James Bence | Michigan State University (original) (raw)

Papers by James Bence

Journal of Heredity

Recent assessments indicate the emergence of naturally produced lake trout (Salvelinus namaycush)... more Recent assessments indicate the emergence of naturally produced lake trout (Salvelinus namaycush) recruitment throughout Lake Huron in the North American Laurentian Great Lakes (>50% of fish <7 years). Because naturally produced fish derived from different stocked hatchery strains are unmarked, managers cannot distinguish strains contributing to natural recruitment. We used 15 microsatellite loci to identify strains of naturally produced lake trout (N = 1567) collected in assessment fisheries during early (2002-2004) and late (2009-2012) sampling periods. Individuals from 13 American and Canadian hatchery strains (N = 1143) were genotyped to develop standardized baseline information. Strain contributions were estimated using a Bayesian inferential approach. Deviance information criteria were used to compare models evaluating strain contributions at different spatial and temporal scales. The best performing models were the most complex models, suggesting that hatchery strain contributions to naturally produced lake trout varied spatially among management districts and temporally between time periods. Contributions of Seneca strain lake trout were consistently high across most management districts, with contributions increasing from early to late time periods (estimates ranged from 52% to 94% for the late period across 8 of 9 districts). Strain contributions deviated from expectations based on historical stocking levels, indicating strains differed with respect to survival, reproductive success, and/or dispersal. Knowledge of recruitment levels of strains stocked in different management districts, and how strain-specific recruitment varies temporally, spatially, and as a function of local or regional stocking is important to prioritize strains for future stocking and management of the transition process from primarily hatchery to naturally produced stocks.

Journal of Great Lakes Research

Ecological applications : a publication of the Ecological Society of America, Jan 22, 2017

Identifying appropriate strategies for sustainable harvest is a challenge for many terrestrial ve... more Identifying appropriate strategies for sustainable harvest is a challenge for many terrestrial vertebrate species because of uncertain system dynamics, limited data to inform population models, and potentially conflicting objectives that seek to harvest and maintain populations at desirable levels. The absence of monitoring and assessment infrastructure needed to regularly estimate abundance accentuates this challenge for many species, and limits application of rigorous state-dependent frameworks for decision making that are commonly advocated in natural resource management. Reference points, which define management targets or triggers for changing management, are often used to guide decision-making, but suffer from ambiguity when developed without explicit consideration of uncertainty or trade-offs among competing objectives. We describe an approach for developing unambiguous target reference points for assessment-limited species using structured decision making, and demonstrate th...

ABSTRACT The influence of stock intermixing on harvest policy performance is a major concern to f... more ABSTRACT The influence of stock intermixing on harvest policy performance is a major concern to fisheries managers because of the potential to overharvest low productivity stocks. If productivity of mixing stocks is not equal, harvest of fish originating in more productive stocks can overshadow declining harvest attributable to less productive stocks, leading to unnoticed depletion of less productive stocks. These “hidden” collapses could lead to sub-optimal fishery outcomes and loss of genetic diversity. Recent research in the Great Lakes suggests that intermixing of distinct lake whitefish (Coregonus clupeaformis) spawning stocks during commercial fishing seasons has increased in recent decades, raising concerns regarding sustainability of some of these stocks. Management agencies in the Great Lakes have already developed plans to move toward managing some intermixing stocks based on pooled assessments, but insufficient research has been conducted to support this action. Our analysis used a management strategy evaluation approach to explore how the current 65% constant-mortality harvest control rule might affect stock sustainability and fishery yield under differing levels of mixing and stock productivity. We also investigated several alternative harvest control rules to compare the existing control rule to more conservative alternatives. For this research, we developed a stochastic simulation model based loosely on lake whitefish stocks in northern lakes Huron and Michigan that incorporated all aspects of the management system (population dynamics, data collection, stock assessment, control rule implementation). Harvest policy performance was evaluated based on mean commercial fishery yield, inter-annual variation in commercial fishery yield, and proportion of years spawning stock biomass declined to less than 20% that of unfished spawning stock biomass (a proxy for risk). Under these performance metrics, desirable attributes of a harvest policy are high and stable yields and low risk of depleting stock biomass. Our analysis suggests that low productivity stocks generally exhibit low average yields and greater risk under the existing control rule when compared to intermediate and high productivity stocks. We also found that patterns of intermixing influenced the performance of control rules for stocks with differed productivity characteristics. Management that accounts for both productivity and rate of intermixing has a better chance of providing desirable fishery outcomes.

