| Summary: | Atka mackerel Pleurogrammus monopterygius make up the single largest biomass of groundfish in the Aleutian Islands, Alaska, and are an important component of this marine ecosystem. Atka mackerel show a significant decrease in size from east to west. We compared fish from two study areas reflecting this size cline: Seguam Pass in the eastern Aleutians and Amchitka Island in the western Aleutians. At any given age, the Atka mackerel at the former location are larger than those at the latter (e.g., 5-year-old fish average 732 g and 39 cm fork length at Seguam Pass but only 575 g and 36 cm at Amchitka Island). Our objectives were to determine the mechanisms underlying the observed differences in growth, such as prey availability, prey quality, and thermal experience. We used a bioenergetics model to examine the effects of diet and temperature on growth. The model estimates of consumption fell within the range of those observed for Atka mackerel, suggesting that the model was an appropriate tool for exploring these effects. The results obtained with the model suggest that prey quality is the main factor in the observed size differences. At Seguam Pass Atka mackerel ate a more energetically rich diet consisting of euphausiids and fish, whereas at Amchitka Island copepods dominated the diet and there was little to no fish consumption. The model results also suggest that thermal experience contributed less to the observed differences in growth than did the composition of the diet. Understanding the mechanisms underlying the growth of Atka mackerel will improve predictions of biomass and yield within the framework of a dynamic ecosystem and a changing climate.
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