Don’t Blame Sea Otters for Abalone Decline

Olive eating black abalone

Sea otter eating black abalone. (Photo credit: David A. Jessup)

Can’t we all just get along?  This question is one we might want to ask of sea otter conservation and advocacy groups and sea urchin and other shellfish advocacy groups. The latter have recently argued before Congress that the recovery of the Southern sea otter (Enhydra lutris nereis), listed as threatened under the Endangered Species Act (ESA), by allowing them to expand their range unimpeded, threatens the recovery of the ESA listed white (Haliotus sorensenii) and black (Haliotus carcherodii) abalone. This is based largely on the premise that sea otters eat abalone, so where sea otters are, abalone are not going to be able to recover.

Sea otters certainly do eat abalone, and a fairly wide variety of other shellfish species. And they tend to eat the larger, more available and easier to capture ones, the same ones shell fishers want. Otters will definitely out compete people for abalone. They can have noticeable and very visible impacts on harvestable abalone abundance when they move into a pristine area, but they can’t reach those that are too deep, or are in between rocks and crevices, and don’t eat smaller ones that still may be of reproductive age.

There is little or no evidence that otters can seriously damage healthy red, black, white or green abalone populations. In fact sea otters and abalone have co-evolved over many thousands of years and it would be extremely disadvantageous to sea otter survival if they could drive abalone to rarity or extinction. It would be a remarkable evolutionary failure.

White abalone range

Historic range of white abalone

The decline of the white abalone throughout most of its range over the last century, and the decline of the black abalone in the southern half of its range over the last 3 decades, had essentially nothing to do with sea otter predation. The white abalones’ historic home range extends from southern Santa Barbara County to all the waters off southern California, Baja Mexico del Norte and into Baja Mexico del Sur. Sea otters were extirpated completely from these waters 150-200 years ago. Although some sea otters can dive as deep as white abalone are often found (100-300 feet), most don’t, rather specializing in easier to obtain prey. And very few sea otters frequent areas many miles off shore where the deep dwelling whites may be found.

Besides being deep dwelling, white abalone are slow reproducing and were never very abundant. They were badly overfished and their habitat has suffered from various forms of pollution and degradation. It should be noted that there is only a very small geographic overlap in the current Southern sea otter range and the historic range of white abalone. Unless the agencies working on white abalone recovery purposely focus their work at the extreme northern end of the historic range, given current rates of sea otter range expansion to the south (about 20 miles in 10 years), sea otters and white abalone are unlikely to occupy the same parts of the Pacific Ocean for many, many years.

White abalone

White abalone

Many of the same agencies and universities (U.S. Fish & Wildlife Service, California Department of Fish & Game, University of California (Davis and Santa Barbara) working on white abalone recovery are also working on Southern sea otter recovery. Communication channels are open. Currently most of the focus on white abalone is on being able to reproduce them and raise them for brood stock. Reintroduction may be in the plans in the next decade or two, but there are vast areas of warmer southern California and Mexican ocean waters available that have no sea otters and will not for decades, and maybe centuries, or ever again.

Black abalone with withering syndrome

Black abalone with withering syndrome

Black abalone have been the victims of a terrible new disease introduced into the waters off the Channel Islands in the early 1980’s. Although the exact origin is unknown, ballast water, aquaculture and northern migration of warmer water species are the three most likely sources. The organism is a rickettsia (Xenohaliotis californiensis) that infects the cells lining the simple gut of the black abalone (also to a lesser degree other abalone species). It blocks absorption of nutrients and the host starves and withers away, hence the name “withering syndrome”. This devastating disease has wiped out 80-90% of black abalone as it spread northward from the Channel Islands.

Not only has withering syndrome devastated abalone, but in doing so it has deprived sea otters of an important, high calorie prey in the southern half of its range, likely retarding their recovery. Because they graze rather than filter feed, abalone do not concentrate toxins, pathogens and parasites like clams, mussels and other shellfish. In fact, studies show that sea otters that specialize in eating abalone and larger crabs (the deep diving food guild) are protected against infection with Toxoplasma gondii and Sarcocystis neurona, two major causes death in prime age animals.

Black abalone at San Nicolas Island

Black abalone at the same spot at low tide on San Nicolas Island before and after withering syndrome hit.

Since they live in shallow waters, sea otters have always utilized black abalone as prey. Otters take enough of them to compete with humans who want to harvest them. But their numbers were not seriously depleted and some level of recreational fisheries take could be sustained where and when both sea otters and black abalone lived together in the 1960’s and 70’s, before the plague of withering syndrome. In fact, healthy populations of sea otters and black abalone still co-exist in areas where the withering syndrome has not been able to spread because of lower water temperatures (the rickettsia doesn’t tolerate lower sea water temperatures like those found in the Northern Big Sur coast, Monterey Bay and northward toward the Golden Gate).

So, yes, sea otters eat the ESA listed black abalone, but they had nothing to do with why that species has declined precipitously and is now protected. And, unless or until withering syndrome is successfully controlled, manipulation of sea otters will do little to bring them back. And yes, theoretically, if given the opportunity, otters might eat some white abalone, and could complicate recovery efforts, but that isn’t really a significant problem facing their recovery now or in the near future.

So, can’t we all just get along?  Can’t we recognize that conflict isn’t really necessary?  Maybe recognize that restoring white and black abalone is very important from many perspectives, that they are part of a healthy of near-shore ocean, but that recovery plans and efforts are at a very early stage?  Can’t we trust the people whose job it is to see to abalone recovery to communicate and coordinate with those working on sea otter recovery?  Isn’t it in the interests of both communities to see all three of these ESA listed species recover and thrive?

Those that love and use the ocean may not have the exactly the same priorities in life, but it’s just possible we have more in common than some at first think.

 

Related Links

H.R. 4043 and the “No Otter Zone”

 

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