Dynamics of growth in the early life history of shortbelly rockfish Sebastes jordani. ... The rel... more Dynamics of growth in the early life history of shortbelly rockfish Sebastes jordani. ... The relationship between length and age of larval and juvenile shortbelly rockfish Sebastes jordani , determined from otolith microstructure, is complex. ...

Resource and Energy Economics, Nov 1, 2010

ABSTRACT We developed offshore fish community models for lakes Huron and Ontario to forecast the ... more ABSTRACT We developed offshore fish community models for lakes Huron and Ontario to forecast the potential consequences to changes in Pacific salmonid Oncorhynchus stocking policies by the states of New York and Michigan and the province of Ontario. The Lake Huron forecasting model included Chinook salmon Oncorhynchus tshawytscha, lake trout Salvelinus namaycush, steelhead Oncorhynchus mykiss, walleye Sander vitreus, and burbot Lota lota as predators; the Lake Ontario forecasting model included Chinook salmon, lake trout, steelhead, Atlantic salmon Salmo salar, brown trout Salmo trutta, and coho salmon Oncorhynchus kisutch as predators. Alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax were the primary prey species for both models. Evaluated stocking policies ranged from a complete cessation in stocking of all Pacific salmonids to a doubling of the 2003-2005 average stocking rate. Policies were evaluated using metrics that captured the desire to have high, stable catches of lake trout and Chinook salmon and large sizes of Chinook salmon. The susceptibility of alewife and rainbow smelt populations being driven to low adult abundance levels was also measured for the policy evaluations simulations. We found that for Lake Huron decreases in Pacific salmonid stocking rates were predicted to increase biomass and fishery yield of lake trout and age-3 Chinook salmon spawning weights throughout the lake; additionally, recreational yield of Chinook salmon was predicted to increase in the Main Basin of Lake Huron. Biomass of alewife and rainbow smelt biomass also increased with decreases in Pacific salmonid stocking rates; however, even with a complete cessation of all Pacific salmon stocking, there still remained a high probability of prey abundance declining to below threshold levels as a result of presumably high wild recruit of Chinook salmon. Results will be discussed in context of large fish community changes that have recently occurred in Lake Huron. The fish community of Lake Ontario was relatively unaffected by changes in Pacific salmon stocking policies because of higher prey productivity and presumably low wild recruitment of Chinook salmon. The results for both lakes Huron and Ontario were sensitive to assumptions about predator wild recruitment, predator search efficiencies, and scaling of prey stock-recruitment relationships. These sensitivities should be kept in mind in any proposed changes in Pacific salmonid stocking rates.

Http Dx Doi Org 10 1577 T07 012 1, Jan 9, 2011

Commonly used approaches to studying mass-length relations and condition indices often do not ade... more Commonly used approaches to studying mass-length relations and condition indices often do not adequately address covariance between mass-length parameters, usually ignore heterogeneity in individual variance for body mass at a given length, and assume that length distributions of fish samples are similar across regions and years. We used body mass at selected lengths as condition indices based on statistical modeling and a hierarchical Bayesian approach to inferences, and our approach allowed us to avoid using restrictive assumptions. We estimated spatial and annual variation in mass-length relations, where the process errors in parameters are drawn from a multivariate distribution. We also estimated region-, year-, and size-group-specific variance for individual variation in mass at given lengths. We applied our approach to study mass-length relations of lake trout Salvelinus namaycush (1977-2005) and Chinook salmon Oncorhynchus tshawytscha (1983-2004) in U.S. waters of Lake Huron. We found that small lake trout were able to maintain their condition in response to declines in prey availability, whereas Chinook salmon of the same size could not. Similarly, large Chinook salmon exhibited a greater and more rapid decline in body condition than did lake trout of the same size. We also found that spatial differences in recent temporal trends in condition of large fish altered the ranks of the three lake trout management regions. Ignoring spatial heterogeneity led to a biased overall average mass-length relation for lake trout. The large temporal variation in mass-length parameters suggested that it was better not to assume isometric growth for a specific year, even though the overall average exponent for Chinook salmon was nearly 3.0. In our study, conventional application of Fulton's condition factor to a relatively narrow size range was able to capture qualitative longterm trends but missed some important short-term changes because of the additional variation of lengthrelated bias and the limitation of sample sizes. Mass-length relations and condition indices constitute a basis for studying physiological condition of fish

