Pollock Recruitment Research Articles

Oceanographic conditions and analysis of walleye pollock <i>Theragra chalcogramma</i> fishery in the Navarin area in 2017–2021

The northwestern Bering Sea, in particular the waters at Cape Navarin, is one of the main areas of the walleye pollock fishery. Environmental variability in the Bering Sea affects on recruitment, abundance, behavior, and seasonal spatial distribution of pollock, which challenge on the fishery management strategy. Understanding of environmental-driven changes in the pollock population has to improve predictions of the population dynamics and affect positively on the commercial harvest. The pollock from the eastern Bering Sea migrate to the Navarin area in summer-fall season. This study demonstrates that great annual differences in the migration activity, spatial distribution of pollock and fishery are related with oceanographic conditions, zooplankton productivity, the population reproduction, year-classes abundance and total biomass. Appearance of the relatively abundant year-classes in 2017–2020 may lead to some increase in commercial part of the pollock population in 2022–2024.

Predicting year class strength for climate-stressed gadid stocks in the Gulf of Alaska

Climate change makes fish stocks more vulnerable to recruitment failure, and early detection of these events is important for an effective management response. Here, we evaluate the value of larval and juvenile surveys, and a thermal spawning habitat index, for predicting recruitment in two economically important gadids, walleye pollock (Gadus chalcogrammus) and Pacific cod (G. macrocephalus), in the Gulf of Alaska. These stocks have been exposed to rapid human-induced ocean warming since 2014, which has apparently contributed to anomalies in age structure, size at age, and other population variables (for pollock) and stock collapse (for cod). We found that warming results in recruitment that falls short of predictions from historical spawner-recruit relationships for both stocks, highlighting that climate change makes recruitment expectations based on historical experience less reliable. However, we also found that recruitment could be successfully predicted with surveys of early-life stages. Using Bayesian regression, we found that juvenile trawl survey data for pollock predicts recruitment to age-1 (as estimated by a stock assessment model), while prediction from larval surveys was less successful. Beach seine estimates of juvenile abundance also predicted pollock recruitment, a surprising result for a species that is typically sampled in offshore habitats. The spawning habitat index and beach seine survey both predicted cod recruitment to age-3 as estimated by the stock assessment model. We did not find a predictive relationship between cod larval abundance and recruitment. However, residuals from the larval model showed low-frequency variability, suggesting nonstationarity (time-dependence) in the predictive relationship. Dynamic Factor Analysis (DFA) models summarizing information across multiple data sets showed reasonable predictive value for both species (Bayesian R2 ≈ 0.4 for log recruitment), and they also allowed recruitment prediction for years with missing observations in some data sets. We conclude that surveying multiple early life stages may be the most useful approach for predicting gadid recruitment.

Climate warming and the loss of sea ice: the impact of sea-ice variability on the southeastern Bering Sea pelagic ecosystem

AbstractWe investigated relationships among three metrics of sea-ice cover in eight regions of the eastern Bering Sea and the abundance of Calanus copepods, jellyfish medusae, and year-class strength of walleye pollock (Gadus chalcogrammus). In summer, Calanus spp. were more abundant over the middle shelf when sea ice lingered late into spring, and, to a lesser extent, when February sea-ice cover was heavy. Between 1982 and 1999, there were no significant (p ≤ 0.05) relationships between the amount or timing of sea-ice cover and pollock recruitment. However, between 2000 and 2015, pollock year-class strength was positively correlated with sea ice in the outer and middle shelves, with 17 of 24 regressions significant. Pollock year-class strength was best predicted by days with sea-ice cover after February. Pollock recruitment was positively influenced by copepod numbers, particularly in the middle shelf, with r2 values from 0.36 to 0.47. We hypothesize that the Calanus spp. present in the southeastern Bering Sea are primarily Calanus glacialis that have been advected south in association with sea ice. None of our sea-ice metrics explained the variance in jellyfish biomass. Jellyfish biomass in our study area in the pollock age-0 year was not correlated with pollock recruitment 3 years later.

Interannual variation in the coastal distribution of a juvenile gadid in the northeast Pacific Ocean: The relevance of wind and effect on recruitment

AbstractDrift 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.

