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1.
Concern about impacts of climate change in the Bering Sea prompted several research programs to elucidate mechanistic links between climate and ecosystem responses. Following a detailed literature review, Hunt et al. (2011) (Deep‐Sea Res. II, 49, 2002, 5821) developed a conceptual framework, the Oscillating Control Hypothesis (OCH), linking climate‐related changes in physical oceanographic conditions to stock recruitment using walleye pollock (Theragra chalcogramma) as a model. The OCH conceptual model treats zooplankton as a single box, with reduced zooplankton production during cold conditions, producing bottom‐up control of apex predators and elevated zooplankton production during warm periods leading to top‐down control by apex predators. A recent warming trend followed by rapid cooling on the Bering Sea shelf permitted testing of the OCH. During warm years (2003–06), euphausiid and Calanus marshallae populations declined, post‐larval pollock diets shifted from a mixture of large zooplankton and small copepods to almost exclusively small copepods, and juvenile pollock dominated the diets of large predators. With cooling from 2006–09, populations of large zooplankton increased, post‐larval pollock consumed greater proportions of C. marshallae and other large zooplankton, and juvenile pollock virtually disappeared from the diets of large pollock and salmon. These shifts in energy flow were accompanied by large declines in pollock stocks attributed to poor recruitment between 2001 and 2005. Observations presented here indicate the need for revision of the OCH to account for shifts in energy flow through differing food‐web pathways due to warming and cooling on the southeastern Bering Sea shelf.  相似文献   

2.
Juvenile marine growth (SW1) of salmon and a new temperature change (TC) index were evaluated as ecosystem indicators and predictors for the post age‐0 year class strength (YCS) of groundfish in the Gulf of Alaska (GOA) and eastern Bering Sea (EBS). Our hypothesis was that SW1, as measured on the scales of adult Pacific salmon (Oncorhynchus spp.), is a proxy for ocean productivity on the continental shelf, a rearing area for young salmon and groundfish. Less negative TC index values are the result of a cool late summer followed by a warm spring, conditions favorable for groundfish YCS. In the GOA, SW1 was a positive predictor of age‐1 pollock (Theragra chalcogramma), but not age‐2 sablefish (Anoplopoma fimbria) YCS, indicating that the growth of the Karluk River sockeye salmon that enter Shelikof Strait is a proxy for ocean conditions experienced by age‐0 pollock. Contrary to our hypotheses, the TC index was a negative predictor of GOA pollock YCS; and the SW1 a negative predictor of EBS pollock and cod YCS since the 1980s. Recent fisheries oceanography survey results provide insight into possible mechanisms to support the inverse SW1 and YCS relationship. For the EBS, the TC index was a significant positive predictor for pollock and cod YCS, supporting the hypothesis that a cool late summer followed by a warm spring maximizes the over‐wintering survival of pollock and cod (Gadus macrocephalus), especially since the 1980s. The TC and SW1 index showed value for the assessment of pollock and cod, but not sablefish.  相似文献   

3.
Ecosystem‐based fisheries management requires the development of physical and biological time series that index ocean productivity for stock assessment and recruitment forecasts for commercially important species. As recruitment in marine fish is related to ocean condition, we developed proxies for ocean conditions based on sea surface temperature (SST) and biometric measurements of chum salmon (Oncorhynchus keta) captured in the walleye pollock (Gadus chalcogrammus) fishery in the eastern Bering Sea in three periods (July 16–30, September 1–15 and September 16–30). The main purpose of this paper was to evaluate Pacific salmon (Oncorhynchus spp.) growth as a possible indicator of ocean conditions that, in turn, may affect age‐1 walleye pollock recruitment. Marine growth rates of Pacific salmon are the result of a complex interplay of physical, biological and population‐based factors that fish experience as they range through oceanic habitats. These growth rates can, therefore, be viewed as indicators of recent ocean productivity. Thus, our hypothesis was that estimated intra‐annual growth in body weight of immature and maturing age‐4 male and female chum salmon may be used as a biological indicator of variations in rearing conditions also experienced by age‐0 walleye pollock; consequently, they may be used to predict the recruitment to age‐1 in walleye pollock. Summer SSTs and chum salmon growth at the end of July and September explained the largest amount of variability in walleye pollock recruitment indicating that physical and biological indices of ocean productivity can index fish recruitment.  相似文献   

