Winter mortality has been documented in a large number of freshwater fish populations, and a smaller, but increasing, number of marine and estuarine fishes. The impacted populations include a number of important North American and European resource species, yet the sources of winter mortality remain unidentified in most populations where it has been documented. Among the potential sources, thermal stress and starvation have received the most research attention. Other sources including predation and pathogens have significant impacts but have received insufficient attention to date. Designs of more recent laboratory experiments have reflected recognition of the potential for interactions among these co-occurring stressors.



Geographic patterns in winter mortality are, in some cases, linked to latitudinal clines in winter severity and variability. However, for many freshwater species in particular, the effects of local community structure (predators and prey) may overwhelm latitudinal patterns. Marine (and estuarine) systems differ from freshwater systems in several aspects important to overwintering fishes, the most important being the lack of isolating barriers in the ocean. While open population boundaries allow fish to adopt migration strategies minimizing exposure to thermal stresses, they may retard rates of evolution to local environments. Geographic patterns in the occurrence and causes of winter mortality are ultimately determined by the interaction of regional and local factors.



Winter mortality impacts population dynamics through episodic depressions in stock size and regulation of annual cohort strength. While the former tends to act in a density-independent manner, the latter can be density dependent, as most sources of mortality tend to select against the smallest members of the cohort and population. Most stock assessment and management regimes have yet to explicitly incorporate the variability in winter mortality. Potential management responses include postponement of cohort evaluation (to after first winter of life), harvest restrictions following mortality events and habitat enhancement.



Future research should place more emphasis on the ecological aspects of winter mortality including the influences of food-web structure on starvation and predation. Beyond illuminating an understudied life-history phase, studies of overwintering ecology are integral to contemporary issues in fisheries ecology including ecosystem management, habitat evaluation, and impacts of climate change.

Causes and consequences of winter mortality in fishes

. Under modern conditions only North Pacific Intermediate Water is formed in the northwest Pacific Ocean. This situation might have changed in the past. Recent studies with general circulation models indicate a switch to deep-water formation in the northwest Pacific during Heinrich Stadial 1 (17.5–15.0 ka) of the last glacial termination. Reconstructions of past ventilation changes based on paleoceanographic proxy records are still insufficient to test whether a deglacial mode of deep-water formation in the North Pacific Ocean existed. Here we present deglacial ventilation records based on radiocarbon-derived ventilation ages in combination with epibenthic stable carbon isotopes from the northwest Pacific including the Okhotsk Sea and Bering Sea, the two potential source regions for past North Pacific ventilation changes. Evidence for most rigorous ventilation of the intermediate-depth North Pacific occurred during Heinrich Stadial 1 and the Younger Dryas, simultaneous to significant reductions in Atlantic Meridional Overturning Circulation. Concurrent changes in δ13C and ventilation ages point to the Okhotsk Sea as driver of millennial-scale changes in North Pacific Intermediate Water ventilation during the last deglaciation. Our records additionally indicate that changes in the δ13C intermediate-water (700–1750 m water depth) signature and radiocarbon-derived ventilation ages are in antiphase to those of the deep North Pacific Ocean (>2100 m water depth) during the last glacial termination. Thus, intermediate- and deep-water masses of the northwest Pacific have a differing ventilation history during the last deglaciation.

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Pulses of enhanced North Pacific Intermediate Water ventilation from the Okhotsk Sea and Bering Sea during the last deglaciation