Large copepods as leading indicators of walleye pollock recruitment in the southeastern Bering Sea: Sample-Based and spatio-temporal model (VAST) results

In the southeastern Bering Sea, years with above-average sea ice extent have been associated with elevated abundances of large lipid-rich copepods in both net tows and age-0 walleye pollock (Gaduschalcogrammus) diets, followed by high overwinter pollock survival and stronger recruitment into the fishery three years later. In this study, we directly compare large copepod abundances of taxa important in age-0 pollock diets (Calanus glacialis/marshallae, Metridia pacifica and Neocalanus spp.) to age-3 pollock abundance from stock assessments for year classes 2002–2015 (excluding 2013). Copepod samples from bongo nets were collected on fishery oceanography surveys in late summer during warm (low ice) and cold (high ice) climate stanzas. In addition to mean estimates of large copepods among stations (sample-based), we implemented a spatial delta-generalized linear mixed model using the Vector Autoregressive Spatio-Temporal (VAST) package to account for variations in survey coverage among years. Positive significant linear relationships were found for both of these estimates of large copepods during the age-0 year and the abundance of these pollock at age-3. Correlations were stronger using VAST model-based indices compared with sample-based indices (R2 = 0.74 versus 0.43, respectively, all copepod taxa combined). A significant relationship was also observed between the bottom cold pool (< 2 °C) area (indicative of sea ice coverage in the prior winter) during the age-0 year and subsequent age-3 pollock abundance (R2 = 0.56) and recruits per spawner, (ln(age-3 abundance /spawning stock biomass), R2 = 0.77). Consequently, the large copepod index or cold pool index may be used to predict future recruitment success of pollock three years in advance. Results provide support for the revised oscillating control hypothesis that suggests as the climate warms, reductions in sea ice and consequent reduced availability of ice-associated algae, an early spring food source, could be detrimental to large copepods and recruitment of the pollock stock in the region.

Empirical orthogonal function regression: Linking population biology to spatial varying environmental conditions using climate projections

Ecologists and oceanographers inform population and ecosystem management by identifying the physical drivers of ecological dynamics. However, different research communities use different analytical tools where, for example, physical oceanographers often apply rank-reduction techniques (a.k.a. empirical orthogonal functions [EOF]) to identify indicators that represent dominant modes of physical variability, whereas population ecologists use dynamical models that incorporate physical indicators as covariates. Simultaneously modeling physical and biological processes would have several benefits, including improved communication across sub-fields; more efficient use of limited data; and the ability to compare importance of physical and biological drivers for population dynamics. Here, we develop a new statistical technique, EOF regression, which jointly models population-scale dynamics and spatially distributed physical dynamics. EOF regression is fitted using maximum-likelihood techniques and applies a generalized EOF analysis to environmental measurements, estimates one or more time series representing modes of environmental variability, and simultaneously estimates the association of this time series with biological measurements. By doing so, it identifies a spatial map of environmental conditions that are best correlated with annual variability in the biological process. We demonstrate this method using a linear (Ricker) model for early-life survival ("recruitment") of three groundfish species in the eastern Bering Sea from 1982 to 2016, combined with measurements and end-of-century projections for bottom and sea surface temperature. Results suggest that (a) we can forecast biological dynamics while applying delta-correction and statistical downscaling to calibrate measurements and projected physical variables, (b) physical drivers are statistically significant for Pacific cod and walleye pollock recruitment, (c) separately analyzing physical and biological variables fails to identify the significant association for walleye pollock, and (d) cod and pollock will likely have reduced recruitment given forecasted temperatures over future decades.

Assessment of stock, spatial distribution, and recruitment of walleye pollock in the northern and eastern Bering Sea

Abundance and biomass of walleye pollock Theragra chalcogramma in the Bering Sea exceeded the mean values in 2015 owing to the high-abundant year-classes of 2008 and 2012 and several medium-abundant year-classes (of 2006, 2009-2011, 2013, and 2014). In contrast to rather stable distribution on its spawning grounds, distribution of feeding pollock in the northern Bering Sea is very variable and depends on both biomass of the population and long-term and short-term variations of environments, as water temperature and zooplankton abundance. In some years including recent times (2005-2007 and 2010-2014), the pollock migrated earlier and faster into the northwestern Bering Sea and returned back to the eastern shelf also early: in late summer - early autumn. This scenario is supposedly conditioned by low zooplankton abundance over the Bering Sea that forces pollock to active feeding migrations, particularly a deficit of preferable food (large-sized zooplankton) on the north-western feeding grounds could be a reason for its leaving this area in late summer. Strength of pollock year-classes is highly variable, too, and depends mostly on young-of-the-year fish survival in winter. As usual, favorable conditions for reproduction and progeny survival, when strong year-classes appear, are observed in the years or short periods of transition from one state of marine environments to another, whereas long periods of either «warm» or «cold» regime are not favorable for forming of high-abundant year-classes of the Bering Sea pollock.