4.
Walleye pollock (Gadus chalcogrammus) supports one of the largest commercial fisheries in the world. Juvenile pollock are important forage fish in the eastern Bering Sea (EBS) ecosystem, often representing the largest fraction in the diets of major Bering Sea piscivores. Large variability in the EBS pollock stock biomass in recent years has been attributed primarily to fluctuations in recruitment. It has been hypothesized that predation rates on forage fishes increase when the cold pool (a body of cold water < 2°C) is extensive and covers much of the middle continental shelf, which tends to concentrate larger predatory fishes in the outer shelf and slope regions. In contrast, young pollock appear to tolerate colder temperatures than older fish and can stay in the cold pool, thereby reducing predation. We used a multispecies modeling approach to examine the effects of the cold pool size on predation of juvenile pollock. We found that predation on age‐1 pollock by age‐3+ pollock decreased, and predation on age‐1 and age‐2 pollock by arrowtooth flounder increased with increasing bottom temperature, which was used as a proxy for the cold pool size. These results suggest that the cold pool creates spatial separation between juvenile pollock and arrowtooth flounder, but not between adult and juvenile pollock. The model developed in this study could be used to examine the effects of other covariates on interspecific interactions, help explain observed changes in fish communities, and understand implications of climate change on ecosystems and their productivity.  相似文献   

5.
Populations of several species of marine birds and mammals in the Bering Sea and Gulf of Alaska have been declining since the mid-1970s, with numbers of one, the Steller sea lion (Eumetopias jubatus) , so depressed it was listed as threatened under the Endangered Species Act in spring 1990. All of the declining populations depend to an important extent on walleye pollock (Theragra chakogramma) for food, although they eat numerous other species as well. In contrast, certain animals that compete with pollock for common prey have been increasing in abundance. All of these changes could be related through food web connections mediated by pollock. Pollock is also important to people–it presently supports the largest single-species commercial fishery in the world, in large part because of its great biomass, which has averaged about 15 × 106t in the Bering Sea over the past 15 years. Pollock consume an inordinate proportion of the pelagic production in the Bering Sea, which further supports the conclusion that it is a key species in the ecosystem. However, there are conflicting hypotheses about the importance of the roles played by pollock as predator and prey, and about the effect that changes in pollock abundance might have on biomass yield at higher trophic levels.  相似文献   

6.
In 2003, the Alaska walleye pollock industry reported product quality issues attributed to an unspecified parasite in fish muscle. Using molecular and histological methods, we identified the parasite in Bering Sea pollock as Ichthyophonus. Infected pollock were identified throughout the study area, and prevalence was greater in adults than in juveniles. This study not only provides the first documented report of Ichthyophonus in any fish species captured in the Bering Sea, but also reveals that the parasite has been present in this region for nearly 20 years and is not a recent introduction. Sequence analysis of 18S rDNA from Ichthyophonus in pollock revealed that consensus sequences were identical to published parasite sequences from Pacific herring and Yukon River Chinook salmon. Results from this study suggest potential for Ichthyophonus exposures from infected pollock via two trophic pathways; feeding on whole fish as prey and scavenging on industry‐discharged offal. Considering the notable Ichthyophonus levels in pollock, the low host specificity of the parasite and the role of this host as a central prey item in the Bering Sea, pollock likely serve as a key Ichthyophonus reservoir for other susceptible hosts in the North Pacific.  相似文献   

7.
Interannual variability in growth of walleye pollock, Theragra chalcogramma, was examined. Adult walleye pollock were collected from the central Bering Sea (Aleutian Basin) from 1978 to 1999. Average fork lengths were found to be approximately 47 cm during the 1970–80s, this increased to 56 cm in the late 1990s. Age was determined for 4805 individuals using the otolith break and burn method. Ages ranged from 5–23 years and the year classes of 1978 and 1989 were dominant in the 1980s and the 1990s, respectively. Fish had significantly larger length-at-age in the 1990s compared to the 1970–80s, and interannual variability in age–length relationship was clearly observed. Taking into consideration a recent decrease of the walleye pollock biomass in the central Bering Sea, density-dependent growth was supported as one possibility of the growth variability. At the same time, we could not rule out the possibility that oceanographic variability affected the growth of walleye pollock in the area.  相似文献   