Http Dx Doi Org 10 1577 1548 8675019 2 0 Co 2, Jan 8, 2011

Populations of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the 1... more Populations of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the 1940s because of predation by sea lampreys Petromyzon marinus and commercial fishing. Efforts to rehabilitate lake trout have emphasized reduction of mortality and the stocking of hatchery-reared lake trout to reestablish populations. We fit a statistical catch-at-age model for lake trout in the southern main basin of Lake Huron using a maximum likelihood approach to estimate mortality rates and abundance during 1984–1993. This represents the first such analysis for lake trout in the Great Lakes, and a flexible application of the approach proved useful for integrating diverse information and assessing population and mortality trends. Sea lamprey-induced mortality and recruitment of lake trout to age 1 were calculated external to model fitting. Recruitment was based on numbers of lake trout stocked because natural recruitment is negligible. Sea lamprey-induced mortality rates were based on observed wounding data on lake trout. Other mortality rates were estimated during model fitting. Mortality rates have varied from year to year but do not show trends over time and are currently well below target maximum levels. Lake trout abundance has declined from a peak spawning biomass of 412,000 kg in 1988 to a low of 295,000 kg in 1993. This decline is a result of lower stocking levels since the mid-1980s. Sea lamprey predation was the dominant source of mortality for lake trout older than age 4. During 1984–1993, we estimated that sea lamprey predation caused 48% and fishing accounted for 16% of deaths of lake trout older than age 2. Although our estimates of sea lamprey-induced mortality are uncertain, current estimates of lethality of sea lamprey attacks would need to be greatly overestimated to overturn our conclusion that sea lamprey mortality has exceeded fishing mortality in southern Lake Huron.

ABSTRACT Over the past 20 years I have helped develop a range of age-structured stock assessments... more ABSTRACT Over the past 20 years I have helped develop a range of age-structured stock assessments in the Laurentian Great Lakes, including for multiple stocks of Chinook Salmon, Lake Trout, Lake Whitefish, Walleye, and Yellow Perch. During this time period the lakes have experienced large environmental and food web changes. These changes have led to alterations in parameters such as selectivity, catchability, and natural mortality. Given the need to allow for temporally varying parameters I participated in simulation studies that evaluated approaches for allowing for such variation, and the results suggested that random walks could capture such changes, with little cost when such changes were not occurring. For real stocks, however, multiple parameters are potentially changing simultaneously. When this is the case modeling multiple parameters by random walks can lead to problematic results, because temporal changes may be captured by the wrong parameter. This can sometimes be identified via retrospective patterns. As an alternative we have worked to capture temporal changes via functions of predictors, rather than as stochastic processes. These issues and thoughts for the next generation of stock assessments will be illustrated with examples from the assessments.

Detecting Ecological Impacts, 1996

Many invertebrate and finfish species are difficult to assess using traditional stock assessment ... more Many invertebrate and finfish species are difficult to assess using traditional stock assessment models. We use a direct approach to set bounds and estimate uncertainty for survey catchability, stock biomass, and fishing mortality. Bounds and uncertainty for inshore longfin squid are based on a vessel calibration study, trawl sensor data, biological information, expert judgment, and capture efficiency. Prior distributions for each factor can be used to calculate a composite prior distribution for survey catchability. This composite prior for squid is asymmetric with a broad flat region implying a range of values that are almost equally probable. There are interesting mathematical and policy aspects because it is easier to make precise assumptions about the upper bound of survey catchability (lower bound for biomass) than the lower bound for survey catchability (upper bound for biomass). Our method could be applied to other stocks, and is especially useful when information about area...

We explored approaches to allowing time-varying catchability, selectivity, and natural mortality ... more We explored approaches to allowing time-varying catchability, selectivity, and natural mortality in statistical catch at age (SCAA) assessment models for lake trout Salvelinus namaycush in the main basin of Lake Huron. Lake trout was the top native predator in the North America Great Lakes, but was nearly extirpated by the early 1950s due to excessive mortality from sea lamprey Petromyzon marinus predation and fishery harvest. Sea lamprey control, hatchery stocking, and fishery regulation have been the three primary strategies for lake trout rehabilitation in the Great Lakes. Despite these efforts, nearly all recruitment to the population over the period modeled (1984-2010) was from releases of hatchery-reared fish. During this period Lake Huron experienced substantial changes in the food web and the fisheries system. We conducted SCAA assessments in three regions of the main basin of Lake Huron. The assessment models allow for time-varying catchability for both of the two fisheries...