Modelling spatially dependent predation mortality of eastern Bering Sea walleye pollock, and its implications for stock dynamics under future climate scenarios

Abstract Arrowtooth flounder (Atheresthes stomias) are an important predator of juvenile walleye pollock (Gadus chalcogramus) in the eastern Bering Sea (EBS) shelf and have increased 3-fold in biomass from 1977 to 2014. Arrowtooth flounder avoid the summer “cold pool” (bottom water ≤2°C) and variability in cold pool size and location has affected their spatial overlap with juvenile walleye pollock. Developing a method to account for the relationship between climate change and pollock mortality can highlight ecosystem dynamics and contribute to better assessments for fisheries management. Consequently, spatially resolved predation mortality rates were estimated within an age-structured walleye pollock stock assessment population model (based on spatial information on diet and abundance from trawl surveys), along with the effect of sea surface temperature (SST) on pollock recruitment. Projections of SST and cold pool area to 2050 were obtained (or statistically downscaled) from nine global climate models and used within an age-structure population model to project pollock abundance given estimated relationships between environmental variables and predator and prey spatial distributions, pollock recruitment, and maximum rate of arrowtooth flounder consumption. The climate projections show a wide range of variability but an overall trend of increasing SST and decreasing cold pool area. Projected pollock biomass decreased largely due to the negative effect of increased SST on pollock recruitment. A sensitivity analysis indicated that the decline in projected pollock biomass would be exacerbated if arrowtooth flounder increased their relative distribution in the EBS northwest middle shelf (an area of relatively high density of juvenile pollock) in warm years.

Wasp waist or beer belly? Modeling food web structure and energetic control in Alaskan marine ecosystems, with implications for fishing and environmental forcing

The Eastern Bering Sea (EBS) and Gulf of Alaska (GOA) continental shelf ecosystems show some similar and some distinctive groundfish biomass dynamics. Given that similar species occupy these regions and fisheries management is also comparable, similarities might be expected, but to what can we attribute the differences? Different types of ecosystem structure and control (e.g. top-down, bottom-up, mixed) can imply different ecosystem dynamics and climate interactions. Further, the structural type identified for a given ecosystem may suggest optimal management for sustainable fishing. Here, we use information on the current system state derived from food web models of both the EBS and the GOA combined with dynamic ecosystem models incorporating uncertainty to classify each ecosystem by its structural type. We then suggest how this structure might be generally related to dynamics and predictability. We find that the EBS and GOA have fundamentally different food web structures both overall, and when viewed from the perspective of the same commercially and ecologically important species in each system, walleye pollock (Gadus chalcogrammus). EBS food web structure centers on a large mass of pollock, which appears to contribute to relative system stability and predictability. In contrast, GOA food web structure features high predator biomass, which contributes to a more dynamic, less predictable ecosystem. Mechanisms for climate influence on pollock production in the EBS are increasingly understood, while climate forcing mechanisms contributing to the potentially destabilizing high predator biomass in the GOA remain enigmatic. We present results of identical pollock fishing and climate-driven pollock recruitment simulations in the EBS and GOA which show different system responses, again with less predictable response in the GOA. Overall, our results suggest that identifying structural properties of fished food webs is as important for sustainable fisheries management as attempting to predict climate and fisheries effects within each ecosystem.

