A’mar, Z. T., Punt, A. E., and Dorn, M. W. 2009. The evaluation of two management strategies for the Gulf of Alaska walleye pollock fishery under climate change. – ICES Journal of Marine Science, 66: ...1614–1632.Management strategy evaluation (MSE) is the process of using simulation testing with feedback to examine the robustness of candidate management strategies to error and uncertainty. The structure of the management strategy can be selected to attempt to satisfy desired (but conflicting) management objectives. MSE was used to assess the performance of the current management strategy and an alternative management strategy (the “dynamic B0” strategy) for the fishery for walleye pollock (Theragra chalcogramma) in the Gulf of Alaska (GOA), when age-1 recruitment was driven by climate. The relationships between age-1 abundance and climate indices (and the uncertainties associated with these relationships) were characterized within an age-structured operating model that was fitted to the data for GOA walleye pollock. Projections into the future were based on the fitted relationships and predictions of those indices from the Intergovernmental Panel on Climate Change (IPCC) models, using the current or the alternative management strategy to determine catch limits. Management performance (the ability to leave the stock close to the management reference level and achieve high and stable catches) deteriorated when age-1 recruitment was forced by climate, although stocks were kept near the reference level on average. In addition, the ability to estimate management-related quantities, such as spawning biomass, deteriorated markedly when recruitment was forced by climate. Performance was sensitive to the choice of IPCC dataset and, in particular, estimation and management performance was poorest (outcomes most variable) for the IPCC datasets that led to the greatest variation in recruitment to the fishery. Although basing management on a “dynamic B0” management strategy led to improved management and estimation performance, the magnitude of the improvement was slight.
Barbeaux, S. J., Horne, J. K., and Dorn, M. W. 2013. Characterizing walleye pollock (Theragra chalcogramma) winter distribution from opportunistic acoustic data. - ICES Journal of Marine Science, 70: ...1162-1173.In 2003, acoustic data from 25 000 km of ship track lines were collected from two fishing vessels participating in the eastern Bering Sea walleye pollock (Theragra chalcogramma) fishery. Although these data were not calibrated or collected on a systematic grid, their broad temporal extent combined with high spatial resolution facilitated the examination of the distribution and behaviour of fished aggregations. To demonstrate their scientific applicability, these data were used to identify the spatio-temporal dynamics of pollock aggregations over scales ranging from hundreds of metres to hundreds of kilometres and from minutes to months. The spatial analysis identified three levels of pollock aggregation. The largest regions of high pollock density had an average diameter of 110 km and were comparable with distinct fishing grounds identified by fishers. The next smaller areas of high pollock density had a diameter between 2.5 and 6 km. Within these areas were clusters of pollock at even higher densities. The extent of the smallest aggregations ranged in diameter from 0.1 km in daylight to 0.6 km at night. Time-series analysis identified vertical and horizontal diel changes in pollock distribution and an overall decline in pollock density over the study period.
There is strong evidence that low frequency between-year environmental variability, in addition to fishing, is able to affect fish population abundance via recruitment. However, scientific advice ...regarding catch limits is often based on control rules that depend on the estimation of biomass reference points which typically do not explicitly consider the effects of trends over time in reference points caused by environmental variability. Harvest rates based on commonly used biological reference points such as the level of unfished spawning biomass (
B
0), the current size of the stock in relation to
B
0, and
B
MSY that are sustainable under current environmental conditions may be unsustainable under different environmental conditions. Although several methods exist for estimating biomass reference points, it is unclear which of these are most robust to the effects of long term, low frequency environmental variability. Therefore, simulation is used to evaluate alternative estimators, which differ in terms of how the stock–recruitment relationship is modeled, and whether explicit estimators or proxies are used for
B
0, the steepness of the stock–recruitment relationship
, and current spawning biomass relative to
B
0. The simulations consider three life histories: a long-lived unproductive rockfish, a moderately long-lived and productive flatfish, and a moderately long-lived and productive hake with highly variable recruitment. Results indicate that in the presence of low frequency autocorrelated forcing of recruitment, biomass reference points should be based on average recruitment and/or dynamic
B
0 if catch and survey data are available for at least one full period of the environmental variable. In contrast, previous analysis suggests that in the absence of autocorrelated environmental forcing of recruitment, and if the available catch and survey data do not span at least, in this case, 50 years which is one full period of the environmental variable, biomass reference points should be based on the fit of the stock–recruitment relationship. Life history affects the estimability of biomass reference points, which are more difficult to estimate for species with more rapid dynamics such as hake. The method used to calculate the reference points given the results of a stock assessment has a larger effect on estimability than the configuration of the stock assessment method, for the three stock assessment model configurations investigated in this study.
