Mortality Sensitivity in Life‐stage Simulation Analysis: A Case Study of Southern Sea Otters

Abstract

Currently, there are no generally recognized approaches for linking detailed mortality and pathology data to population‐level analyses of extinction risk. We used a combination of analytical and simulation‐based analyses to examine 20 years of age‐ and sex‐specific mortality data for southern sea otters (Enhydra lutris), and we applied results to project the efficacy of alternative conservation strategies. Population recovery of the southern sea otter has been slow (rate of population increase λ = 1.05) compared to other recovering populations (λ = 1.17–1.20), and the population declined (λ = 0.975) between 1995 and 1999. Age‐based Leslie matrices were developed to explore explanations for the slow recovery and recent decline in the southern sea other population. An elasticity analysis was performed to predict effects of proportional changes in stage‐specific reproductive or survival rates on the rate of population increase. A life‐stage simulation analysis (LSA) was developed to evaluate the impact of changing age‐ and cause‐specific mortality rates on λ. The information used to develop these models was derived from death assemblage, pathology, and live population census data to examine the sensitivity of sea otter population growth to different sources of mortality (e.g., disease and starvation, direct human take [fisheries, gun shot, boat strike, oil pollution], mating trauma and intraspecific aggression, shark bites, and unknown). We used resampling simulations to generate random combinations of vital rates for a large number of matrix replicates and drew on these to estimate potential effects of mortality sources on population growth (λ). Our analyses suggest management actions that are likely and unlikely to promote recovery of the southern sea otter and more broadly indicate a methodology to better utilize cause‐of‐death data in conservation decision‐making.

Energetic demands of immature sea otters from birth to weaning: implications for maternal costs, reproductive behavior and population-level trends

“…Numerous studies have attributed the sluggish recovery of southern sea otters to elevated mortality, with the highest mortality rates occurring in postpartum pups and juveniles in their first year post-weaning, but with a disproportionately high mortality rate also observed in prime-age females (Estes et al, 2003;Tinker et al, 2006). Because adult female mortality in particular has a profound influence on the population trajectory of the southern sea otter (Gerber et al, 2004;Tinker et al, 2006), it is of concern for overall population stability and growth.…”

Estimating Cause-Specific Mortality Rates Using Recovered Carcasses

“…This is the complement of equation 1 in Gerber et al (2004). Note that because of the constraint q J~q { P J{1 j~1 q j , there are only J-1 free q j parameters to estimate through the likelihood method (although an estimate for the final source can be made by subtraction).…”

“…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).…”

“…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). This approach has previously been applied in research on sea otter mortality patterns (Gerber et al, 2004); however, no derivation of these mortality rate estimators was provided. Here, we derive causespecific mortality rate estimates and their variances based on maximum likelihood.…”

Loss of Genetic Diversity in Wild Populations

“…California sea otters historically have had the lowest population growth rate of any sea otter population as well as the highest reported incidence of disease within adult animals, those that make up the breeding population (Gerber et al, 2004;Hanni et al, 2003;Kreuder et al, 2003;Miller et al, 2004;Thomas & Cole, 1996). In addition, a high incidence of infectious diseases among this population suggests a weakened immune system or immunosuppression (Gerber et al, 2004;Hanni et al, 2003;Kreuder et al, 2003;Miller et al, 2004). It seems at least conceivable that the low genetic diversity and corresponding high corticosterone levels in this population have played a role in susceptibility to infectious disease via immunosuppression.…”