School of Fisheries and Ocean Sciences

Acoustic data and net samples were collected duringlate springand early fall 1997-1999 to assess zooplankton and micronekton abundance and distribution relative to the Inner Front at three samplingg rids (Port Moller, Cape Newenham and Nunivak Island) on the inner shelf of the southeast BeringSea. Epibenthic scatteringlayers were observed duringMay-June and August-September in all three years. Acoustic data were scaled to

The eastern Bering Sea shelf experienced a sequence of warm years after a regime shift in the late 1970s. Following a series of unusually warm years in the early 2000s, the climate shifted again in 2007 to a series of extremely cold years that were marked by intense ice coverage and late ice retreat. Spatial and temporal variability in zooplankton communities during the recent cold period was investigated as part of the collaborative BEST-BSIERP program. An increasing presence of the Arctic hyperiid Themisto libellula, which had not been reported from the southeastern Bering Sea since the 1970s, was observed in the Middle Shelf Domain, indicating a developing structural shift in the zooplankton community in response to continuous cold conditions. Simultaneously, T. libellula became an increasingly dominant prey in the diets of zooplanktivorous fish and seabirds, demonstrating the important role for T. libellula in the pelagic food web. Our analysis suggests that T. libellula is capable of controlling copepod populations, thus it may become a potential contributor to top-down regulation of Calanus spp. in the eastern Bering Sea.

Spatial distribution, egg production and growth of the predominant euphausiids in the vicinity of the Pribilof Islands, Bering Sea, were studied during August 2004. Thysanoessa longipes was abundant in the oceanic water, aggregations of Thysanoessa inermis were observed both in the oceanic region and on the shelf near the Pribilofs, while Thysanoessa raschii was found near the islands. Euphausia pacifica and Thysanoessa spinifera occurred in much smaller quantities in the oceanic water. Euphausiid abundances on the middle shelf were several orders of magnitude lower than in the cold summer of 1999. The instantaneous growth rate (IGR) technique was applied to measure somatic growth. Juvenile T. inermis showed the most intensive growth (∼8% of dry body weight per day). In contrast, the somatic growth of mature specimens of both T. inermis and T. raschii was slow (<1% of dry body weight per day) suggesting the investment of resources into seasonal lipid deposition and reproduction. Egg release corresponded to an average of 1.1% of female dry body weight per day for T. raschii, indicating that enough resources were available for the animals to maintain reproductive activity. Our data suggest that the area near the Pribilofs, known for its high biological productivity, may provide a refuge for the shelf euphausiid population during warmer summers, when conditions on the shelf are unfavorable for their growth and reproduction.