8.
I examined the age, growth, maturity, mortality, and body condition of walleye pollock, Theragra chalcogramma, in the northeastern Japan Sea (northern Japan Sea population) and evaluated their resilience to exploitation. Walleye pollock were collected in pre-spawning (October 1991-1995) and post-spawning (April 1990-1996) seasons. Estimated ages ranged from 3 to 18 years for both sexes. A von Bertalanffy growth model showed that females had longer asymptotic fork length (460 mm) than males (425 mm). Fifty percent of females and males were mature at 348 mm (4.6 years) and 322 mm (3.9 years), respectively. The instantaneous natural mortality rate was estimated to be 0.22. These life-history traits in the northern Japan Sea population were compared to those in the Bering Sea, the Gulf of Alaska, and the Japan Pacific populations. As a result, female walleye pollock in this population matured at small body sizes, grew rapidly toward small maximum sizes, and had short reproductive lifespans with low size-specific fecundity and poor body condition. Low prey availability and habitat temperatures are considered as a possible mechanism for the small maximum sizes in this population. The potential rate of population increase of both the northern Japan Sea population and other pollock populations tended to be lower than other exploited populations of non-viviparous marine fishes, suggesting potentially lower resilience to exploitation in this population and walleye pollock populations in general.  相似文献   

9.
Acoustic trawl surveys were conducted in 2000 and 2001 in two troughs located off the eastern coast of Kodiak Island in the Gulf of Alaska as part of a multiyear, multidisciplinary experiment to examine the influence of environmental conditions on the spatial distribution of adult and juvenile walleye pollock (Theragra chalcogramma) and capelin (Mallotus villosus). Continuous underway sea surface temperature samples and water column profiles collected in 2000 and 2001 showed the presence of a sharp shelf‐break front in Chiniak Trough and a mid‐trough front in Barnabas Trough. At distances <22 km from shore, the water column was well mixed, whereas a well‐defined mixed layer was present beyond approximately 22 km from shore. Satellite drifter tracks in Barnabas Trough entered along the upstream edge of the trough and appeared to follow the frontal boundary across the middle portion of the trough. A storm in 2001 weakened stratification and cooled surface water temperature by 1.6–2.1°C. Wind mixing associated with the storm event mixed subsurface chlorophyll a to the surface and enhanced nutrients in the surface waters. The storm event revealed spatial partitioning of summer production in Barnabas Trough, with production concentrated in regions inside the mid‐trough front. In contrast, post‐storm summer production was distributed throughout Chiniak Trough. The spatial distribution of walleye pollock and capelin differed and appeared to be related to differences in habitat characteristics. Acoustic survey data identified four acoustic sign types: age‐1 pollock, adult pollock, capelin, capelin–age‐0 pollock mix. The spatial distribution of these four sign types appears to be influenced by the oceanographic and topographic features of the two troughs. Adult pollock were broadly distributed throughout Chiniak Trough, whereas adult pollock were aggregated on the coastal side of the frontal system in Barnabas Trough. In 2000, capelin occurred with age‐0 pollock. In Chiniak Trough, capelin were most abundant along steep topographic gradients at the edges of the trough and in a deep region near Cape Chiniak, whereas the capelin–age‐0 mix (2000) or capelin (2001) concentrations were observed in slope water intrusions over the outer shelf in Barnabas Trough. Results suggest that habitat selection of walleye pollock and capelin are controlled by different processes. Capelin distributions appear to be limited by oceanographic conditions while other factors appear to be more important for pollock.  相似文献   

