Walleye pollock

Climate change is expected to strongly impact Alaskan (USA) marine ecosystems, particularly those of the northern Bering Sea. To understand how climate change may alter predator demand for prey resources, we quantified the relationship between temperature and allometric rates of prey consumption for 3 major groundfish predators: walleye pollock (Gadus chalcogrammus), Pacific cod (Gadus macrocephalus), and arrowtooth flounder (Atheresthes stomias). We contrasted regional patterns in foraging rates of more than 120 thousand fish collected annually from 1981-2011 from eastern Bering Sea (EBS), Gulf of Alaska (GOA), and Aleutian Islands (AI). From field observations, we estimate that juveniles and adults of each species eat 1.15–3.94 and 0.84 -3.13 times their body weight (BW) per year, respectively (i.e., 0.2%-1.2% BW d-1) with arrowtooth from in all 3 regions and adult EBS Pacific cod exhibiting the highest rates. If historical patterns signify future conditions, we estimate that a 2 oC projected increase in water temperature would cause 26% and 14% increases in daily rations of adult EBS pollock and Pacific cod, respectively, whereas we predict a 37% decline in the daily rations of EBS arrowtooth. Similarly, we predict that GOA pollock, Pacific cod and arrowtooth rations would increase markedly (70%, 34%, and 65%, respectively). Although daily ration of AI arrowtooth may increase 31%, our models predict 41% and 3% declines in AI pollock and Pacific cod rations, respectively. These results portend (sometimes counterintuitive) region- and species-specific shifts in Alaska groundfish predator foraging rates and trophodynamic interactions concomitant with climate change.

Modelled connectivity between Walleye Pollock (Gadus chalcogrammus) spawning and age-0 nursery areas in warm and cold years with implications for juvenile survival. Adult and early life stage distributions of the commercially important demersal fish Walleye Pollock (Gadus chalcogrammus) have varied in relation to the warm and cold environmental conditions on the eastern Bering Sea (EBS) shelf. Previous modelling studies indicate that transport alone does not account for the disparate juvenile distributions in warm and cold years, but that spawning locations are important. Our objective was to determine the potential connectivity of EBS pollock spawning areas with juvenile nursery areas between warm and cold years from an 18-year hindcast (1995 – 2012). We calculated the connectivity between larval sources and juvenile positions that were produced by a coupled biological-physical individual-based model that simulated transport, growth, and vertical behavior of pollock from the egg until the juvenile stage. Three connectivity patterns were seen in most simulations: along-isobaths to the northwest, self-retention, and transport around the Pribilof Islands. The major differences in connectivity between warm and cold years, more northwards in warm years and more off-shelf in cold years, mimicked wind-driven flow characteristics of those years that were related to winter mean zonal position of the Aleutian Low. Connectivity relationships were more sensitive to spatial alterations in the spawning areas in cold years, while they were more responsive to spawn timing shifts in warm years. The strongest connectivity to advantageous juvenile habitats originated in the well-known spawning areas, but also in a less well-studied region on the Outer Shelf. This northern Outer Shelf region emerged as a very large sink of pollock reaching the juvenile transition from all spawning sources, suggesting more thorough sampling across multiple trophic levels of this potentially important juvenile pollock nursery is needed.

To determine the effects of diet mixing on digestive performance, harbour seals (Phoca vitulina L., 1758) were offered either pure diets of Pacific herring (Clupea pallasii Valenciennes, 1847) or walleye pollock (Theragra chalcogramma (Pallas, 1814)) or a 1:1 mix of herring and pollock. Regardless of diet, retention time decreased approximately 40% as intake quadrupled. The mean apparent digestible dry matter (ADDM) was greatest on mixed diets during low feeding frequency trials; ADDM during high feeding frequency trials was significantly reduced as intake increased for animals on the single-species (pollock or herring) diets. As intake increased, up to 45% more digestible energy was assimilated from the mixed diet than from either single-species diet. The findings of this study suggest that a mixed diet consisting of prey differing in lipid and protein amounts increased digestible energy intake in harbour seals. Our measures of intake and ADDM in harbour seals revealed digestive fl...

