Sea Otter Mortality
A primary focus of California Department of Fish and Wildlife’s Marine Wildlife Veterinary Care and Research Center (CDFW-MWVCRC) is providing pathology and laboratory diagnostic investigation services for sea otter mortalities and other marine wildlife mortality events, and archiving of tissues and specimens. This research provides information to the Office of Spill Prevention and Response (OSPR), CDFW, and other agencies on factors limiting the recovery of the southern sea otter population, and information on background mortality rates of otters and other marine wildlife (useful for detecting unusual mortality events). The U.S. Fish & Wildlife Service (USFWS) Recovery Plan for the Southern Sea Otter identifies the CDFW as the lead agency for investigation of mortality.
Today, the majority of all southern sea otters found dead off the coast of California are brought to the MWVCRC in Santa Cruz for necropsy or are examined in the field by CDFW sea otter biologists. The MWVCRC has a dedicated necropsy laboratory for post-mortem examinations of sea otters and other marine wildlife. For more information on what we’ve learned by investigating sea otter pathology, check out MWVCRC’s Technical Reports and Publications pages.
Causes of Southern Sea Otter Mortality
The graphic below provides a summary of primary cause of death for a subset of sea otters examined at the CDFW-MWVCRC from 1999 through 2006, including only fresh carcasses of subadult or adult animals. Although these animals represent a small fraction of the total number of sea otters that die in California each year, this mortality information provides an approximation of the range and relative proportion of factors responsible for mortality during this time period.
It is often challenging to determine the primary cause of death in sea otters. Postmortem examinations often reveal numerous health issues, and determining which one was ultimately responsible for the animal’s death may be difficult. This includes only primary cause of death, although other factors may have also played an important role in each animal’s death. Additional detail on each mortality category is provided below.
Direct Anthropogenic. This category includes mortality that was directly caused by humans, including trauma determined to be from boat strike (the majority of cases), gunshot, and entanglement in fishing line or nets. As mentioned in several categories below, other causes of death may also be considered to be indirectly anthropogenic.
Biotoxins. This category includes acute or subacute poisoning by toxins produced by marine plankton (domoic acid) or freshwater cyanobacteria (including microcystin). These compounds are produced by naturally occurring microorganisms, but research is ongoing into whether human land management practices (e.g., runoff of excess nutrients) influence bloom severity and toxin production.
Bacterial Infection. This category includes all bacterial infections, including species that originate in the marine environment and those that flow to the ocean from land-based animal feces or sewage. Anthropogenic watershed degradation may enhance flow of bacteria and other pathogens into sea otter habitat.
Protozoal Infection. This category includes infection by land-based protozoal parasites including Toxoplasma gondii (shed in feces of domestic/feral cats, bobcats and mountain lions) and Sarcocystis neurona (shed in feces of opossums). The tough, environmentally persistent eggs of both parasites enter the ocean through coastal watershed runoff, and can concentrate in sea otter prey. Because opossums and domestic/feral cats are not native to California, most cases of protozoal infection of sea otters could be considered to be indirectly anthropogenic.
Starvation/Emaciation. This category includes animals that were unable to meet the extreme energetic demands required of sea otters. Included in this category are animals that succumb to “End Lactation Syndrome,” in which females rearing pups are unable to meet the energetic demands of feeding themselves and their pups.
Mating Trauma. When sea otters mate, the male grasps the female by her nose. This category includes females that have such severe nose wounds from mating that they succumb to infection, impaired breathing, or blood loss.
Cardiovascular Disease. This category includes animals that died with severe heart muscle damage or heart failure (excluding bacterial infections of the heart) Biotoxins are important contributors to cardiovascular disease in sea otters, and many cases of cardiovascular disease in wild sea otters may result from chronic or recurrent biotoxin exposure.
Acanthocephalan Parasites. This category includes animals that die due to perforations of the intestines by acanthocephalan parasites, also known as thorny-headed worms. Otters are exposed to these parasites by eating infected sand crabs and mole crabs.
Shark Bite. This category includes animals that died of wounds related to attacks by sharks, in all cases presumed to be white sharks. Many otters that are bitten by sharks die due to bleeding, or when the wounds become infected with bacteria.
Other Presumed Natural. This category includes other miscellaneous causes of death, including gastrointestinal disorders, disease related to old age, fungal infections, and fight trauma. Most fungal infections in sea otters are caused by Coccidioides immitis, a land based fungus that also causes “Valley Fever” in humans.
Undetermined. In a few cases, the cause of death was uncertain or undetermined.
- California sea otter numbers take a slight dip from last year, but average count exceeds 3,090 for third consecutive year
- Increasing White Shark Bites Now Leading Cause of Sea Otter Mortality in California
- Sea Otter Mortality: Learning to Read Between the Lines
- What are Sea Otter Necropsies and Why are They Needed?