Journal of Heredity

Recent assessments indicate the emergence of naturally produced lake trout (Salvelinus namaycush)... more Recent assessments indicate the emergence of naturally produced lake trout (Salvelinus namaycush) recruitment throughout Lake Huron in the North American Laurentian Great Lakes (>50% of fish <7 years). Because naturally produced fish derived from different stocked hatchery strains are unmarked, managers cannot distinguish strains contributing to natural recruitment. We used 15 microsatellite loci to identify strains of naturally produced lake trout (N = 1567) collected in assessment fisheries during early (2002-2004) and late (2009-2012) sampling periods. Individuals from 13 American and Canadian hatchery strains (N = 1143) were genotyped to develop standardized baseline information. Strain contributions were estimated using a Bayesian inferential approach. Deviance information criteria were used to compare models evaluating strain contributions at different spatial and temporal scales. The best performing models were the most complex models, suggesting that hatchery strain contributions to naturally produced lake trout varied spatially among management districts and temporally between time periods. Contributions of Seneca strain lake trout were consistently high across most management districts, with contributions increasing from early to late time periods (estimates ranged from 52% to 94% for the late period across 8 of 9 districts). Strain contributions deviated from expectations based on historical stocking levels, indicating strains differed with respect to survival, reproductive success, and/or dispersal. Knowledge of recruitment levels of strains stocked in different management districts, and how strain-specific recruitment varies temporally, spatially, and as a function of local or regional stocking is important to prioritize strains for future stocking and management of the transition process from primarily hatchery to naturally produced stocks.

Journal of Great Lakes Research

Ecological applications : a publication of the Ecological Society of America, Jan 22, 2017

Identifying appropriate strategies for sustainable harvest is a challenge for many terrestrial ve... more Identifying appropriate strategies for sustainable harvest is a challenge for many terrestrial vertebrate species because of uncertain system dynamics, limited data to inform population models, and potentially conflicting objectives that seek to harvest and maintain populations at desirable levels. The absence of monitoring and assessment infrastructure needed to regularly estimate abundance accentuates this challenge for many species, and limits application of rigorous state-dependent frameworks for decision making that are commonly advocated in natural resource management. Reference points, which define management targets or triggers for changing management, are often used to guide decision-making, but suffer from ambiguity when developed without explicit consideration of uncertainty or trade-offs among competing objectives. We describe an approach for developing unambiguous target reference points for assessment-limited species using structured decision making, and demonstrate th...

ABSTRACT The influence of stock intermixing on harvest policy performance is a major concern to f... more ABSTRACT The influence of stock intermixing on harvest policy performance is a major concern to fisheries managers because of the potential to overharvest low productivity stocks. If productivity of mixing stocks is not equal, harvest of fish originating in more productive stocks can overshadow declining harvest attributable to less productive stocks, leading to unnoticed depletion of less productive stocks. These “hidden” collapses could lead to sub-optimal fishery outcomes and loss of genetic diversity. Recent research in the Great Lakes suggests that intermixing of distinct lake whitefish (Coregonus clupeaformis) spawning stocks during commercial fishing seasons has increased in recent decades, raising concerns regarding sustainability of some of these stocks. Management agencies in the Great Lakes have already developed plans to move toward managing some intermixing stocks based on pooled assessments, but insufficient research has been conducted to support this action. Our analysis used a management strategy evaluation approach to explore how the current 65% constant-mortality harvest control rule might affect stock sustainability and fishery yield under differing levels of mixing and stock productivity. We also investigated several alternative harvest control rules to compare the existing control rule to more conservative alternatives. For this research, we developed a stochastic simulation model based loosely on lake whitefish stocks in northern lakes Huron and Michigan that incorporated all aspects of the management system (population dynamics, data collection, stock assessment, control rule implementation). Harvest policy performance was evaluated based on mean commercial fishery yield, inter-annual variation in commercial fishery yield, and proportion of years spawning stock biomass declined to less than 20% that of unfished spawning stock biomass (a proxy for risk). Under these performance metrics, desirable attributes of a harvest policy are high and stable yields and low risk of depleting stock biomass. Our analysis suggests that low productivity stocks generally exhibit low average yields and greater risk under the existing control rule when compared to intermediate and high productivity stocks. We also found that patterns of intermixing influenced the performance of control rules for stocks with differed productivity characteristics. Management that accounts for both productivity and rate of intermixing has a better chance of providing desirable fishery outcomes.

