Sea Otter Mortality: Learning to Read Between the Lines

Mortality Pie Chart: 1998 - 2003

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Scientists working on Southern sea otter conservation use various measures to assess long-term population trends. Because ongoing, high mortality is one important obstacle to sea otter recovery, those that die are examined closely by disease specialists and veterinary pathologists. Their findings are compiled into summary reports that help sea otter biologists and other scientists to recognize disease outbreaks, map long-term mortality trends and recognize health issues that are affecting the population. One common and easy to understand way to summarize the data is mortality pie charts, like the one shown above. Pie charts divide major causes of death like infection or trauma into discrete sections that resemble slices of pie.  The size of each pie “slice” relative to all others is a rough approximation of the importance of that condition or problem as a cause of death for the living population in the time period specified.

White shark tooth embedded in sea otter.

White shark tooth embedded in sea otter. (Click to enlarge)

In the pie chart above, you can see that the yellow slices, representing all causes of infectious disease make up over half of the deaths of 241 Southern sea otters that were examined between 1998 and 2003. Some of the larger yellow slices illustrate common infectious diseases of sea otters, such as abdominal infections by thorny-headed worms (acanthocephalids ) and brain infections (encephalitis) caused by single-celled protozoa like Toxoplasma and Sarcocystis. Traumatic injuries, as indicated by the blue pie slices, are another common general cause of death. Shark bite, boat strike, and trauma due to mating activity and fighting between otters are the major causes in this category. This pie chart clearly illustrates that sea otter deaths due to trauma and infectious disease are very important. Because high mortality is inhibiting Southern sea otter population recovery, determining major causes of mortality is critical for optimizing conservation. Pie charts can help guide those efforts.

However, mortality pie charts can also cause major misconceptions. For example, Southern sea otters commonly die with several significant health problems that are affecting them concurrently, such as trauma and thorny-headed worm infection, or pre-existing brain disease and trauma. However, the “1-slice, 1-cause” rule that pie charts utilize to depict data may result in under recognition of concurrent causes of mortality. This is  important for fully understanding conservation needs. Toola, one of the sea otters at the Monterey Bay Aquarium suffered from seizures and tremors due to  brain damage apparently caused by Toxoplasma (diagnosis based on clinical findings). Sea otters with brain damage may be more likely to get bitten by sharks or struck by boats because their ability to respond appropriately and escape is compromised.

Certain causes of death may be under-represented when using pie charts to analyze mortality trends, because our ability to recognize “cases” is hindered by a lack of precise scientific knowledge. One excellent example is sea otter death due to poisoning/toxins, which represents only 2.9% of total deaths in the above figure. Our ability to detect these toxins is hindered by limitations on funding and access to the most sensitive and accurate diagnostic tests. What we don’t know about toxins is greater than what we do know!

Disease, like all other biological processes, is not static in time or space. Clear patterns exist for disease and pathogen exposure in sea otters. Some areas of the coast are “high-risk” for death due to a specific cause, like Sarcocystis, while other regions pose a much lower risk.  Striking differences in disease risk also occur by sea otter age, sex and reproductive status. These variations in risk of disease exposure cannot be depicted in a pie chart. Recognizing these differential risk patterns and their causes are critical to optimizing research and its contribution to conservation efforts.

In summary, mortality pie charts like the one above provide helpful information to guide mitigation efforts for threatened and endangered species like Southern sea otters. However, they should never be viewed by themselves as a complete and accurate summary of mortality trends.  Much more interesting and important information lies just beneath the surface and between the lines !


Related Links:

VIDEO: What are Sea Otter Necropsies and Why are They Needed?

Sea Otter Mortality from White Sharks in California


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About is dedicated to raising awareness about California's threatened sea otters and the role research plays in the species recovery and conservation. It's a collaboration of the Monterey Bay Aquarium, U.C. Santa Cruz, U.C. Davis, U.S. Geological Survey, U.S. Fish & Wildlife Service, California Department of Fish and Wildlife, among others. is also home of the world's first HD live stream of southern sea otters in the wild.