•Steepness measures the strength of recruitment compensation and is hard to estimate.•Regional meta-analysis predicting steepness for Pacific rockfishes have been run biannually 2007–2017.•Estimated ...steepness priors have increased from 0.57(2007) to 0.72 (2017), following periods with poor or strong recruitment.•Increases have contributed to conclusions that several rockfishes are now rebuilt.•We discuss alternatives to current practice for using steepness for Pacific rockfishes.
Theoretical and applied research suggests that survival rates during early life stages will increase when spawning biomass is reduced in marine fishes (termed “recruitment compensation”). However, the magnitude of recruitment compensation is generally difficult to estimate for individual fish stocks, and its average value for marine fishes remains highly contested. Scientists and managers for Pacific rockfishes (Sebastes spp.) on the US West Coast have used a regional meta-analysis to estimate the likely distribution of the steepness parameter of the Beverton-Holt stock-recruit relationship using stock assessment models since 2007, and the method has been updated every two years as new assessments are conducted (i.e., five biennial updates). Here, we provide a short history of this approach, its methodological assumptions, changes in results over time, and ongoing efforts to validate its assumptions. While the regional meta-analysis has been successful in ensuring a consistent approach to treatment of steepness across assessments, the estimates of mean steepness have been unexpectedly variable as the meta-analysis has been updated. Specifically, we show that the estimated average value of steepness for West Coast rockfish increased markedly from 2007 (average: <0.6) to 2011 (average:>0.75), before decreasing somewhat again in the 2017 update. We also show that this value has a strong impact on rockfish rebuilding plans, and showcase the example of canary and widow rockfishes, where the estimated rates of rebuilding are strongly influenced by the assumed value of steepness. We conclude by discussing the bias-variance tradeoff between using global and regional meta-analysis, as well as the likely implications of difficult-to-validate assumptions including: (1) no recruitment autocorrelation within each stock; (2) no correlations among stocks; and (3) no bias from individual stocks resulting from mis-specification of the stock assessment models used in the meta-analysis.
Fishery management plans for U.S. fisheries are required to specify status determination criteria (e.g. whether the stock is overfished and whether overfishing is occurring) and typically use harvest ...control rules to adjust target and limit fishing mortality and catch levels to prevent overfishing, achieve optimum yield and rebuild overfished stocks. The status determination criteria are based on the concept of the fishing mortality rate (
F
MSY) that maximizes long-term catch as the upper limit on the allowable rate of fishing and the associated
B
MSY, the spawning biomass which produces MSY, is the target for rebuilding of overfished stocks. In practice, proxies for the biological reference points
F
MSY and
B
MSY are often employed. Although several methods exist for estimating these quantities, it is unclear which performs best. Simulation is therefore used to evaluate alternative estimators for these quantities. These estimators differ in terms of whether a stock–recruitment relationship is estimated, and whether a prior based on Bayesian meta-analysis is used as a penalty on steepness, a critical parameter of the stock–recruitment relationship. The simulations consider three life histories: a long-lived unproductive rockfish, a moderately long-lived and productive flatfish, and a hake, which is also moderately long-lived and productive, but exhibits highly variable recruitment. Results indicate that estimator performance varies among reference points. However, estimators of
B
0, the average spawning biomass in the absence of exploitation, and stock depletion based on a fitted stock–recruitment relationship generally perform best.
B
0 is estimated either better (the rockfish and flatfish) or similarly (the hake) to stock depletion. Estimating
B
MSY from the fit of the stock–recruitment relationship performed best for the rockfish and flatfish life histories; average recruitment estimators proved to be best for the flatfish and hake life histories. Proxy methods of calculating
B
MSY generally performed relatively poor in comparison to the non-proxy measures. The performance of estimators of biological reference points was generally better for the rockfish and flatfish life histories, which were similar, than for the hake life history. Estimator performance was generally poorer in the presence of high recruitment variability.
Atrial fibrillation (AF) is the most common cardiac arrhythmia, and the total number of AF patients is constantly increasing. The mechanisms leading to and sustaining AF are not completely understood ...yet. Heterogeneities in atrial electrophysiology seem to play an important role in this context. Although some heterogeneities have been used in in-silico human atrial modeling studies, they have not been thoroughly investigated. In this study, the original electrophysiological (EP) models of Courtemanche et al., Nygren et al. and Maleckar et al. were adjusted to reproduce action potentials in 13 atrial regions. The parameter sets were validated against experimental action potential duration data and ECG data from patients with AV block. The use of the heterogeneous EP model led to a more synchronized repolarization sequence in a variety of 3D atrial anatomical models. Combination of the heterogeneous EP model with a model of persistent AF-remodeled electrophysiology led to a drastic change in cell electrophysiology. Simulated Ta-waves were significantly shorter under the remodeling. The heterogeneities in cell electrophysiology explain the previously observed Ta-wave effects. The results mark an important step toward the reliable simulation of the atrial repolarization sequence, give a deeper understanding of the mechanism of atrial repolarization and enable further clinical investigations.
