Incorporating Diverse Data and Realistic Complexity Into Demographic Estimation Procedures for Sea Otters

Abstract

Reliable information on historical and current population dynamics is central to understanding patterns of growth and decline in animal populations. We developed a maximum likelihood-based analysis to estimate spatial and temporal trends in age/sex-specific survival rates for the threatened southern sea otter (Enhydra lutris nereis), using annual population censuses and the age structure of salvaged carcass collections. We evaluated a wide range of possible spatial and temporal effects and used model averaging to incorporate model uncertainty into the resulting estimates of key vital rates and their variances. We compared these results to current demographic parameters estimated in a telemetry-based study conducted between 2001 and 2004. These results show that survival has decreased substantially from the early 1990s to the present and is generally lowest in the north-central portion of the population's range. The greatest temporal decrease in survival was for adult females, and variation in the survival of this age/sex class is primarily responsible for regulating population growth and driving population trends. Our results can be used to focus future research on southern sea otters by highlighting the life history stages and mortality factors most relevant to conservation. More broadly, we have illustrated how the powerful and relatively straightforward tools of information-theoretic-based model fitting can be used to sort through and parameterize quite complex demographic modeling frameworks.

The High Cost of Motherhood: End-Lactation Syndrome in Southern Sea Otters (Enhydra Lutris Nereis) on the Central California Coast, Usa

“…The mother's age was also important, likely reflecting the fact that more experienced females were more likely to successfully navigate the challenges of pup rearing (Riedman et al 1994, Tinker et al 2006a. Variation in the magnitude of the hazard rate over the course of the pup dependency period (Figure 7) was consistent with expectations of life history theory .…”

“…The Bayesian-based proportional hazards analysis produced estimates of age-specific survival that were consistent with previously published sea otter vital rates (Tinker et al 2006b), but allowed for a greater flexibility in examining the contributions of various fixed and random effects to variation in survival. This was important as it allowed us to evaluate predictions associated with various hypotheses about factors limiting sea otter population growth in central California.…”

“…Combining all of our estimates of birth rates and survival rates of pups and independent otters, we calculated the expected population growth rates at Big Sur and Monterey using a projection matrix (following methods described in Tinker et al 2006a), and compared these with the observed trends at each site from 2006-2010, based on census data from USGS and CDFW (www.werc.usgs.gov/seaottercount). At Big Sur, the growth rate predicted by a matrix model parameterized with vital rate estimates from our analysis of tagged otters was -3.0%, very similar to the observed trend of -2.5%.…”

“…The probability of pup mortality varied as a function of pup age, as has been previously reported (Riedman et al 1994, Tinker et al 2006a, although the CAR model we used allowed for a more detailed examination of the nature of this variation (Figure 7). The instantaneous hazard rate was highest immediately after birth (when many new-born pups are abandoned), dropped sharply over the next week but then increased again between 3-6 weeks of age, and then dropped to low values for the period between 6 and 14 weeks of age.…”

“…We evaluated survival rates of our radio-tagged sample of animals over the monitoring period, conducting comparisons between BSR and MON, as well as comparisons with similar data from previous monitoring studies (Tinker et al 2006b). We also evaluated birth rates and pup survival rates (or weaning success rates) in these populations.…”