Sleeper sharks eat dead whales, not sea lions

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A study of a large Pacific shark species that was previously believed to be a predator of Steller sea lions has shown that the fish actually feeds predominantly on dead whale carcasses.

The Pacific sleeper shark, Somniosus pacificus, is often found around "rookeries" of Steller sea lions, Eumetopias jubatus, and previous stomach analyses have revealed the odd chunk of seal or sea lion flesh, which led experts to believe that Steller sea lions formed the staple diet of the species in the area.

But a new study published in the Journal of Fish Biology this week says that the Pacific sleeper shark actually feeds on Salmon while young and the carcasses of dead whales when mature. Steller sea lions do not appear to form part of its diet in any way.

A team of scientists caught Pacific sleeper sharks around a Steller sea lion rookery in the north-east Pacific at the time of year when the sea lion pups were most vulnerable to predation. They analysed the stomach contents of the sharks and used a sophisticated molecular technique to determine what they had eaten for their last meal.

In August, bony fish formed the bulk of the sharks' diet, while they switched to eating squid and octopus in May. Although they found on average 15% of the sharks had some form of marine mammal meat in their stomachs throughout the year, none of the tissues sampled had came from the Steller sea lions that the sharks were living in close proximity to.

"Molecular genetic analysis identified Grey whale, Eschrichtius robustus, and Harbour seals, Phoca vitulina, remains in some Pacific sleeper shark stomachs. Most mammals were cetacean and at least 70% of the cetaceans were probably scavenged," the authors wrote.

"Although Pacific sleeper shark and Steller sea lion ranges overlapped, so predation could potentially occur, the diet suggested that predation on Steller sea lions is unlikely, at least when pups first enter the water or during weaning."

The scientists believe that the sleeper shark may feed on Harbour seals, possibly while they are alive, but added that they were an infrequent prey item for the species. They believe that the fish is an opportunistic consumer which feeds on teleost fish, such as Pacific salmon, while young, and then shifts to scavenging on dead whale and dolphin carcasses as it reaches adulthood.

The Pacific sleeper shark, which can reach a size of around 4.5m, and potentially as much as 7m, is also believed to be one of the few predators of the Colossal and Giant squids. The shark species is poorly known since it lives in very deep waters of up to 2000m.

It is believed to be quite abundant and has been recorded from temperate waters from the Californian coast to Mexico, across to New Zealand, Japan and along the Siberian Coast where it occurs in very cold waters.