Ecology of Greenland halibut (Reinhardtius hippoglossoides) during the early life stages in the Eastern Bering Sea and Aleutian Islands Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s4655k011

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  • Greenland halibut (Reinhardtius hippoglossoides) have decreased in the eastern Bering Sea and Aleutian Islands since the 1970s. The reasons behind the poor recruitment are unknown, and important knowledge gaps of the ecology of the early life stages have been identified. The objectives of thesis research were (1) to examine geographic distributions and drift pathways from egg to settled juveniles (age-1) by a retrospective analysis in order to understand connectivity and different habitat utilization through ontogeny, (2) to describe vertical egg distribution using a one-dimensional model in order to understand patterns of vertical egg distribution and their association with water column hydrography and currents through the egg developmental stages, (3) to investigate diet of Greenland halibut larvae and pelagic juveniles diet using stomach content in order to understand foraging behavior, and (4) to examine otolith microstructure of Greenland halibut larvae and pelagic juveniles using otolith analysis in order to establish aging guidelines and other features related to bio-ecological changes (e.g. hatching, mouth opening etc.). The results of the thesis indicate that Greenland halibut early life stages from eggs to newly settled juveniles have a long duration in the plankton and are subject to extended drift pathways. In the eastern Bering Sea and Aleutian Islands, Greenland halibut spawn along the continental slope between Akun Island and the Pribilof Islands, and between Umnak Island and Akun Island in the Aleutian Islands. Spawning most likely occurs in winter from December to January. At-sea observations of egg density ranged from 1023.5 kg/m³ to 1027.8 kg/m³. Based on modeling results, variability of egg density would affect vertical egg distributions. Eggs were found between 50 m and 400 m, in particular, most eggs were concentrated between 200 m and 300 m. Hatching probably occurs in deep water, below 500 m, and larvae rapidly rise in the water column to depths above 45 m. The mechanisms of slope-shelf connectivity appear to be related to the Bering Slope Current in the vicinity of both Bering and Pribilof Canyons. Some larvae cross to the continental shelf from March to May through Bering Canyon, while others are transported through Pribilof Canyon. Pelagic juveniles are mostly found around the Pribilof Islands over the middle shelf (50 m - 100 m isobaths) in July, and settling likely occurs during late summer on the middle shelf near St. Matthew Island. However, given that age-1 juveniles were primarily found on the outer shelf, juvenile Greenland halibut appear to actively move to deeper water, presumably selecting appropriate habitat based on a suite of environmental factors after settlement. Greenland halibut larvae and pelagic juveniles are vigorous carnivores and their feeding habits change as they grow. The clearly defined rings observed in the otoliths might be biological transition marks. This study shows that Greenland halibut have extensive horizontal ontogenetic migrations in the Bering Sea, and utilize a range of geographic areas over the Bering slope and shelf at different life history stages. Based on these results, it is hypothesized that settlement success and recruitment of Greenland halibut may be influenced by variability in currents and flows of the Bering Sea slope and shelf during their transport and by size variations of suitable nursery locations.
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