10.
Differences in zooplankton populations in relation to climate have been explored extensively on the southeastern Bering Sea shelf, specifically in relation to recruitment of the commercially important species walleye pollock (Gadus chalcogrammus). We addressed two research questions in this study: (i) Does the relative abundance of individual copepod species life history stages differ across warm and cold periods and (ii) Do estimated secondary production rates for copepods differ across warm and cold periods? For most copepod species, warmer conditions resulted in increased abundances in May, the opposite was observed in colder conditions. Abundances of smaller‐sized copepod species did not differ significantly between the warm and cold periods, whereas abundances of larger‐sized Calanus spp. increased during the cold period during July and September. Estimated secondary production rates in the warm period were highest in May for smaller‐sized copepods; production in the cold period was dominated by the larger‐sized Calanus spp. in July and September. We hypothesize that these observed patterns are a function of temperature‐driven changes in phenology combined with shifts in size‐based trophic relationships with primary producers. Based on this hypothesis, we present a conceptual model that builds upon the Oscillating Control Hypothesis to explain how variability in copepod production links to pollock variability. Specifically, fluctuations in spring sea‐ice drive regime‐dependent copepod production over the southeastern Bering Sea, but greatest impacts to upper trophic levels are driven by cascading July/September differences in copepod production.  相似文献   

11.
Eastern Bering Sea pollock have two distinctly different stable spawning grounds—along the shelf and in the eastern and central Aleutian Islands between 400 and 500 m water columns. Pollock spawning behavior supports the hypothesis that the shelf and deepwater “basin” spawning pollock are completely independent reproductive stocks. Deepwater pollock inhabit the shelf and, once mature at age 5–6 years, migrate from the shelf onto the continental slope into the Zhemchug, Pribilof, and Bering canyons by the end of winter. Bering Sea pollock recruitment and year class abundance have high annual variability, but there are no clear relationships between pollock year class strength and water temperature, ice distribution or survival on early ontogenesis stages (eggs and larvae). Young-of-the-year fish survival varies dramatically during winter supporting the hypothesis that the Bering Sea pollock recruitment and strength of year class have high annual variability depending on young-of-the-year fish survival during winter. The annual change of physical oceanography condition, productivity and species composition of zooplankton community are associated with great differences in pollock seasonal migrations and distribution, reproduction, survival of recruits at early stages of development and finally with abundance of year classes and total biomass. Implementation of ecosystem-based fishery management most important for application of pollock research both of Russian national program and on base of International Agreements.  相似文献   

12.
A study of common minke and Bryde's whales was conducted in the western North Pacific in the 2000 and 2001 summer seasons to estimate prey selection of cetaceans as this is an important parameter in ecosystem models. Whale sighting and sampling surveys and prey surveys using quantitative echosounder and mid‐water trawl were carried out concurrently in the study. Biomasses of Japanese anchovy, walleye pollock and krill, which were major prey species of common minke and Bryde's whales, were estimated using an echosounder. The results suggested that common minke whale showed prey selection for Japanese anchovy while they seemed to avoid krill in both the offshore and coastal regions and walleye pollock in the continental shelf region. Selection for shoaling pelagic fish was similar to that in the eastern North Atlantic. Bryde's whale showed selection for Japanese anchovy in August 2000 and July 2001, while it showed prey selection for krill in May and June in 2001.  相似文献   

13.
Between 1988 and 1993, 12 satellite-tracked buoys were deployed in four eddies in the south-eastern Bering Sea. Our success in finding eddies resulted from placing buoys in high concentrations of walleye pollock (Them-gra chalcogramma) larvae. We utilize data from hydro-graphic surveys, satellite-tracked buoys and moored current meters to describe the eddies. Small (< 25 km diameter) eddies likely transit along the slope of the eastern Bering Sea every 45–60 days. In previous studies such small features were not observed because their size fell within typical separation of hydrographic stations and the weak sea surface temperature gradients are not resolved by satellite-borne infrared imagery.  相似文献   

14.
Drift of propagules occurs within many populations inhabiting flow fields. This affects the number of propagules that rejoin their source population (recruitment) and plays a role in adaptive spatial redistribution. We focus on the cause and consequence of interannual variation in geographic distribution of population density among five cohorts of young‐of‐the‐year (age‐0) juvenile walleye pollock Gadus chalcogrammus in the western Gulf of Alaska (GOA). The coastal GOA is a wind‐driven advective system. Walleye pollock spawn during spring and their eggs and larvae drift southwestward; by late summer, age‐0 juveniles are variously distributed over the shelf. We found that high population densities of age‐0 juveniles (ca. 6 months old) near the southwestward exit of the Alaska Coastal Current from the GOA corresponded with high abundance of larvae from the major spawning area upstream, but did not translate into high abundance at older ages. Further, offshore and upwelling‐favorable winds were associated with the high downstream abundance and presumed export. In contrast, downwelling‐favorable (northeasterly) wind during and shortly after spawning (April–May) was associated with high recruitment at age 1. Finally, we found that recruitment also increased with apparent retention of age‐0 juveniles in favorable habitat upstream near the main spawning area. We hypothesize that wind‐related retention in superior upstream habitat favors recruitment. Our results argue for including wind‐driven transport in future walleye pollock recruitment models. We encourage more work on the juvenile stage of marine fishes aimed at understanding how transport and species‐specific habitat suitability interact to affect population response to large‐scale forcing.  相似文献   

