Browsing UAF Graduate School by Subject "Pacific halibut"
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Ecological interactions among important groundfishes in the Gulf of AlaskaComplex ecological interactions such as predation and competition play an important role in shaping the structure and function of marine communities. In fact, these processes can have greater impacts than those related to fishing. We assessed ecological interactions among economically important fishes in the Gulf of Alaska - a large marine ecosystem that has recently undergone considerable shifts in community composition. Specifically, we developed an index of predation for Walleye Pollock (Gadus chalcogrammus) to examine spatiotemporal changes in consumption, quantify portfolio effects, and better understand diversity-stability relationships within the demersal food web. We also evaluated the potential for competition between two important pollock predators, Arrowtooth Flounder (Atheresthes stomias) and Pacific Halibut (Hippoglossus stenolepis). We found highly variable predation intensity on Gulf of Alaska pollock. The combination of a single dominant predator and synchronous consumption dynamics indicated strong top-down control in the region. Spatial heterogeneity, however, may offset trophic instability at the basin scale. Assessments of resource partitioning provided little indication for competition between Arrowtooth Flounder and Pacific Halibut of similar lengths. Morphological differences between the two flatfish predators prompted an exploration into whether our conclusions about resource partitioning were dependent upon the size metric used. From this study, we found a relatively early onset of piscivory for Arrowtooth Flounder. Relationships between predator size and prey size also suggested gape limitation among Pacific Halibut sampled. Trophic niche separation was more pronounced for fishes with larger gapes, indicating greater potential for competition among smaller Arrowtooth Flounder and Pacific Halibut in Southeast Alaska. Reexamining basin-scale relationships between spatial and dietary overlap according to gape size would further elucidate the effects an increasing Arrowtooth Flounder population has had on changes in Pacific Halibut size-at-age. Results from this dissertation improve our understanding about the impacts of complex ecological interactions on population and community dynamics, and how those interactions may change in time, space, and under different environmental conditions.
Multi-scale movement of demersal fishes in AlaskaInformation on the movement of migratory demersal fishes such as Pacific halibut, Pacific cod, and sablefish is needed for management of these valuable fisheries in Alaska, yet available methods such as conventional tagging are too coarse to provide detailed information on migration characteristics. In this dissertation, I present methods for characterizing seasonal and annual demersal fish movement at multiple scales in space and time using electronic archival and acoustic tags. In Chapter 1, acoustic telemetry and the Net Squared Displacement statistic were used to identify and characterize small-scale movement of adult female Pacific halibut during summer foraging in a Marine Protected Area (MPA). The dominant movement pattern was home range behavior at spatial scales of less than 1 km, but a more dispersive behavioral state was also observed. In Chapter 2, Pop-up Satellite Archival Tags (PSATs) and acoustic tags were deployed on adult female Pacific halibut to determine annual movement patterns relative to MPA boundaries. Based on observations of summer home range behavior, high rates of year-round MPA residency, migration timing that largely coincided with winter commercial fisheries closures, and the demonstrated ability of migratory fish to return to previously occupied summer foraging areas, the MPA is likely to be effective for protecting both resident and migrant Pacific halibut brood stock year-round. In Chapter 3, I adapted a Hidden Markov Model (HMM) originally developed for geolocation of Atlantic cod in the North Sea for use on demersal fishes in Alaska, where maximum daily depth is the most informative and reliable geolocation variable. Because depth is considerably more heterogeneous in many regions of Alaska compared to the North Sea, I used simulated trajectories to determine that the degree of bathymetry heterogeneity affected model performance for different combinations of likelihood specification methods and model grid sizes. In Chapter 4, I added a new geolocation variable, geomagnetic data, to the HMM in a small-scale case study. The results suggest that the addition of geomagnetic data could increase model performance over depth alone, but more research is needed to continue validation of the method over larger areas in Alaska. In general, the HMM is a flexible tool for characterizing movement at multiple spatial scales and its use is likely to enrich our knowledge about migratory demersal fish movement in Alaska. The methods developed in this dissertation can provide valuable insights into demersal fish spatial dynamics that will benefit fisheries management activities such as stock delineation, stock assessment, and design of space-time closures.
Using fishers' knowledge to explore spatial fishing patterns, perceptions of regulations, and environmental change in AlaskaIn this dissertation, an interdisciplinary approach was used to examine fisher knowledge from recreational charter and subsistence fishers targeting Pacific halibut (Hippoglossus stenolepis) in Alaska. The first chapter identified biological, regulatory, social, and economic drivers of spatial fishing patterns by charter operators in two communities in Alaska. In Homer, the most frequently cited reasons for changes in the location and/or extent of fishing were changes in trip type and the price of fuel, while in Sitka, the most frequently cited reasons for spatial shifts were changes to Pacific halibut regulations and gaining experience or exploring new locations. The second chapter examined perceptions of charter operators to traditional and novel recreational fishery management tools. Results highlighted that controls on individual harvest can be perceived to have unintended consequences for charter businesses, such as effects on profitability and distance traveled. The third chapter explored variability in local ecological knowledge (LEK) of fish abundance and body size trends among charter operators and subsistence harvesters. Results suggested that peoples' perceptions of fish abundance and body size can be affected by attributes of their fishing experience and highlighted the importance of including people with different types of experience in the environment when using LEK to document environmental changes. Together, these chapters contribute to an improved understanding of the human dimensions of small-scale fisheries in Alaska, including perceptions of fishers regarding the management system and shifts in fishing behavior in response to environmental, socioeconomic, and regulatory change. Additionally, this project documented and evaluated variation in local ecological knowledge to contribute new information on data-limited marine fish species in Alaska.