Walleye pollock (Theragra chalcogramma) is the target of one of the world's largest fisheries and is an important prey species in the eastern Bering Sea (EBS) ecosystem. Little is known about the potential effects of fishing on the school characteristics and spatial distribution of walleye pollock. Few dedicated research surveys have been conducted during pollock fishing seasons, so analysis of fishery data is the only feasible approach to study these potential effects. We used acoustic data col - lected continuously by one fishing vessel in January-February 2003, which operated north of Unimak Island. Results from comparisons between two fishing periods showed significant changes of pollock distribution at different scales. The schools were smaller and denser during the second period. Furthermore, the spatial distribution of schools became sparser, as evidenced by the lower frequency of schools per elementary distance sampling unit and the increase in average

As one of the major oceanic sinks of anthropogenic CO2, the Southern Ocean plays a critical role in the climate system. However, due to the scarcity of observations, little is known about physical and biological processes that control air-sea CO2 fluxes and how these processes might respond to climate change. It is well established that primary production is one of the major drivers of air-sea CO2 fluxes, consuming surface Dissolved Inorganic Carbon (DIC) during Summer. Southern Ocean primary production is though constrained by several limiting factors such as iron and light availability, which are both sensitive to mixed layer depth. Mixed layer depth is known to be affected by current changes in wind stress or freshwater fluxes over the Southern Ocean. But we still don't know how primary production may respond to anomalous mixed layer depth neither how physical processes may balance this response to set the seasonal cycle of air-sea CO2 fluxes. In this study, we investigate th...

We conducted surveys in Bristol Bay on the southeastern Bering Sea shelf during late August to mid-September 2000-2003, as part of the Bering-Aleutian Salmon International Survey (BASIS). Our main objective was to compare biological and physical oceanographic characteristics to the distributions of juvenile sockeye salmon (Oncorhynchus nerka) and age-0 walleye pollock (Theragra chalcogramma) during fall in the southeastern Bering Sea.

Amidwater trawl sur­ vey was conducted during July 1991, to examine the large-scale distribution patterns of late larval and early juve­ nile walleye pollock, Theragra chalco­ gramma, and associated fish taxa in the western GulfofAlaska. Gear compari­ sons between the anchovy and Methot trawls were conducted to evaluate which was the more efficient sampler for the size range of T.

Problem: Infer relative danger from spatially segregated predators in large-scale marine systems, using individual differences in prey foraging behaviour. Mathematical models: Optimization of trade-offs between predation risk and energy gain. Key assumption: Foraging individuals choosing to incur higher risk of predation should experience higher energetic gain than individuals choosing safer foraging options. Organisms: Alaskan harbour seals foraging under predation risk from killer whales and Pacific sleeper sharks in shallow and deep strata, respectively. Sleeper sharks are elusive predators and their interactions with prey are difficult to measure directly. Method: Our method infers the relative danger from different predator types that is theoretically most plausible according to intermediate consumer behaviour. We predicted net energy gain and predation risk per foraging dive, parameterizing an analytical model with data on seal behaviour and the spatial distributions of predat...

The controversy surrounding a proposal to allocate walleye pollock Theragra chalcogramma and Pacific cod Gadus macrocephalus harvested in US fisheries off Alaska is reviewed. The walleye pollock allocation, which generated the most concern, would apportion specific shares of the allowable catch to each of the two harvesting sectors, inshore and offshore, along with a set-aside for Alaska community development. The potential transfer is of enormous economic value for industry participants and underscores ...

Quantitative assessment of semidemersal fish such as walleye pollock ( Theragra chalcogramma ) is difficult because the proportion of walleye pollock available to standardized surveys varies temporally and spatially. The US National Marine Fisheries Service’s Alaska Fisheries Science Center conducts bottom trawl (BT) surveys to estimate the demersal portion of the walleye pollock population and acoustic trawl (AT) surveys to estimate the pelagic portion. Both surveys are conducted during daylight hours to minimize variability due to diel changes in vertical distribution. To test if daytime near-bottom light intensity affects the proportion of walleye pollock available to the BT survey, we concurrently measured light and walleye pollock abundance on the Bering Sea shelf. Logistic regression models demonstrated that both light and depth affected walleye pollock abundance estimates by either BT or AT surveys, with more walleye pollock available to the BT survey under high illumination ...