Causes of Mortality in California Sea Otters During Periods of Population Growth and Decline

Abstract

Elevated mortality appears to be the main reason for both sluggish growth and periods of decline in the threatened California sea otter population. We assessed causes of mortality from salvage records of 3,105 beach‐cast carcasses recovered from 1968 through 1999, contrasting two periods of growth with two periods of decline. Overall, an estimated 40%‐60% of the deaths were not recovered and 70% of the recovered carcasses died from unknown causes. Nonetheless, several common patterns were evident in the salvage records during the periods of population decline. These included greater percentages of (1) prime age animals (3–10 yr), (2) carcasses killed by great white shark attacks, (3) carcasses recovered in spring and summer, and (4) carcasses for which the cause of death was unknown. Neither sex composition nor the proportion of carcasses dying of infectious disease varied consistently between periods of population increase and decline. The population decline from 1976 to 1984 was likely due to incidental mortality in a set‐net fishery, and the decline from 1995 to 1999 may be related to a developing live‐fish fishery. Long‐term trends unrelated to periods of growth and decline included a decrease in per capita pup production and mass/length ratios of adult carcasses over the 31‐yr study. The generally high proportion of deaths from infectious disease suggests that this factor has contributed to the chronically sluggish growth rate of the California sea otter population.

Patterns of Mortality in Southern Sea Otters (Enhydra Lutris Nereis) From 1998–2001

“…Based on these estimates and the specific inclusion criteria for this study, approximately one of ten deceased southern sea otters was evaluated. Because the age distribution of carcasses reported here closely matches the age distribution of all carcasses recovered recently (Estes et al, 2003), the carcasses included in this study are likely to be representative of all recovered carcasses. Consequently, the major causes of death reported here are probably similar to what would be observed if detailed cause of death information could be obtained from decomposed or heavily scavenged carcasses.…”

“…Shark-inflicted mortality has been longrecognized as an important cause of mortality in sea otters since carcasses were first evaluated in 1968 and the number of shark-bitten beachcast carcasses has varied considerably by year (Ames et al, 1996). However, the annual proportion of sharkattacked sea otter carcasses relative to the annual population count has increased through time particularly during periods of population decline (Estes et al, 2003). Because interactions with sharks may be modified by encephalitis in otters, prevalence of shark attack may be linked to prevalence of encephalitis.…”

“…A directed search effort would be necessary to collect fresh carcasses from these unpopulated and inaccessible areas. An estimated one-half of all recently deceased sea otters were recovered as beachcast carcasses, and one-fourth of these were retrieved in adequately fresh condition to allow detailed necropsy for specific cause of death determination (Estes et al, 2003). Based on these estimates and the specific inclusion criteria for this study, approximately one of ten deceased southern sea otters was evaluated.…”

“…Sea otter populations experiencing a decline from increased levels of mortality typically have a high percent of prime-aged adults among beachcast carcasses (Estes et al, 2003). Certainly, the prevalence of acanthocephalan-related disease in juveniles and the prevalence of T. gondii encephalitis, cardiac disease, and shark attack in prime aged adults have the potential to exert a population-level effect and may be limiting sea otter population growth in California.…”

Estimating Cause-Specific Mortality Rates Using Recovered Carcasses

“…For other species, long-term diagnostic databases have been developed in an ad hoc manner. Although these data have been valuable for evaluating the relative importance of different causes of mortality , Estes et al, 2003, determining the role of cause-specific mortality in population dynamics requires cause-specific mortality rates (Gerber et al, 2004). Herein, we show how necropsy data can be combined with independent estimates of total mortality rates to estimate crude cause-specific mortality rates (Heisey and Fuller, 1985).…”