Possible mechanism linking ocean conditions to low body weight and poor recruitment of age-0 walleye pollock (Gadus chalcogrammus) in the southeast Bering Sea during 2007

Changes to physical and chemical oceanographic structure can lead to changes in phytoplankton biomass and growth, which, in-turn, lead to variability in the amount of energy available for transfer to higher trophic levels (e.g., forage fish). In general, age-0 (juvenile) walleye pollock (Gadus chalcogrammus) have been shown to have low fitness (determined by energy density and size), in warm years compared to average or cold years in the southeastern Bering Sea. Contrary to these findings, the year 2007 was a cold year with low fitness of age-0 pollock compared to the transition year of 2006 (transitioning from warm to cold conditions) and cold years, 2008–2011. In late summer/early fall (mid-August through September), significantly lower surface silicic acid concentrations coupled with low phytoplankton production and chlorophyll a (Chl a) biomass were observed in 2007 among 2006–2012 (P<0.05). We postulate that the low silicic acid concentrations may be an indication of reduced surface nutrient flux during summer, leading to low primary productivity (PP). The nutrient replenishing shelf/slope water exchange that occurred during late October–February (2006–2007) indicates that deep-water nutrient/salinity reserves for the start of the 2007 growing season were plentiful and had similar concentrations to other years (2006–2012). The spring bloom magnitude appeared to be slightly below average, and surface silicic acid concentrations at the end of the spring bloom period in 2007 appeared similar to other years in the middle domain of the southeastern Bering Sea. However, during summer (June–August) 2007, high stratification and the low number of storm events resulted in low flux of nutrients to surface waters, indicated by the low surface silicic acid concentrations at the end of summer (mid-August through September). Surface silicic acid may be useful as an indicator of surface nutrient enrichment (and subsequent PP) during summer since other macronutrients (e.g. nitrate) are usually near or below detection limits at this time, and diatoms are generally scarce during summer. Surface silicic acid concentration was also positively associated with the size of juvenile fish (age-0 pollock weight and length). This reinforces the theory that nutrient availability and primary productivity are important to energy allocation for higher trophic levels during summer, and possibly provides links between stratification and wind mixing, surface nutrient input, PP and juvenile fish size and condition.

Chum salmon (Oncorhynchus keta) growth and temperature indices as indicators of the year-class strength of age-1 walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea

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.

The critical first year of life of walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea: Implications for recruitment and future research

Walleye pollock (Gadus chalcogrammus) support a large commercial fishery in the eastern Bering Sea despite large interannual and decadal swings in population abundance. These oscillations challenge the fishery, prompting significant effort directed to understanding the species and its recruitment. Conceptual paradigms of walleye pollock recruitment recognize that understanding the factors affecting survivorship during the first year of life is central to understanding population fluctuation. Since the first year is critical to year-class strength of this key economically and ecologically important species, we review the state of knowledge of pre-recruit walleye pollock ecology in the eastern Bering Sea during this critical first-year period, including spawning, changes in vertical and horizontal distributions, feeding, growth, body condition, transport, and predation. We then critically examine the recruitment paradigms based on the component processes that have been proposed to explain mechanisms of recruitment control. We identify paradigm strengths or weaknesses relative to our current state of knowledge, discussing relevance and validity. Finally, we identify gaps in knowledge and propose areas of future research effort.

Eastern Bering Sea pollock recruitment, abundance, distribution and approach to fishery management

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.

Spatial modeling of Bering Sea walleye pollock with integrated age-structured assessment models in a changing environment

Climate change may affect the spatial distribution of fish populations in ways that would affect the accuracy of spatially aggregated age-structured assessment models. To evaluate such scenarios, spatially aggregated models were compared with spatially explicit models using simulations. These scenarios were based on hypothetical climate-driven distribution shifts and reductions in mean recruitment of walleye pollock (Gadus chalcogrammus) in the eastern Bering Sea. Results indicate that biomass estimates were reasonably accurate for both types of estimation models and precision improved with the inclusion of tagging data. Bias in some aggregated model scenarios could be attributed to unaccounted-for process errors in annual fishing mortality rates and was reduced when estimating effective sample size or time-varying selectivity. Spatially explicit models that allow estimation of variability in movement and ontogenetic parameters (specified as a random walk process) were shown to be feasible, whereas models that misspecified ontogenetic movement and climate change effects resulted in biased biomass and movement parameter estimates. These results illustrate that more complex models may characterize processes better but may be less robust for management advice.

