Life History Transmitters (LHX) in Steller sea lions: assessing the effects of health status, foraging ability, and environmental variability on juvenile survival and population trends.
Background Information:
The polar regions cover areas of intense biological interest. The southern circumpolar seas have been fairly untouched by commercial enterprises and remain one of the few ecosystems subjected to comparatively little impact from human activities. Conversely, arctic regions around and including the Bering Sea comprise delicate ecosystems threatened by profound regime shifts (Schumacher 2000). The Bering Sea region represents one of the biologically and economically most important ecosystems in the United States, providing over fifty percent of fish and shellfish catches in a multi-billion dollar industry (Fritz et al. 1995, Fritz & Ferrero 1998). In a troubling development, most apex predators in this ecosystem have exhibited dramatic population declines over the past three decades (Loughlin 1998). Steller sea lions, as one such species have declined to about ten percent of peak population levels and are currently listed as endangered in the western portion of their range, along the Aleutian Islands and in the Bering Sea (Loughlin 1998, Fritz & Ferrero 1998). Northern fur seals, harbor seals and several seabird species have exhibited less dramatic but nonetheless severe declines. Extensive removal of fish biomass through commercial trawling has been hypothesized as one possible factor involved in the decline of Aleutian and Bering Sea pinnipeds (Merrick & Loughlin 1997, Calkins et al. 1998). Despite years of intense research efforts by many agencies and institutions, no conclusive data exists to shed light on the hypothesized link between commercial fisheries, nutritional stress and reduced reproductive output of pinnipeds, or to allow for analysis of proximate mechanisms linking hypothesized cause and effect. Significant fisheries management decisions are being made under dearth of adequate data.
One of the leading hypotheses for the continuing decline of Steller sea lions in the Aleutian Islands (AI) and Gulf of Alaska (GoA), is a decrease in juvenile survival by 10 - 20% (York 1994). This decrease in juvenile survival is seen as consistent with a continuing population decline of about 5% per annum (Merrick et al. 1987) and an increase in the average age of adult females of 1.55 years over a ten-year period (Calkins & Pitcher 1982). Merrick and Loughlin (1997) further hypothesized nutritional stress related to a reduced juvenile foraging efficiency as a possible cause for reduced juvenile survival. This would be consistent with research on other otariid species where foraging juveniles were shown to be more susceptible to a reduction in prey accessibility than even lactating adult females (Horning & Trillmich 1997b, 1999a,b).
The hypothesized reduction in juvenile survival, however, is derived from a model based on a Leslie population matrix (York 1994). This matrix is in turn based on observed rates of decline, observed changes in average age of adult females, and estimates of age-specific fecundity (Calkins & Pitcher 1982). This hypothesized reduction in juvenile survival, resulting in a reduced recruitment into the reproductively active rookery population, has been used to focus investigative efforts relating to the Steller sea lion decline primarily on juvenile animals (Merrick & Loughlin 1997, Didier 1997a,b). To warrant a continued focus of investigative effort on juvenile stellers, juvenile mortality figures need to be accurately determined. Mortality figures are crucial indicators of future population trends, especially since survival of juvenile animals is bound to impact recruitment and thus reproductive output of rookeries. On a population level, survival figures are integrators over several possible proximate effects that could contribute to the population decline, such as disease and pollution, predation as well as a reduction in foraging efficiency due to changes in prey abundance and/or quality. Thus, survival figures can be utilized to monitor a population, irrespective of which proximate causes contribute (with some exceptions) to the population decline. Furthermore, mortality rates are expected to reflect nutritional stress or other proximate factors detrimental to a population several years before ultimate effects such as reduced pupping rates / pup counts become apparent (the latter presumably through a drop in recruitment).
In the LHX Project, we will determine juvenile mortality, and thus directly test a crucial component of the leading hypothesis for the continuing population decline in Stellers, using implanted Satellite-linked Life History Transmitters (LHXs). While LHX technology can be applied to other age groups, we are working with juvenile animals because reduction in survival has been hypothesized for juveniles. Nutritional stress should impact juveniles at least as much as lactating females (see Horning & Trillmich 1997b, 1999a,b). Based on recent advances in capture techniques pioneered by ADF&G (McAllister 1998), juveniles are also the most accessible age group (other than very young pups) for telemetry studies. Using the ADF&G in-the-water capture technique, rookery disturbance will be minimal. We will obtain longitudinal dive effort data, from near-weaning until time of death, from animals that survive the first few years, with an expected lifespan of the data-logging mortality transmitters of >5 years. In addition to the accurate assessment of juvenile Steller sea lion survival, we will obtain dive effort and dive behavior data from animals that do not survive, and presumably contribute the most to the population decline. One of the biggest puzzles in the decline of this species remains the virtually complete absence of any 'corpus delicti', for very few near-death animals or carcasses are ever encountered. This problem will be directly circumvented through the use of satellite-linked mortality transmitters.
Dive effort data has been inversely linked to relative prey abundance / accessibility in otariid seals (Croxall et al. 1988, Costa et al. 1989). In the sole extant study on the development of diving and foraging in otariids (Horning & Trillmich 1997b), juvenile fur seals were found to increase diving and foraging effort during seasons of reduced prey abundance. However, in case of predictable short-term resource reduction diving activity was temporarily diminished. Younger animals - who were more likely to starve during periods of severe nutritional stress - showed a greater seasonal increase in dive effort under moderate reduction in prey accessibility, than older animals (Horning & Trillmich 1997b, 1999a,b). During short periods of predictable reduction in prey accessibility, younger animals reduced dive activity to a greater extent than older ones (Horning & Trillmich 1999a,b). Thus, a comparison of temporal patterns of changes in dive activity and forage effort, along weekly, seasonal and annual scales is crucial to assess a possible reduction in foraging efficiency, related to changes in prey abundance & accessibility, from dive effort data. Information on seasonality of death will elucidate the likelyhood of competion by fisheries playing a role in juvenile survival: if timing of death is related to timing of major fishing episodes in respective areas, a relationship between these two factors can be postulated. To test additional proximate factors that could contribute to the Steller population decline, we will depart from the regional comparison, based on the statistical likelyhood of sampling a larger number of non-survivors in regions of declining vs stable populations. Instead, we will utilize a new paradigm by directly comparing survivors to non-survivors. We will directly test the relationship between early body condition, health - and in cooperation with independent projects - near-weaning diving propensity, indicators of clinical health and immuno-competence, as well as pollutant levels - and survival in individual animals. While the nature of the collected data should permit an early estimate of the incidence of traumatic death (predation, incidental take) vs non-traumatic death (starvation, disease), we intend to further develop & refine methods to detect cause of death through implanted LHXs
© M. Horning 1999.
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