Michael J. Wilberg

Incorporating Time-Varying Catchability into Population Dynamic Stock Assessment Models

Catchability is an important parameter in many stock assessment models because it relates an index of abundance to stock size. We review the theory and evidence for time-varying catchability, its effects on stock assessment estimates, and methods to include time-varying catchability in stock assessments. Numerous studies provide strong evidence that time-varying catchability is common in most fisheries and many fishery-independent surveys and can be caused by anthropogenic, environmental, biological, and management processes. Trends in catchability over time can cause biased estimates of stock size and fishing mortality rates in stock assessment models that do not compensate for them. Methods that use descriptive and functional relationships have been developed to incorporate time-varying catchability in stock assessment models. We recommend that the default assumption for stock assessments should be that catchability varies over time and that multiple methods of including time-varying catchability should be applied. Additional studies are needed to determine relative performance of alternative methods and to develop methods for selecting among models.

Yellow Perch Dynamics in Southwestern Lake Michigan during 1986–2002

Abstract We examined the role of harvest in the collapse of the population of yellow perch Perca flavescens in southwestern Lake Michigan during the mid to late 1990s. After the great decrease in this population at that time, commercial fisheries in Illinois and Wisconsin were closed during 1996–1997 (and have remained closed), and stricter regulations were placed on recreational fisheries. Reproductive failure has been implicated as the primary cause of the population collapse, but the role of fishing in the collapse was not rigorously investigated in previous studies. We conducted an age-, size-, and sex-structured stock assessment of yellow perch to estimate population size and examine historical trends in fishing mortality in Illinois and Wisconsin waters of southwestern Lake Michigan. Model estimates indicated that yellow perch abundance in 2002 was less than 10% of the 1986 abundance in Wisconsin and about 20% of the respective population in Illinois. Annual mortality rates for females age 4 and old...

Comment on "Impacts of Biodiversity Loss on Ocean Ecosystem Services"

Worm et al. (Research Articles, 3 November 2006, p. 787) used a power relation to predict a global collapse of fisheries by the year 2048. However, a linear regression of the data for the past 40 years yields an excellent fit, with a predicted date of collapse of 2114. Thus, long-term projections of fisheries collapse are highly dependent on the specific statistical model used.

Applying Structured Decision Making to Recreational Fisheries Management

Abstract Multiple objectives associated with recreational fishing, combined with multiple uncertainties, pose real challenges for management. We suggest that applying formal decision-making frameworks to partnership-based policy evaluations can prove beneficial to recreational fisheries management. We describe how a sequence of workshops can be used to engage scientists, managers, and other stakeholders and execute the steps of a structured decision-making process that centers on collaborative development of a quantitative forecasting model. This approach should aim to specify objectives and corresponding performance measures, identify critical uncertainties, and aid decision makers in making more informed choices among possible management options. An inclusive, participatory process can increase the transparency by which management decisions are made, provide an opportunity for tradeoffs to be discussed in a tangible way, and promote consensus building. Constructing and documenting a well-specified repre...

FishSmart: An Innovative Role for Science in Stakeholder- Centered Approaches to Fisheries Management

Until recently, marine fisheries managers have predominately interacted with a single user group—commercial fisheries. However, changes in participation in fisheries and progress toward ecosystem-based approaches have introduced new stakeholders into the management process. Yet, there are few examples of successful approaches of how to engage the spectrum of stakeholders interested in management policy and decisions. Here we describe one such approach that was used in the fisheries for king mackerel (Scomberomorus cavalla) along the U.S. southeast coast. The approach combined consensus building in facilitated workshops and decision analysis in which stakeholders could compare the consequences of alternative management options on trends in the king mackerel population and the fisheries it supports. The process resulted in a workgroup of stakeholders that developed a clear vision for its desired future of king mackerel fisheries and several alternative management options. Decision analysis was used to selec...