15.
Accounting for variation in prey mortality and predator metabolic potential arising from spatial variation in consumption is an important task in ecology and resource management. However, there is no statistical method for processing stomach content data that accounts for fine‐scale spatio‐temporal structure while expanding individual stomach samples to population‐level estimates of predation. Therefore, we developed an approach that fits a spatio‐temporal model to both prey‐biomass‐per‐predator‐biomass data (i.e. the ratio of prey biomass in stomachs to predator weight) and predator biomass survey data, to predict “predator‐expanded‐stomach‐contents” (PESCs). PESC estimates can be used to visualize either the annual landscape of PESCs (spatio‐temporal variation), or can be aggregated across space to calculate annual variation in diet proportions (variation among prey items and among years). We demonstrated our approach in two contrasting scenarios: a data‐rich situation involving eastern Bering Sea (EBS) large‐size walleye pollock (Gadus chalcogrammus, Gadidae) for 1992–2015; and a data‐limited situation involving West Florida Shelf red grouper (Epinephelus morio, Epinephelidae) for 2011–2015. Large walleye pollock PESC was predicted to be higher in very warm years on the Middle Shelf of the EBS, where food is abundant. Red grouper PESC was variable in north‐western Florida waters, presumably due to spatio‐temporal variation in harmful algal bloom severity. Our approach can be employed to parameterize or validate diverse ecosystem models, and can serve to address many fundamental ecological questions, such as providing an improved understanding of how climate‐driven changes in spatial overlap between predator and prey distributions might influence predation pressure.  相似文献   

16.
Walleye pollock is the second most extensively fished species in the world. The major fishing grounds include the Bering and Okhotsk Seas. Large-scale fishing started in the 1960s and continues to date with average annual landings over this 50-year period of 1.5 million tons. Yet over this period catches were characterized by considerable volatility. This volatility makes rational management of stock and planning of annual fishing activities difficult. The changes in annual catches correlate with the changes in the biomass of walleye pollock. Existing data suggest a close link between climate change in the northern Pacific and biomass, which allows quantitative estimates of future trends in the biomass, and consequently annual catch, of walleye pollock. Cooling of the northern Pacific is expected to increase the biomass in the Sea of Japan and decrease it in the Bering Sea and Sea of Okhotsk. The opposite is predicted to occur if the northern Pacific experiences warming.  相似文献   

17.
Larval and early juvenile fishes were sampled from the eastern Bering Sea (EBS) shelf from 2001 to 2005, and in 2007. Data from these collections were used to examine spatial and temporal patterns in species assemblage structure and abundance. The years 2001–2005 were unusual because the EBS water temperature was ‘warm’ compared with the long‐term mean temperature. In contrast, 2007 was a ‘cold’ year. The abundance of the five most numerous taxa at 12 stations common to all years sampled (1996–2005, 2007) were significantly different among years. Larval and early juvenile stage Theragra chalcogramma (walleye pollock), a commercially important gadid, were by far the most abundant fish in all years. Bottom depth alone best explained assemblage structure in most years, but in others, bottom depth and water column temperature combined and percent sea‐ice coverage were most important. Abundance of T. chalcogramma larvae increases with water column temperature until 5°C and then becomes level. Higher abundances of Gadus macrocephalus (Pacific cod) larvae occur in years with the greatest percent sea‐ice cover as indicated by GAM analysis. Larvae of Lepidopsetta polyxystra (northern rock sole) increase in abundance with increasing maximum wind speed, but decrease at a later date during the last winter storm. The data are consistent with the hypothesis that oceanographic conditions, specifically water temperature and sea‐ice coverage, affect the spatial and temporal pattern of larval abundances. In general, ichthyoplankton species assemblages can be important early indicators of environmental change in the Bering Sea and potentially other subarctic seas as well.  相似文献   