Climate and Demography Dictate the Strength of Predator-Prey Overlap in a Subarctic Marine Ecosystem

There is growing evidence that climate and anthropogenic influences on marine ecosystems are largely manifested by changes in species spatial dynamics. However, less is known about how shifts in species distributions might alter predator-prey overlap and the dynamics of prey populations. We developed a general approach to quantify species spatial overlap and identify the biotic and abiotic variables that dictate the strength of overlap. We used this method to test the hypothesis that population abundance and temperature have a synergistic effect on the spatial overlap of arrowtooth flounder (predator) and juvenile Alaska walleye pollock (prey, age-1) in the eastern Bering Sea. Our analyses indicate that (1) flounder abundance and temperature are key variables dictating the strength of flounder and pollock overlap, (2) changes in the magnitude of overlap may be largely driven by density-dependent habitat selection of flounder, and (3) species overlap is negatively correlated to juvenile pollock recruitment when flounder biomass is high. Overall, our findings suggest that continued increases in flounder abundance coupled with the predicted long-term warming of ocean temperatures could have important implications for the predator-prey dynamics of arrowtooth flounder and juvenile pollock. The approach used in this study is valuable for identifying potential consequences of climate variability and exploitation on species spatial dynamics and interactions in many marine ecosystems.

Correlation between recruitment and fall condition of age-0 pollock (Theragra chalcogramma) from the eastern Bering Sea under varying climate conditions

Fishery managers require an understanding of how climate influences recruitment if they are to separate the effects of fishing and climate on production. The southeastern Bering Sea offers opportunities to understand climate effects on recruitment because inter-annual oscillations in ice coverage set up warm or cold conditions for juvenile fish production. Depth-averaged temperature anomalies in the Bering Sea indicate the past nine years have included three warm (2003–2005), an average (2006), and five cold (2007–2011) years. We examined how these climatic states influenced the diet quality and condition (size, energy density and total energy) of young-of-the-year (YOY) pollock (Theragra chalcogramma) in fall. The implications of fall condition were further examined by relating condition prior to winter to the number of age-1 recruits-per-spawner the following summer (R/S). The percentage of lipid in pollock diets was threefold higher in cold years compared with warm years, but stomach fullness did not vary. Consequently, fish energy densities were 33% higher in cold years (P<0.001) than in warm years. In contrast, neither fish size (P=0.666), nor total energy (P=0.197) varied with climatic condition. However, total energy was significantly (P=0.007) and positively correlated with R/S (R2=0.736). We conclude that recruitment to age-1 in the southeastern Bering Sea is improved under environmental conditions that produce large, energy dense YOY pollock in fall.

Interannual changes in the zooplankton community structure on the southeastern Bering Sea shelf during summers of 1994–2009

On the southeastern Bering Sea shelf, mesozooplankton plays an important role in material transfer between primary producers and fisheries resources. The biomass of mesozooplankton in this region is known to vary annually, but little is known about annual changes in community structure and species composition. In the present study, regional and long-term changes in abundance, biomass and community structure of copepods and chaetognaths on the shelf were evaluated based on NORPAC net samples collected during summers of 1994–2009. During the study period, regime shifts occurred from high interannual variability regime (1994–1999) to low interannual variability regime with high temperature (2000–2005), then to a low interannual variability regime with low temperature (2007–2009). A total of 24 calanoid copepod species belonging to 21 genera were identified from samples. Copepod abundance ranged from 150 to 834,486inds.m−2, was greatest on the Middle shelf, and was higher in cold years, than in warm years. Copepod biomass ranged from 0.013 to 150gDMm-2, and was also higher in cold years than in warm years. Based on the results of cluster analysis, the copepod community was divided into six groups (A–F). The regional and interannual distributions of each group were distinct. Interannual changes in abundance of the dominant copepod on the Outer shelf and Middle shelf were highly significant (p<0.0001), and their abundances were negatively correlated with temperature and salinity. Interannual changes in copepod community that occurred between cold and warm years are thought to have been caused by differences in the magnitude and timing of the spring phytoplankton bloom between the two regimes. Abundance and biomass of the chaetognath Parasagitta elegans ranged from 30 to 15,180inds.m-2 and from 11 to 1559mgDMm−2, respectively. Chaetognath abundance was significantly correlated with the abundance of the dominant copepods (p<0.0001). Differences in cold and warm years may also affect recruitment of walleye pollock. We conclude that on the southeastern Bering Sea shelf, the magnitude and timing of primary production, which is related to climate change, may significantly affect how it is transferred through the food web.