Monte Carlo simulations and equilibrium analyses are used to explore the behavior of threshold management strategies. The analyses explore the medium- to long-term implications of uncertainty about ...the steepness of the stock–recruitment relationship, variation as well as possible temporal autocorrelation in recruitment, and estimation and implementation uncertainty. The results highlight the trade-offs among various management strategies, in particular the trade-offs between average catch, inter-annual variation in catch, and the risk of dropping below the Pacific Fishery Management Council (PFMC) overfished threshold of 25% of the average unfished spawning biomass. Uncertainty regarding steepness is the major source of variation in the final size of the resource and whether it is below the overfished threshold, although the extent of recruitment variability also impacts these quantities. The extent of inter-annual variation in catches is determined primarily by the amount of implementation error. Although the values for the various performance measures depend on all of the factors examined to some extent, the impact of interactions among these factors is generally small.
There is increasing consensus of the need for ecosystem-based fisheries management (EBFM), which accounts for trophic interactions and environmental conditions when managing exploited marine ...resources. Continued development and testing of analytical tools that are expected to address EBFM needs are essential for guiding the management of fisheries resources in achieving and balancing multiple social, economic, and conservation objectives. To address these needs, we present and compare alternative climate-informed multi-species statistical catch-at-age models to account for spatio-temporal differences in stock distributions, with application to four groundfish species (walleye pollock Gadus chalcogrammus, Pacific cod Gadus macrocephalus, arrowtooth flounder Atheresthes stomias, and Pacific halibut Hippoglossus stenolepis) in the Gulf of Alaska, USA. We integrate across multiple forms of uncertainty regarding the data and distribution of Pacific halibut using an ensemble modelling approach. Models developed here can be used to supplement current tactical fisheries management and inform on the trade-offs between harvesting across groundfish in the Gulf of Alaska. This approach may be applicable for other situations where spatial and temporal overlap is extensive among closely coupled species.
An emerging goal of national and international fisheries management organizations is the development of an ecosystem-based approach to fisheries management. The mechanisms determining changes in an ...ecosystem and their impacts on marine populations are not well established for most species, but the expectations of managers are that these impacts should be accounted for when selecting management strategies. Predation is an important component of ecosystem dynamics, and predator–prey functional responses can be used to represent predation in multi-species population dynamics models. The predators of walleye pollock,
Theragra chalcogramma, in the Gulf of Alaska include arrowtooth flounder (
Atheresthes stomias), Pacific cod (
Gadus macrocephalus), and Pacific halibut (
Hippoglossus stenolepis). Management strategy evaluation is used to examine the impact of changes over time in natural mortality-at-age related to trends in predator abundance on the performance of the current management strategy for the fishery for walleye pollock in the Gulf of Alaska. The current management strategy is evaluated under nine scenarios which capture uncertainty regarding the predator–prey functional relationship and the level of future fishing mortality on the predators. The current management strategy, which ignores the possibility of changes over time in natural mortality, kept the spawning biomass high relative to the target level and had a low risk of fishing mortality exceeding the overfishing limit, due to low catches relative to the total biomass.
Shen, H., Dorn, M. W., Wespestad, V., and Quinn, T. J. 2009. Schooling pattern of eastern Bering Sea walleye pollock and its effect on fishing behaviour. – ICES Journal of Marine Science, 66: ...1284–1288 Walleye pollock (Theragra chalcogramma) form persistent midwater and near-bottom schools in the daytime during the winter spawning season in the eastern Bering Sea (EBS). Two spawning areas in the EBS, north of Unimak Island and near the Pribilof Islands, are the main fishing grounds. To study the schooling pattern of pollock and its effect on fishing behaviour on these two fishing grounds, a principal component analysis with instrumental variables was carried out using acoustic and observer data from 2003 and 2005. Significant differences between the school descriptors distinguished the schooling patterns among areas and years. The harvester, that is to say, the fishing vessel and its crew taken together, searched for fish aggregations, which were caught in a different manner when the schooling pattern changed. School density had a greater effect than school size on fishing behaviour. Aggregations were less dense in 2003 than in 2005, and the harvester tended to fish with longer tows, at higher speeds, when it encountered less dense aggregations.
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