Posted by: The ocean update | August 21, 2015

Great White Sharks Are Killing More and More Endangered Otters—But Not Eating Them (California, USA)

Sea otters were nearly wiped out by hunting by the early 20th century. Their recovery has been hampered by bites from great white sharks. MIKE BAIRD VIA WIKIMEDIA COMMONS

Sea otters were nearly wiped out by hunting by the early 20th century. Their recovery has been hampered by bites from great white sharks. MIKE BAIRD VIA WIKIMEDIA COMMONS

August 21st, 2015 (Douglas Main). There is no evidence that great white sharks eat sea otters, except perhaps in rare circumstances. Despite being the heaviest member of the weasel family, these hairy guys are still quite small compared to the seals great whites prefer—they’re just not large enough to make it worth a shark’s while to dine on them.

And yet, in the last decade, great whites are increasingly chomping on these animals and killing them—bites from these sharks now account for 60 percent of the documented deaths of these endangered animals on the coast of California, according to a study published in August in the journal Marine Mammal Science. In the southern part of the range, north of Point Conception, it’s worse, with more than two-thirds of otters that wash ashore dead bearing great white bites.

This is driving a decline in the number of southern sea otters in several parts of their range, says M. Tim Tinker, a wildlife biologist with the Western Ecological Research Center, run by the U.S. Geological Service.

“This is the first instance we know of in which a predator has become the primary source of mortality for a non-prey species,” Tinker says.

“It is very surprising to see so many bitten and discarded otters,” since the animal is not a food source for great whites, says Salvador Jorgensen, a research scientist at the Monterey Bay Aquarium who wasn’t involved in the paper. “This is a very unique situation, unlike anything I have seen.”

So what’s going on?

Southern sea otters (Enhydra lutris nereis), as well as northern and Asian sea otters, were nearly hunted to extinction in many areas of the Pacific by the early 20th century. Since being classified as a threatened species in the 1970s, the protections afforded them by federal laws such as the Marine Mammal Protection Act have helped them rebound somewhat; their range now stretches south nearly to Santa Barbara, and north to Pigeon Point, near Santa Cruz. Great white shark populations also appear to be increasing off of California.

These increases may help explain the trend: There are more otters and perhaps more great white sharks to bite them. But it’s not that simple, because the number of deaths from bites has grown much faster than the otters’ population levels.

One theory is that these biting deaths are driven by young great whites who don’t know better. It could be youthful sharks “trying to make their first cracks at marine mammals,” says Christopher Lowe, a shark expert with California State University Long Beach. The trends and timeline of otter bites “match what we have seen in terms of white shark population increase based on the number of baby white sharks being seen or caught in southern California.” However, what doesn’t make sense is why a hungry young white shark would bother to bite a potential meal and not eat it, he adds. It’s possible that the taste of the otter is enough to convince the young shark that it doesn’t contain enough calories—the “low-fat version of a brownie,” Lowe jokes.

Another theory is that “as the white shark population grows and there is increased competition for choice feeding opportunities” of seals, that “they are trying different locations and investigating new potential prey,” Lowe says.

One a related note, the trend could be due in part to a shift in the distribution of sharks, which are increasingly found near large populations of seals, places that otters can also often be found. This trend has happened during the same time period—the last 60 years—that the risk of a human getting bitten by a great white shark has decreased by 90 percent, Jorgensen said. So the shift of sharks away from beaches with people to areas with lots of seals has helped humans, but not otters, the data suggests.

Lilian Carswell, a scientist with the U.S. Fish and Wildlife Service, says that great whites are now limiting the range expansion of the southern sea otter, which once stretched as far north as Oregon south to midway down the Baja Peninsula. One glimmer of hope for expansion is a small population of animals between Point Conception and Gaviota State Beach, west of Santa Barbara. There are about 50 animals in this area, where there is abundant otter food (aka invertebrates like clams and crabs), and may offer some protection from shark bites, she says.

Another bright spot for sea otters is a population on San Nicolas Island, the most remote of the Channel Islands (a group of isles south of Santa Barbara). This group is is small but doesn’t “appear to be affected by shark bite mortality and has been growing at 16.4 percent per year over the past five years,” Carswell says.

The increased biting of otters by sharks "is a very unique situation, unlike anything I have seen,” says researcher Salvador Jorgensen. Here is a mother otter with rare twin pups. MIKE BAIRD VIA WIMIKEDIA COMMONS

The increased biting of otters by sharks “is a very unique situation, unlike anything I have seen,” says researcher Salvador Jorgensen. Here is a mother otter with rare twin pups. MIKE BAIRD VIA WIMIKEDIA COMMONS

 

Photographs taken during sea otter necropsies illustrating evidence considered diagnostic of shark bites. (A) A white shark tooth fragment found in tissue associated with puncture wound (note the serrated tooth edge, a diagnostic characteristic of white shark teeth). (B) Parallel scratches found on a sea otter bone, assumed to be the result of contact with the serrated edge of a white shark tooth. (C) Abdominal region of a sea otter, showing a series of stab-like puncture wounds arranged in an arc on either side of the body, assumed to be the result of a shark bite.

Photographs taken during sea otter necropsies illustrating evidence considered diagnostic of shark bites. (A) A white shark tooth fragment found in tissue associated with puncture wound (note the serrated tooth edge, a diagnostic characteristic of white shark teeth). (B) Parallel scratches found on a sea otter bone, assumed to be the result of contact with the serrated edge of a white shark tooth. (C) Abdominal region of a sea otter, showing a series of stab-like puncture wounds arranged in an arc on either side of the body, assumed to be the result of a shark bite.

Citation : Tinker, M. T., Hatfield, B. B., Harris, M. D. and Ames, J. A. (2015), Dramatic increase in sea otter mortality from white sharks in California. Marine Mammal Science. doi: 10.1111/mms.12261

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