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| − | A better understanding of habitat requirements is mandated by federal fisheries law. One way to approach this issue is to examine factors that influence distribution. We are investigating how distribution changes with abundance, which is of particular interest in exploited populations that undergo large changes in abundance. Three theories have been proposed for the relationship between marine fish abundance and distribution: the Constant Density Model (CDM), the Proportional Density Model (PDM), and MacCall’s Basin Model (MBM) which incorporates the Ideal Free Distribution model of Fretwell and Lucas. The CDM predicts that the population’s range expands and contracts with changes in global abundance while local density does not vary. Habitat quality varies and the best habitats, conferring the highest fitness, remain occupied at lowest abundance. The PDM predicts that range remains constant and that local density varies with changes in abundance. Local density is highest in habitats that confer the highest fitness. The MBM predicts that there is density-dependent habitat use, such that both local density and species range vary with changes in abundance. Fitness should be the same across habitats because of these density-dependent processes.
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| − | We are testing these theories using geospatial modeling and National Marine Fisheries Service trawl survey data. We test for changes in range by mapping non-zero catches at high and low population levels. We test for fitness changes across habitats by assessing spatial autocorrelation in fitness metrics (weight-at-length, weight-at-age). Results from analysis of yellowtail flounder (Limanda ferruginea) show evidence of spatial autocorrelation for both males and females except during spring spawning season when populations are low. Data from both the spring and fall survey shows that the area occupied by the species increases with increasing population size. While there are other predictions made by these models which we have not yet tested, these results support the constant density model.
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