Graduate Thesis Or Dissertation
 

The Factors Affecting the Diets of Groundfish in the Gulf of Alaska : Ecological and Modeling Considerations

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  • Marine systems undergo changes in community composition over time as a result of a variety of environmental and anthropogenic factors. Understanding these community changes and the factors that drive them is critical for ecosystem management of marine resources. The Gulf of Alaska (GOA) is a large marine ecosystem that includes a variety of species that support large scale fisheries. This is also a system in which large scale community shifts, or regime shifts, have occurred due to the Pacific Decadal Oscillation (PDO) and fishing pressure. Given the economic and conservation importance of this marine system, the GOA has been modeled using a variety of multispecies, or ecosystem, models. While this work has been critical in understanding the ecosystem dynamics of the GOA and helped generate management recommendations for commercial species, these models often make assumptions regarding trophic-dynamics, particularly that predator-prey relationships follow a standard functional response and do not change through time in response to environmental variables. However, empirical evidence suggests that a predator's diet can be influenced by a variety of factors, abiotic and biotic, at large and small spatial scales. Our overall objective was to investigate the potential impact environmental variables may have in structuring this ecosystem by using statistical analyses of diets and an ecosystem modeling framework. We focused on three commercially and ecologically important groundfish predators: Pacific Cod (Gadus macrocephalus), Pacific Halibut (Hippoglossus stenolepis), and Sablefish (Anoplopoma fimbria). We also focused on a key prey species, Walleye Pollock (Gadus chalcogrammus), and used environmental data collected during trawl surveys and PDO data generated for the entire North Pacific region. The first study completed was focused solely on the consumption of Walleye Pollock, a critical fishery species and ecosystem link. We used data from trawl surveys to determine the potential influence of local environmental variables on the predation rate of Walleye Pollock in the system by these groundfish. Using an information-theoretic approach, we found that predator length was positively related to Walleye Pollock presence and proportion of total diet weight in all predators. Increased temperatures positively affected consumption of Walleye Pollock by Pacific Halibut, but not the other predators. We found evidence for a number of inter-predator effects of co-occurring predators, both positive (facultative) and negative (competitive). Observed prey density was not statistically significant with respect to consumption for these predators, suggesting that trawls sample the environment differently than Walleye Pollock predators or species interactions are more complex than those used in previous multispecies models. In our second study, we considered the entire diet of these predators, rather than one key prey. Furthermore, because PDO had been described as leading to community changes in the system previously, we hypothesized that it could also be driving shifts in diets in the three groundfish predators we studied. We used a multivariate statistical approach to compare the diets of these predators by PDO state (warm or cold years) and also included local environmental covariates. Overall, we found that diets observed in PDO cold years were significantly different than diets in fish in warm years. In general, predators were found to be consuming more Walleye Pollock and euphausiids in warm years, and more benthic invertebrates and other forage fish in cold years. Local environmental covariates contributed to the diets observed in these predators, however no general pattern was observed. Our results also show the benefit of using diet data from large scale monitoring efforts as indicators of community shifts in a large marine ecosystem through time. Ultimately, we used our statistical analyses regarding diet and PDO state to drive a modeling exercise using alternate representations of the diets of the predators in the GOA. We investigated the potential impact of shifting diets in groundfish in an ecosystem-modeling framework, using Ecopath trophic-mass balance models. We changed the diets of key groundfish predators in the model based on our previous results for three alternative model parametrizations; 1) average conditions over the time period, 2) cold PDO state, and 3) warm PDO state. We noted a number of differences in model estimated ecosystem indices. Biomass accumulation estimates indicated that some ecologically and commercially important species groups would be expected to greatly diverge in population size if models were based on data from warm vs. cold PDO years compared to the averaged climatic state. In general, predator overlap was at its lowest in the cold year model, as predators had more diverse diets and therefore predation was more diffuse in the system in general. These results indicate the potential importance of environmental context when collecting diet data to be used in ecosystem models designed to provide fishery management recommendations. As ecosystem models are used more commonly, taking the time to investigate the factors that structure diets and how predation changes due to environment can yield more representative, and potentially more accurate biomass projections and recommendations for the GOA and likely many other managed marine ecosystems.
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