We simulated and evaluated multistate capture–recapture models to estimate mortality rates using telemetry data. Four field designs were considered: (A) fixed receivers to estimate total instantaneous mortality (Z), (B) manual searches to estimate instantaneous fishing (F) and natural (M) mortality, (C) fixed receivers combined with external high‐reward tags to estimate F and M, and (D) manual searches combined with external high‐reward tags to estimate M and fishing mortality rates associated with harvest (Fh) and catch‐and‐release death (Fcr) as well as the probability of death due to catch and release (α). Estimates generally appeared to be unbiased for a simulated study with five periods and releases of telemetered fish at the start of periods 1–4. Compared to estimating Z, larger sample sizes are needed to achieve reliable estimates of component rates (F and M). Estimates of component rates were more precise when that source of mortality was directly observed (M in design B, F in design C). The field design using fixed receivers and high‐reward tags should be especially useful in practice, because manual searches are not required to estimate F and M. Multistate models are useful for clarifying the connection between field observations and ecological processes. Reliable estimates of mortality rates, coupled with information on behavior, habitat use, and movement, make telemetry a highly valuable tool for improving fisheries management and stock assessment.
The distribution of threatened Gulf of Mexico sturgeon Acipenser oxyrinchus desotoi
Anthropogenic dewatering of aquatic habitats can cause stranding and mortality of burrowed larval lampreys; however, the effects of dewatering have not been quantified. We assessed: (a) changes in spatial distribution, abundance, and emergence of larvae dewatered at Leaburg Reservoir (OR); (b) emergence and mortality of larvae dewatered in a laboratory; and (c) bias, precision, and interpretation of field results by simulation and modeling of laboratory results. In the field, we examined the distribution, abundance (by N-mixture model), and density of larvae by electrofishing at randomly selected sites before dewatering and after refill, and assessed the emergence rate by observation and excavation during dewatering. Due to dewatering in the field, about 42% of larvae emerged and spatial distribution changed toward sites dewatered less than 20 hours. Estimated average density decreased from 10.8 larvae/m 2 before dewatering to 2.3 larvae/m 2 after refilling, suggesting that abundance declined by 79%; simulation suggested this decline ranged 71-84% (interquartile range). In the laboratory, we examined the emergence and mortality rates of larvae dewatered 0-48 hrs. The emergence rate in the laboratory was similar to that in the field. Mortality rate increased with hours dewatered and was higher for emerged than burrowed larvae. Laboratory estimates of mortality rate predicted a 61% decline in abundance if only burrowed larvae survived and a 54% decline if both burrowed and emerged larvae survived. Abundance declines in the field could be from mortality (e.g., desiccation, predation) and relocation to watered habitat. Our results indicate dewatering can substantially affect spatial distribution and abundance of larval lampreys in freshwater ecosystems.
Dewatering of fine sediments in rivers and streams can kill many thousands of larval lampreys (Order Petromyzontiformes) burrowed in these habitats. The larval life stage for lampreys lasts 3‐ 10 years, and because larvae often aggregate in large numbers, negative impacts from dewatering could potentially deplete local populations and affect multiple year classes. Larval lampreys have not traditionally been considered during in‐stream projects, but recent efforts to increase awareness of lamprey habitats have resulted in guidance on dewatering approaches to limit impacts to lampreys. Salvage efforts to rescue and relocate lampreys aim to mitigate losses, but a lack of understanding of lamprey responses limits optimization of dewatering and salvage procedures. We summarize the state of the science for nine factors that influence larval lamprey (Entosphenus and Lampetra spp.) responses to dewatering, including: burrowing depth, the prevalence and timing of emergence, movements, survival, and the influence of slope, dewatering rate, light, and lamprey size. Research suggests that shoreline slope influences movement capability; hot and sunny conditions increase the risk of mortality; salvage activities cause minimal direct mortality; and smaller larvae are especially vulnerable to negative impacts from dewatering because they are more likely to emerge and are less capable of movement. Critical uncertainties associated with dewatering include cues that drive emergence, influence of sediment composition and stratigraphy, vertical distribution of larvae in natural settings, use of the hyporheic zone, the scale of predation losses, and the effectiveness and impacts of salvage activities. Balancing investments in salvage efforts and lamprey exclusion efforts (e.g., screening) and developing field survey approaches to evaluate use of the hyporheic zone by lampreys are identified management implications and research needs. Addressing the critical uncertainties discussed here and providing updated, science‐based guidance on dewatering and salvage practices are suggested management actions to support lamprey conservation.
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