacificus and determined that a minimum of 16 individuals were consumed over the 2 encounters. This represents the first confirmed prey species of offshore killer whales based on field observations of foraging and the first record of any Somniosus species in the prey of Orcinus. We also show quantitatively that apical tooth wear is far greater in offshores than in resident and transient killer whales, and propose that such wear is at least in part due to abrasion from dermal denticles embedded in shark skin. Further studies are needed to determine whether offshore killer whales are as specialised ecologically as resident and transient killer whales, and whether sharks play a dominant role in their diet.

Confirmation of the occurrence of a second killer whale morphotype in South African waters

“…Even for animals with unworn teeth, such dexterity would be seemingly impossible without cooperation in prey-handling between individuals, but animals with worn teeth would seem incapable of participating in any meaningful way in prey dismemberment. Consequently, they must either subsist on smaller prey items or, as proposed by Ford et al (2011), be provisioned effectively by other school members, as has been previously documented for killer whales (e.g. Hoelzel 1991;Ford and Ellis 2006).…”

“…Caldwell andBrown (1964) speculated that such wear might arise from an undercutting motion of the teeth against each other, but the mechanism for this seems unlikely, and it is now considered more probable that the attrition is diet-related, in particular to the handling of fish with highly abrasive skin such as elasmobranchs (Ford et al 2011). This hypothesis would seem to be supported by the stomach contents of the flat-toothed female in our study.…”

In cold blood: evidence of Pacific sleeper shark (Somniosus pacificus) predation on Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska

“…However, a delayed secondary ingestion at the surface still would expose LHX tags to SSTs and ambient light, and the probability of a simultaneous encounter of a sea lion and 2 distinct predator species at depth would be very low, in part because it would expose Pacific sleeper sharks to predation risk (Ford et al, 2011).…”

Cultural evolution of killer whale calls: background, mechanisms and consequences

“…Here, three distinct killer whale assemblies or ecotypes have been described: Residents specialize on fish, live mainly in coastal waters and usually travel in large stable social units of closely related animals Ivkovich et al, 2010); Transients or Bigg's killer whales hunt primarily marine mammals and travel in smaller, more fluid social groups (Baird & Dill, 1996;Ford et al, 1998), and Offshores appear to specialize on sharks in their diet (Ford et al, 2011a) and typically occur in large groups (50+) with an unknown social structure. As the name indicates these whales are usually encountered further away from shore.…”