Abstract
Most of the world’s sea otters reside in Alaska, but there has never been an assessment of long-term mortality patterns for this keystone predator. We examined data collected from 780 northern sea otter (Enhydra lutris kenyoni)carcasses recovered in Alaska from 2002 to 2012 to evaluate the causes of mortality and risk factors associated with death. A smaller group (n= 144, 18%) of fresh non-frozen carcasses were included in a more detailed mortality analysis. Forty-four% of the fresh dead otters were determined to have died from infectious endocarditis, meningoencephalitis, and/or septicemia due to systemic streptococcosis (“Strepsyndrome”).Streptococcus lutetiensis, a member of theStreptococcus bovis/equinus(SB/E) group was most commonly isolated, although other members of the SB/E group were identified. There were fewer cases whereS. phocaeand other streptococci were isolated. A regression analysis revealed age and location risk factors forStrepsyndrome. Subadults were the highest risk age group, and otters recovered from the Kachemak Bay region were 3.6 times (95% CI: 2.2–5.9) more likely to die fromStrepsyndrome than otters recovered elsewhere. Diagnosis of thisStrepsyndrome had not been reported in other marine mammals in Alaska. Sporadic cases of septicemia and infectious endocarditis due toS. lutetiensishave been seen in the southern sea otter population in California. Other causes of death for the fresh otters included neurologic diseases (10%), trauma (8%), nutritional diseases (7%), cardiovascular diseases (7%), gastrointestinal diseases /parasites (6%), undetermined (5%), septicemia (3%), and neoplasia (3%). All other causes of death (oiling, hepatobiliary, fungal, marine biotoxins, pulmonary) were at or below 1%. Twenty percent of fresh animals were positive for phocine distemper virus (PDV) by polymerase chain reaction (PCR, 11/55), 18% seropositive for PDV (7/38), and paramyxovirus-like particles were demonstrated by electron microscopy within inclusion bodies from one animal. Low concentrations of the harmful algal bloom toxins domoic acid and saxitoxin were also detected in 26% and 22% of fresh animals, respectively. Protozoal disease was rare. These patterns of disease differ from sea otters in other regions and possible reasons are discussed.
Highlights
The SC and SE stocks are not listed under the Endangered Species Act (ESA), an observed increase in the number of sea otter carcasses being recovered from the SC stock in Kachemak Bay with mortality due to high proportions of infectious endocarditis (IE) resulted in a Marine Mammal Unusual Mortality Event (UME) being declared by the U.S Working Group on UMEs in 2006
The objective of this study was to examine the causes of mortality in northern sea otters recovered in Alaska from 2002 to 2012, using stranding data to evaluate risk factors associated with mortality
A logistic regression analysis evaluated Strep syndrome vs. all other causes of death and revealed that otters recovered from the Kachemak Bay region were 3.6 times more likely to die from Strep syndrome than otters recovered elsewhere
Summary
Data from marine mammal stranding programs have been used to understand causes of mortality (Kreuder et al, 2003; Joly et al, 2009; Bogomolni et al, 2010; Bossart, 2011) and types of human interactions (Goldstein et al, 1999) and have led to the discovery of novel diseases (Carrasco et al, 2014b; Shen et al, 2017; Siqueira et al, 2017). In Alaska, a northern sea otter (Enhydra lutris kenyoni) stranding program was initiated in 2002 by the U.S Fish and Wildlife Service (USFWS) to examine causes of strandings and mortality in the three Alaska sea otter population stocks (Figure 1); Southwest (SW), Southcentral (SC), and Southeast (SE) (Gorbics and Bodkin, 2001). The SC and SE stocks are not listed under the ESA, an observed increase in the number of sea otter carcasses being recovered from the SC stock in Kachemak Bay with mortality due to high proportions of infectious endocarditis (IE) resulted in a Marine Mammal Unusual Mortality Event (UME) being declared by the U.S Working Group on UMEs in 2006
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