18.
We describe the spatial distribution and dispersal pathways of Greenland halibut (Reinhardtius hippoglossoides) early life stages based on historical field data from the eastern Bering Sea and adjacent water along the eastern Aleutian Islands. Our results indicate that Greenland halibut from preflexion larvae to newly settled juveniles have a long pelagic duration and are subject to extended drift pathways. Hatching may occur in deep water, below 530 m, and larvae rise in the water column as they grow. Flexion/postflexion larvae are mostly found around the Pribilof Islands over the middle shelf (50–100‐m isobaths) in July, and settling occurs during late summer on the middle shelf near St. Matthew Island. However, given that age‐1 individuals were primarily found on the outer shelf, it appears that Greenland halibut actively move to deeper water with age (or size). The mechanisms of slope–shelf connectivity in preflexion larvae may be related to the Bering Slope Current in the vicinity of both Bering and Pribilof Canyons. This study shows that Greenland halibut early life stages have extensive horizontal ontogenetic migrations in the Bering Sea, and utilize a range of geographic areas over the basin and slope along the Aleutian Islands and in the eastern Bering Sea. 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.  相似文献   

19.
Walleye pollock Theragra chalcogramma (pollock hereafter) is a key ecological and economic species in the eastern Bering Sea, yet detailed synthesis of the spatial and temporal patterns of pollock ichthyoplankton in this important region is lacking. This knowledge gap is particularly severe considering that egg and larval distribution are essential to reconstructing spawning locations and early life stages drift pathways. We used 19 yr of ichthyoplankton collections to determine the spatial and temporal patterns of egg and larval distribution. Generalized additive models (GAMs) identified two primary temporal pulses of pollock eggs, the first occurring from 20 February to 31 March and the second from 20 April to 20 May; larvae showed similar, but slightly lagged, pulses. Based on generalized cross‐validation and information theory, a GAM model that allowed for different seasonal patterns in egg density within three unique areas outperformed a GAM that assumed a single fixed seasonal pattern across the entire eastern Bering Sea. This ‘area‐dependent’ GAM predicted the highest densities of eggs (i.e., potential spawning locations) in three major areas of the eastern Bering Sea: near Bogoslof Island (February–April), north of Unimak Island and the Alaska Peninsula (March–April), and around the Pribilof Islands (April–August). Unique temporal patterns of egg density were observed for each area, suggesting that pollock spawning may be more spatially and temporally complex than previously assumed. Moreover, this work provides a valuable baseline of pollock spawning to which future changes, such as those resulting from climate variability, may be compared.  相似文献   

20.
Information on the annual variability in abundance and growth of juvenile groundfish can be useful for predicting fisheries stocks, but is often poorly known owing to difficulties in sampling fish in their first year of life. In the Western Gulf of Alaska (WGoA) and Eastern Bering Sea (EBS) ecosystems, three species of puffin (tufted and horned puffin, Fratercula cirrhata, Fratercula corniculata, and rhinoceros auklet, Cerorhinca monocerata, Alcidae), regularly prey upon (i.e., “sample”) age-0 groundfish, including walleye pollock (Gadus chalcogramma, Gadidae) and Pacific cod (Gadus microcephalus, Gadidae). Here, we test the hypothesis that integrating puffin dietary data with walleye pollock stock assessment data provides information useful for fisheries management, including indices of interannual variation in age-0 abundance and growth. To test this hypothesis, we conducted cross-correlation and regression analyses of puffin-based indices and spawning stock biomass (SSB) for the WGoA and EBS walleye pollock stocks. For the WGoA, SSB leads the abundance of age-0 fish in the puffin diet, indicating that puffins sample the downstream production of the WGoA spawning stock. By contrast, the abundance and growth of age-0 fish sampled by puffins lead SSB for the EBS stock by 1–3 years, indicating that the puffin diet proxies incoming year class strength for this stock. Our study indicates connectivity between the WGoA and EBS walleye pollock stocks. Integration of non-traditional data sources, such as seabird diet data, with stock assessment data appears useful to inform information gaps important for managing US fisheries in the North Pacific.  相似文献   

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