Walleye are an important Lake Erie species with both economic and societal value to Canada and the U.S. Accurate mortality estimates are important for sustainable management of the species, especially for yield estimation and quota setting. Walleye harvest limits in Lake Erie are determined by applying a harvest control rule to the results of a stock assessment model performed by the Walleye Task Group (WTG). The WTG uses a complex statistical catch-at-age model to estimate abundance of walleye annually, incorporating fishery-dependent data from Ontario, Ohio, and Michigan, along with data from three fishery-independent gill net surveys, and assumes a natural mortality rate of 0.32. Deriving estimates of natural mortality, spatially and temporally, is challenging, but would allow for variability to be investigated and be valuable to fishery management. Acoustic detections can indicate the alive or dead status of a tagged fish at a specific point in time and space, but it is unclear how to use these data to quantify mortality of the population. There were four objectives of this dissertation: 1) Develop an estimation approach and software code for estimating mortality components for walleye in the Great Lakes based on ongoing acoustic telemetry study designs, 2) Assess the performance of the different methods and explore through stochastic simulations the sensitivity of each approach, 3) Evaluate mortality rates of Lake Erie walleye, and 4) Provide guidance on next steps for this research.Chapter 1 addresses objectives one and two, and presents an evaluation of two different approaches to estimating mortality, a spatial and a non-spatial approach. Both approaches were evaluated using simulated data across a range of scenarios, including different study designs (receivers arranged in grids versus lines), number of receivers, and mortality levels. Accuracy and precision in mortality estimates were sensitive to assumed mortality rates and receiver configurations; the high-density receiver grid resulted in the lowest error rates. Estimates were consistently positively biased. Chapter 2 addresses objective three by estimating mortality rates of Lake Erie walleye using the spatial and non-spatial approaches developed for Chapter 1 and the acoustic telemetry data collected from the Great Lakes Acoustic Telemetry Observation System (GLATOS) network. We applied the spatial and non-spatial mortality estimation methods to three groups of acoustically tagged Lake Erie walleye. The total mortality rate estimated by these methods was similar to the total mortality estimated in the assessment model for Lake Erie Walleye currently used by the management agencies, yet the high reported fish harvest in the acoustic telemetry data suggests that the natural mortality rate may be lower than that assumed by the assessment model. The differences between the three tagging groups also suggests spatial differences may exist in the mortality experienced by different spawning populations. However, the poor performance of the spatial models discourage confidence in those results and inhibited inferences on the specific spatial patterns of mortality. Chapter 3 addresses the fourth objective and provides guidance on the next steps for this line of research. The bias and performance issues in Chapters 1 and 2, particularly for the spatial models, led to the conclusion that new estimation methods are needed for using acoustic telemetry data to estimate mortality components. Accounting for the underlying movement patterns of the study species, incorporating other auxiliary data, and using a larger data set of acoustic detections may allow for a more complete spatial investigation of Lake Erie walleye mortality patterns.
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