Dynamics of growth in the early life history of shortbelly rockfish Sebastes jordani. ... The rel... more Dynamics of growth in the early life history of shortbelly rockfish Sebastes jordani. ... The relationship between length and age of larval and juvenile shortbelly rockfish Sebastes jordani , determined from otolith microstructure, is complex. ...

Resource and Energy Economics, Nov 1, 2010

ABSTRACT We developed offshore fish community models for lakes Huron and Ontario to forecast the ... more ABSTRACT We developed offshore fish community models for lakes Huron and Ontario to forecast the potential consequences to changes in Pacific salmonid Oncorhynchus stocking policies by the states of New York and Michigan and the province of Ontario. The Lake Huron forecasting model included Chinook salmon Oncorhynchus tshawytscha, lake trout Salvelinus namaycush, steelhead Oncorhynchus mykiss, walleye Sander vitreus, and burbot Lota lota as predators; the Lake Ontario forecasting model included Chinook salmon, lake trout, steelhead, Atlantic salmon Salmo salar, brown trout Salmo trutta, and coho salmon Oncorhynchus kisutch as predators. Alewife Alosa pseudoharengus and rainbow smelt Osmerus mordax were the primary prey species for both models. Evaluated stocking policies ranged from a complete cessation in stocking of all Pacific salmonids to a doubling of the 2003-2005 average stocking rate. Policies were evaluated using metrics that captured the desire to have high, stable catches of lake trout and Chinook salmon and large sizes of Chinook salmon. The susceptibility of alewife and rainbow smelt populations being driven to low adult abundance levels was also measured for the policy evaluations simulations. We found that for Lake Huron decreases in Pacific salmonid stocking rates were predicted to increase biomass and fishery yield of lake trout and age-3 Chinook salmon spawning weights throughout the lake; additionally, recreational yield of Chinook salmon was predicted to increase in the Main Basin of Lake Huron. Biomass of alewife and rainbow smelt biomass also increased with decreases in Pacific salmonid stocking rates; however, even with a complete cessation of all Pacific salmon stocking, there still remained a high probability of prey abundance declining to below threshold levels as a result of presumably high wild recruit of Chinook salmon. Results will be discussed in context of large fish community changes that have recently occurred in Lake Huron. The fish community of Lake Ontario was relatively unaffected by changes in Pacific salmon stocking policies because of higher prey productivity and presumably low wild recruitment of Chinook salmon. The results for both lakes Huron and Ontario were sensitive to assumptions about predator wild recruitment, predator search efficiencies, and scaling of prey stock-recruitment relationships. These sensitivities should be kept in mind in any proposed changes in Pacific salmonid stocking rates.

Http Dx Doi Org 10 1577 T07 012 1, Jan 9, 2011

Commonly used approaches to studying mass-length relations and condition indices often do not ade... more Commonly used approaches to studying mass-length relations and condition indices often do not adequately address covariance between mass-length parameters, usually ignore heterogeneity in individual variance for body mass at a given length, and assume that length distributions of fish samples are similar across regions and years. We used body mass at selected lengths as condition indices based on statistical modeling and a hierarchical Bayesian approach to inferences, and our approach allowed us to avoid using restrictive assumptions. We estimated spatial and annual variation in mass-length relations, where the process errors in parameters are drawn from a multivariate distribution. We also estimated region-, year-, and size-group-specific variance for individual variation in mass at given lengths. We applied our approach to study mass-length relations of lake trout Salvelinus namaycush (1977-2005) and Chinook salmon Oncorhynchus tshawytscha (1983-2004) in U.S. waters of Lake Huron. We found that small lake trout were able to maintain their condition in response to declines in prey availability, whereas Chinook salmon of the same size could not. Similarly, large Chinook salmon exhibited a greater and more rapid decline in body condition than did lake trout of the same size. We also found that spatial differences in recent temporal trends in condition of large fish altered the ranks of the three lake trout management regions. Ignoring spatial heterogeneity led to a biased overall average mass-length relation for lake trout. The large temporal variation in mass-length parameters suggested that it was better not to assume isometric growth for a specific year, even though the overall average exponent for Chinook salmon was nearly 3.0. In our study, conventional application of Fulton's condition factor to a relatively narrow size range was able to capture qualitative longterm trends but missed some important short-term changes because of the additional variation of lengthrelated bias and the limitation of sample sizes. Mass-length relations and condition indices constitute a basis for studying physiological condition of fish