Seven piston cores were obtained from the Bering Sea and the western subarctic Pacific during Cruise KH99-3 of R.V. Hakuho-maru in August 1999: Cores BOW-8A, BOW-9A, BOW-12A, AB, UMK-3A, GAT-3A in the Bering Sea, and Core ES in the western subarctic Pacific. The quantitative high-resolution analyses of radiolarians in the cores revealed significant variations in both their faunal assemblages and accumulation rates (AR). A trend of conspicuous turn-over of the surface dwellers and intermediate dwellers (increase of one at expense of the other) after the last glacial maximum (LGM) is clearly visible in the Bering Sea data set. However, this turn-over is not present at Site ES in the western subarctic Pacific core. These changes in radiolarian assemblages around the LGM suggest that the surface waters had extremely low temperature and low salinity and that the subsurface dichothermal layer (temperature minimum layer) was thickened all the way across the entire east–west span of the Bering Sea. Furthermore, sea-ice coverage extended to the central Aleutian Basin around the LGM. This suggests that the water-mass structure in the western subarctic Pacific around the LGM was basically the same as that of the eastern Bering Sea based on the species composition of the surface dwellers. However, the detailed development of intermediate water masses differed from that of the eastern Bering Sea. During Marine Isotope Stage (MIS) 5 to 3, such species turn-over events did not occur and species abundance stayed relatively uniform in the studied cores. Therefore, the present water-mass structure is interpreted to have been formed after the LGM. Based on the relative abundances of Cycladophora davisiana (the most dominant species dwelling in the intermediate water, and a useful tracer of cold and well-oxygenated intermediate-water in the North Pacific), the past North Pacific Intermediate Water (NPIW) had the following source regions during the last 100 kyr: (1) both the Okhotsk Sea and the Bering Sea during MIS 5 to 3; (2) the Bering Sea around the LGM; and (3) shifting from the Bering Sea to the Okhotsk Sea after the LGM. Relatively low radiolarian AR at Site BOW-8A, which is the shallowest site studied here (present water depth: 884 m), were observed during the glacial periods. This is due to decrease of radiolarian production in the thinned upper water layer above the dicothermal layer caused by sea-level drop. Cycladophora davisiana probably lived deeper during the glacials than today, as it was absent in the shallow piston core. r 2005 Elsevier Ltd. All rights reserved.

Late Quaternary paleoceanographic changes in the Bering Sea and the western subarctic Pacific based on radiolarian assemblages

Identification of habitat used for skate egg deposition has been rarely studied or reported worldwide. Four nursery sites for the Alaska skate Bathyraja parmifera, 2 for the Aleutian skate B. aleutica and 2 for the Bering skate B. interrupta were identified along the upper continental slope in the eastern Bering Sea. All sites were located near undersea canyons from 145 to 380 m depth in relatively flat sandy to muddy bottom habitat. Bottom temperatures were relatively constant throughout the year, varying from 3.7 to 4.6°C. Egg case densities varied between nursery sites and were encountered at the Alaska skate nursery in Bering Canyon at densities greater than 800 000 eggs km -2 . Based on egg case composition, sites were predominantly used by a single skate species for egg deposition; however, up to 6 skate species used the habitat commonly. Seasonal sam- pling indicated that sites were continuously occupied throughout the year, and embryo length com- position showed multiple cohorts developing simultaneously. Data from bottom trawl surveys suggest juvenile skates occupy habitats different than nursery sites. The movement of juvenile skates out of nursery habitat after hatching may lessen predation by common predators such as the Pacific cod Gadus macrocephalus and the Pacific halibut Hippoglossus stenolepis.

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Identification of skate nursery habitat in the eastern Bering Sea

Changes in the flux and location of overturning circulation may have large effects on marine ecosystems and CO 2 exchange between the ocean and atmosphere. However, unlike the Atlantic, little is known about ocean circulation and ventilation under glacial boundary conditions in the North Pacific, especially in regard to intermediate water circulation. Here we present new records of neodymium (Nd) isotopes (e Nd ) in Fe-Mn oxyhydroxides of Bering Sea sediments (884 m water depth). We found a systematic variation between radiogenic values (up to +0.8 e Nd ) during cold periods and relatively less radiogenic values ( Nd ) during warm periods. There are no water masses with such radiogenic Nd isotope signatures in the intermediate or deep North Pacific. Potential sources of radiogenic values in the subarctic North Pacific are limited to surface waters adjacent to the Aleutian Arc and Kamchatka Peninsula. Therefore, the radiogenic e Nd values of Fe-Mn oxyhydroxides observed at the intermediate depth during glacial periods are best explained by subduction of the surface water to the intermediate depth (at least ∼800 m) due to brine rejection. Our data strongly indicate that the northwestern Bering Sea (off northeastern Kamchatka) was a possible source region of glacial intermediate water in the Bering Sea and the subarctic North Pacific.

Intermediate water formation in the Bering Sea during glacial periods: Evidence from neodymium isotope ratios

Data from 35,700 hydrographic stations were grouped in areas of 1° latitude and 2° longitude and monthly, seasonal, and annual means calculated. We examined temperature, salinity, and water masses of the Bering Sea, studying their vertical structure, temporal variability, and features of their spatial and temporal distribution. We also considered data from coastal observations, information on the meteorological regime, and river outflow.

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Vladimir A. Menovshchikov

Papers

Thermohaline Structure and Water Masses in the Bering Sea