- Ampelisca eschrichtii are among the most important prey of the Western North Pacific gray whales, Eschrichtius robustus. The largest and densest known populations of this amphipod occur in the gray whale’s Offshore feeding area on the Northeastern Sakhalin Island Shelf. The remote location, ice cover and stormy weather at the Offshore area have prevented winter sampling. The incomplete annual sampling has confounded efforts to resolve life history and production of A. eschrichtii. Expanded comparisons of population size structure and individual reproductive development between late spring and early fall over six sampling years between 2002 and 2013 however, reveal that A. eschrichtii are gonochoristic, iteroparous, mature at body lengths greater than 15 mm and have a two-year life span. The low frequencies of brooding females, the lack of early stage juveniles, the lack of individual or population growth or biomass increases over late spring and summer, all indicate that growth and reproduction occur primarily in winter, when sampling does not occur. Distinct juvenile and adult size cohorts additionally indicate growth and juvenile production occurs in winter through spring under ice cover. Winter growth thus requires that winter detritus or primary production are critical food sources for these ampeliscid populations and yet, the Offshore area and the Eastern Sakhalin Shelf ampeliscid communities may be the most abundant and productive amphipod population in the world. These A. eschrichtii populations are unlikely to be limited by western gray whale predation. Whether benthic community structure can limit access and foraging success of western gray whales is unclear.
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- This research was initiated by Dr. V.I. Fadeev† (24.10.1952-21.06.2014) - Head of the Laboratory of the Ecology of Shelf Communities of the A.V. Zhirmunsky Institute of Marine Biology, IMB. Dr. V.I. Fadeev participated in most expeditions to the Sakhalin Shelf and sampling of the amphipod material for this report, read and approved of an early draft of this manuscript but passed away before the final version was complete. The surviving authors (NLD, JWC and VBD) accept all responsibility for the integrity and validity of the data collected and analyzed. We thank E.P. Dats (Vladivostok State University of Economics and Service, VSUES) for assistance with software, editing formulas and calculations and support at all stages as a friend and a colleague. We are grateful to L.N. Khorol'skaya (IMB) for her assistance in measuring amphipods. We thank Dr. L.L. Budnikova and R.G. Bezrukov (TINRO-Center) for June 2002 collection material on A. eschrichtii from the Offshore area. We thank Dr. V.V. Ivin for help with statistical analysis in software PRIMER (IMB). We thank Dr. M.S. Kornienko for the gray whale photo included in our graphical abstract. We thank Prof. A.I. Buyanovsky (VNIRO) for critical reading and discussion of the results and Dr. Dave Rugh (National Marine Mammal Laboratory, NOAA, Seattle, Washington) for critically reviewing and assisting with our summaries of EGWs. We thank Dr. Vladimir Efremov (Exxon Neftegas Limited) for his encouragement and review of the manuscript. We also thank Dr. James Sumich (Hatfield Marine Science Center, Newport, Oregon), the PLOS ONE editor and an anonymous reviewer for their timely, helpful reviews.