Climate impacts on eastern Bering Sea foodwebs: a synthesis of new data and an assessment of the Oscillating Control Hypothesis

Abstract Hunt, G. L., Coyle, K. O., Eisner, L. B., Farley, E. V., Heintz, R. A., Mueter, F., Napp, J. M., Overland, J. E., Ressler, P. H., Salo, S., and Stabeno, P. J. 2011. Climate impacts on eastern Bering Sea foodwebs: a synthesis of new data and an assessment of the Oscillating Control Hypothesis. – ICES Journal of Marine Science, 68: 1230–1243. Walleye pollock (Theragra chalcogramma) is an important component of the eastern Bering Sea ecosystem and subject to major fisheries. The Oscillating Control Hypothesis (OCH) predicted that recruitment of pollock year classes should be greatest in years with early ice retreat and late blooms in warm water, because more energy would flow into the pelagic (vs. benthic) community. The OCH further predicted that, with pollock population growth, there should be a shift from bottom-up to top-down regulation. New data support the predictions that in those years with early ice retreat, more primary production accrues to the pelagic compartment and that large numbers of age-0 pollock survive to summer. However, in these years, production of large crustacean zooplankton is reduced, depriving age-0 pollock of lipid-rich prey in summer and autumn. Consequently, age-0 pollock energy reserves (depot lipids) are low and predation on them is increased as fish switch to age-0 pollock from zooplankton. The result is weak recruitment of age-1 recruits the following year. A revised OCH indicates bottom-up constraints on pollock recruitment in very warm periods. Prolonged warm periods with decreased ice cover will likely cause diminished pollock recruitment and catches relative to recent values.

Expected declines in recruitment of walleye pollock (Theragra chalcogramma) in the eastern Bering Sea under future climate change

AbstractMueter, F. J., Bond, N. A., Ianelli, J. N., and Hollowed, A. B. 2011. Expected declines in recruitment of walleye pollock (Theragra chalcogramma) in the eastern Bering Sea under future climate change. – ICES Journal of Marine Science, 68: 1284–1296. A statistical model is developed to link recruitment of eastern Bering Sea walleye pollock (Theragra chalcogramma) to variability in late summer sea surface temperatures and to the biomass of major predators. The model is based on recent advances in the understanding of pollock recruitment, which suggest that warm spring conditions enhance the survival of early larvae, but high temperatures in late summer and autumn are associated with poor feeding conditions for young-of-year pollock and reduced recruitment in the following year. A statistical downscaling approach is used to generate an ensemble of late summer temperature forecasts through 2050, based on a range of IPCC climate projections. These forecasts are used to simulate future recruitment within an age-structured stock projection model that accounts for density-dependent effects (stock–recruitment relationship), the estimated effects of temperature and predation, and associated uncertainties. On average, recruitment in 2040–2050 should expectedly decline by 32–58% relative to a random recruitment scenario, depending on assumptions about the temperature relationship, the magnitude of density-dependence, and future changes in predator biomass. The approach illustrated here can be used to evaluate the performance of different management strategies and provide long-term strategic advice to managers confronted with a rapidly changing climate.

Causes of walleye pollock (Theragra chalcogramma) recruitment decline in the northern Sea of Japan: implications for stock management

Recruitment of the northern Japan Sea stock (JSS) of walleye pollock has been decreasing since around 1990. In this study, I analyzed the factors causing this decrease in recruitment by investigating the relationship between recruitment, spawning stock biomass (SSB) and environmental factors using a generalized additive model (GAM). GAM fit to the data showed the importance of SSB, sea surface temperature (SST), ocean current strength (Tsushima Warm Current) and wind intensity (Asian monsoon) in determining the recruitment. Of these, the relationship between SSB and recruitment was positive and not negatively density-dependent. On the other hand, the recruitment was negatively related to SST and ocean current strength, and a dome-shaped relationship was observed between wind intensity and recruitment. Since around 1990, the values of SST and ocean current strength have mostly been high and that of wind intensity mostly low. In addition, SSB has been decreasing since the late 1990s. It is likely that the recruitment decline of JSS after approximately 1990 has been caused by warm water temperature, strong Tsushima Warm Current and weak Asian monsoon, and that the recent decrease in SSB has amplified this recruitment decline. According to the model’s estimation, a recruitment recovery due to environmental improvement will be highly restricted as long as SSB remains at its current low level. Significant recovery of SSB is urgently needed for JSS.