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.
The concept of delayed transmission Satellite-linked Life History Transmitters (LHX):
Data on survivorship is crucial for the effective monitoring and management of many species of marine endotherms, in particular endangered species and those potentially exposed to detrimental ecological and anthropogenic environmental changes, or climate related regime shifts. In addition, data on individual survivorship is needed to assess the efficacy of programs designed to ameliorate the impact of such changes and shifts.
Due to wide dispersal or migrations, and the open ocean ranging of marine endotherms, such data is extremely difficult to obtain. Survival of free-ranging animals is typically assessed through mark and recapture studies, or through the use of mortality transmitters. Mark and recapture studies are expensive and logistically complex to conduct, and highly disruptive in the rookeries of shy species such as Stellers. In addition, such studies cannot directly distinguish between dispersal / emigration, and mortality. Conventional mortality transmitters are externally attached VHF transmitters. Several problems are associated with such devices: on pinnipeds and seabirds, external units typically do not remain attached beyond the annual molt, limiting tracking to a maximum of one year. Battery-size and -capacity constraints also limit the life span of such units. Implanting mortality transmitters would avoid such problems. Implanted devices have been successfully used on a wide range of marine endotherms. However, reception range and thus area coverage from implants is reduced compared to external devices. Transmitting life span is still limited to 2-3 years.
A possible solution to extend coverage range for mortality transmitters is the use of satellite-linked devices. Satellite-linked data loggers, using theArgos system aboard NOAA satellites (accessible through Service Argos) for obtaining location fixes and transmission of stored data have been successfully and extensively used on oceanic vertebrates. At present however, transmission to a satellite from implanted devices is not feasible (Horning et al. 1999).
To circumvent this problem, the concept of implanted, delayed transmission, Satellite-linked Life History Transmitters (LHX) was developed in the Laboratory for Applied Biotelemetry & Biotechnology at Texas A&M University. Under funding from the North Pacific Marine Research Program, LHX devices are being developed by Wildlife Computers, in cooperation with our LABB. The LHX units consist of an ARGOS-compatible transmitter, a microprocessor driven controller, and five sensors for pressure, temperature, motion, light-level and conductivity. These LHX devices continuously monitor these built-in sensors to establish death of an instrumented animal, then store time and date of death in memory. Subsequently, the LHX devices will transmit this data to an orbiting ARGOS satellite, once the positively buoyant device has been released from the decomposing or consumed body. Through the absence of any transmissions, until after death, battery life is greatly extended to well beyond five years.
One of the problems associated with this concept of "delayed-transmission" satellite-linked mortality transmitters, is the impossibility of periodically transmitting an 'alive' signal, which is traditionally used to verify continued operation of the transmitter, and to guarantee the quality of the data obtained. In the absence of such transmissions of 'alive' signals, the accurate assessment of the transmission failure rate of implanted mortality transmitters becomes crucial. Mortality data obtained in form of positive 'deceased' signals needs to be corrected by estimates of failure rates. In the LHX project, the use of dual redundant implants is one of several approaches used to quantitatively assess instrument and transmission failure rates. Cumulative system failure rates are determined by comparing the ratio of dual versus single hits from two redundant LHX devices implanted into each study animal.
A required step before we can consider implanting LHX devices into free-ranging Steller sea lions - a declining and endangered species - is the validation of the LHX concept under highly controlled conditions, on captive animals.
This validation will consist of two steps:
We will accomplish these validations through a combination of tests. We will initially test LHX devices on carcasses of California sea lions, to ascertain technical functionality and operation of the units. Next, we will impant dual redundant LHX devices into rehabilitated California sea lions at The Marine Mammal Center (TMMC) at Sausalito, CA - a major marine mammal rehabilitation organization. TMMC is highly interested in the LHX project. Our technology - if validated - will allow rehabilitation centers such as TMMC to accurately assess the success rates of their rehab & release programs, through long-term post-release tracking and survival monitoring of rehabilitated animals. After implantation of two redundant LHX devices into rehabilitated California sea lions, the animals will be kept under close observation for periods from two to eight weeks, and will then be released along the California coast. We will then monitor for a signal from the LHX implants, through Service Argos, for the next five years.
Significance of the LHX concept:
The concept of a satellite-linked, delayed transmission mortality tag represents a new experimental paradigm in the study of ocean-ranging marine endotherms. Through the use of LHX technology, we can directly assess effects of a large number of proximate causes on the survival of individual animals. For the testing of hypotheses pertinent to proximate causes of Steller sea lion decline, our new paradigm will offer a greater resolution, and will be able to detect smaller effects, than the classic regional comparison between stable and declining populations. In the LHX project, we expect to test predictive powers on future survival, of parameters that can be measured in juvenile animals when they are most accessible, prior to weaning. This could become an important tool in predicting future population trends several years before these trends become apparent at the census level. The LHX project will likely enhance future capabilities of modeling the response of an apex predator population to a variety of environmental changes - whether manmade or associated with non-anthropogenic environmental variability. In providing - for the first time in a marine mammal - long-term, multi-year dive effort data, LHX technology will provide the means to relate short-term (seasonal) and long-term (inter annual, ontogenetic) variability in survival and foraging behavior to environmental variability along multiple temporal and spatial scales. In addition, LHX technology will be a valuable tool for long-term population monitoring, when applied to other age groups including adults. The LHX project follows specific recommendations by the Bering Sea Ecosystem Research Plan Draft (BSERP 1998), and several Steller Sea Lion Recovery Team Research Peer Review Workshops (Didier, ed. 1997a,b; Springer et al. 1999; Williams et al 1999).
© M. Horning 1999.
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