Http Dx Doi Org 10 1577 1548 8675019 2 0 Co 2, Jan 8, 2011

Populations of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the 1... more Populations of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the 1940s because of predation by sea lampreys Petromyzon marinus and commercial fishing. Efforts to rehabilitate lake trout have emphasized reduction of mortality and the stocking of hatchery-reared lake trout to reestablish populations. We fit a statistical catch-at-age model for lake trout in the southern main basin of Lake Huron using a maximum likelihood approach to estimate mortality rates and abundance during 1984–1993. This represents the first such analysis for lake trout in the Great Lakes, and a flexible application of the approach proved useful for integrating diverse information and assessing population and mortality trends. Sea lamprey-induced mortality and recruitment of lake trout to age 1 were calculated external to model fitting. Recruitment was based on numbers of lake trout stocked because natural recruitment is negligible. Sea lamprey-induced mortality rates were based on observed wounding data on lake trout. Other mortality rates were estimated during model fitting. Mortality rates have varied from year to year but do not show trends over time and are currently well below target maximum levels. Lake trout abundance has declined from a peak spawning biomass of 412,000 kg in 1988 to a low of 295,000 kg in 1993. This decline is a result of lower stocking levels since the mid-1980s. Sea lamprey predation was the dominant source of mortality for lake trout older than age 4. During 1984–1993, we estimated that sea lamprey predation caused 48% and fishing accounted for 16% of deaths of lake trout older than age 2. Although our estimates of sea lamprey-induced mortality are uncertain, current estimates of lethality of sea lamprey attacks would need to be greatly overestimated to overturn our conclusion that sea lamprey mortality has exceeded fishing mortality in southern Lake Huron.

ABSTRACT Over the past 20 years I have helped develop a range of age-structured stock assessments... more ABSTRACT Over the past 20 years I have helped develop a range of age-structured stock assessments in the Laurentian Great Lakes, including for multiple stocks of Chinook Salmon, Lake Trout, Lake Whitefish, Walleye, and Yellow Perch. During this time period the lakes have experienced large environmental and food web changes. These changes have led to alterations in parameters such as selectivity, catchability, and natural mortality. Given the need to allow for temporally varying parameters I participated in simulation studies that evaluated approaches for allowing for such variation, and the results suggested that random walks could capture such changes, with little cost when such changes were not occurring. For real stocks, however, multiple parameters are potentially changing simultaneously. When this is the case modeling multiple parameters by random walks can lead to problematic results, because temporal changes may be captured by the wrong parameter. This can sometimes be identified via retrospective patterns. As an alternative we have worked to capture temporal changes via functions of predictors, rather than as stochastic processes. These issues and thoughts for the next generation of stock assessments will be illustrated with examples from the assessments.

Detecting Ecological Impacts, 1996

Many invertebrate and finfish species are difficult to assess using traditional stock assessment ... more Many invertebrate and finfish species are difficult to assess using traditional stock assessment models. We use a direct approach to set bounds and estimate uncertainty for survey catchability, stock biomass, and fishing mortality. Bounds and uncertainty for inshore longfin squid are based on a vessel calibration study, trawl sensor data, biological information, expert judgment, and capture efficiency. Prior distributions for each factor can be used to calculate a composite prior distribution for survey catchability. This composite prior for squid is asymmetric with a broad flat region implying a range of values that are almost equally probable. There are interesting mathematical and policy aspects because it is easier to make precise assumptions about the upper bound of survey catchability (lower bound for biomass) than the lower bound for survey catchability (upper bound for biomass). Our method could be applied to other stocks, and is especially useful when information about area...

We explored approaches to allowing time-varying catchability, selectivity, and natural mortality ... more We explored approaches to allowing time-varying catchability, selectivity, and natural mortality in statistical catch at age (SCAA) assessment models for lake trout Salvelinus namaycush in the main basin of Lake Huron. Lake trout was the top native predator in the North America Great Lakes, but was nearly extirpated by the early 1950s due to excessive mortality from sea lamprey Petromyzon marinus predation and fishery harvest. Sea lamprey control, hatchery stocking, and fishery regulation have been the three primary strategies for lake trout rehabilitation in the Great Lakes. Despite these efforts, nearly all recruitment to the population over the period modeled (1984-2010) was from releases of hatchery-reared fish. During this period Lake Huron experienced substantial changes in the food web and the fisheries system. We conducted SCAA assessments in three regions of the main basin of Lake Huron. The assessment models allow for time-varying catchability for both of the two fisheries...