Demographics and Parasitism of American Eels in the Chesapeake Bay, USA

Abstract The Chesapeake Bay supports the largest U.S. harvest of American eels Anguilla rostrata, yet little is known about the underlying demographics and production rates that sustain these harvests. Chesapeake Bay subestuaries (Sassafras, Chester, Choptank, Patuxent, Potomac, and James rivers) are expected to provide productive but varying growth habitats for yellow-stage American eels due to differences in land use, prey availability, and salinity. We compared length, age, growth, mortality, condition, and health (prevalence and incidence of parasitism by the swim bladder nematode Anguillicola crassus) of yellow American eels (n = 850) in the six principal subestuaries. As has been observed for other systems, female American eels in the Chesapeake Bay were larger, older, and heavier and had higher growth rates than male, intersexual, or undifferentiated American eels. Prevalence of male and intersexual individuals in the upper bay region was higher than that in the lower bay and higher than that repor...

Surplus Production Model Accuracy in Populations Affected by a No-Take Marine Protected Area

Abstract Marine protected areas (MPAs) are increasingly used as a tool in fisheries management. However, implementation of an MPA violates common assumptions for fishery stock assessments that provide estimates of abundance and fishing mortality for management. Thus, it is important to understand the effects of MPAs on estimates from stock assessments. We conducted a simulation study to determine the effects of MPAs on accuracy of surplus production model (SPM) stock assessments. We simulated the dynamics of a population that had part of its range in an MPA, and we assessed the population with spatially aggregated and spatially explicit SPMs under a range of conditions, including different MPA sizes (percentage of the total stock area), different rates of migration between MPA and non-MPA regions, and scenarios with high and low observation error in the indices of abundance. We also considered a scenario in which there was no available index of abundance within the MPA. We used the median of the absolute value of relative error and the median relative error from 200 replicates/scenario to test SPM accuracy. In most cases, spatially explicit SPMs performed better in both accuracy and bias than spatially aggregated SPMs. The accuracy of the assessments also increased as MPA size increased except under the scenario of no index of abundance within the MPA; for that scenario, accuracy increased as MPA size decreased. Monitoring of the stock within the MPA is essential for conducting accurate stock assessments in areas with MPAs.

Survival of Juvenile Lake Trout Stocked in Western Lake Huron during 1974–1992

Abstract The population of lake trout Salvelinus namaycush in the main basin of Lake Huron collapsed in the late 1940s from the combined effects of overfishing and predation by sea lampreys Petromyzon marinus. Stocking juvenile lake trout has been one of the key management strategies in efforts toward lake trout rehabilitation. However, the survival of juvenile stocked lake trout has decreased over time in lakes Erie, Ontario, and Superior. We examined catch per effort (CPE) for age-5 fish divided by the number of that year-class that was previously stocked to determine whether the survival of juvenile stocked lake trout changed in Michigan waters of Lake Huron. During 1974–1992, this standardized CPE decreased significantly in the northwestern part of Lake Huron but did not change significantly in the central or southwestern parts of the lake. The trend in the northwestern part of the lake probably reflects decreasing juvenile survival attributable to increases in sea lamprey abundance. Our study suggest...

Sustainable exploitation and management of autogenic ecosystem engineers: application to oysters in Chesapeake Bay

Autogenic ecosystem engineers are critically important parts of many marine and estuarine systems because of their substantial effect on ecosystem services. Oysters are of particular importance because of their capacity to modify coastal and estuarine habitats and the highly degraded status of their habitats worldwide. However, models to predict dynamics of ecosystem engineers have not previously included the effects of exploitation. We developed a linked population and habitat model for autogenic ecosystem engineers undergoing exploitation. We parameterized the model to represent eastern oyster (Crassostrea virginica) in upper Chesapeake Bay by selecting sets of parameter values that matched observed rates of change in abundance and habitat. We used the model to evaluate the effects of a range of management and restoration options including sustainability of historical fishing pressure, effectiveness of a newly enacted sanctuary program, and relative performance of two restoration approaches. In general, autogenic ecosystem engineers are expected to be substantially less resilient to fishing than an equivalent species that does not rely on itself for habitat. Historical fishing mortality rates in upper Chesapeake Bay for oysters were above the levels that would lead to extirpation. Reductions in fishing or closure of the fishery were projected to lead to long-term increases in abundance and habitat. For fisheries to become sustainable outside of sanctuaries, a substantial larval subsidy would be required from oysters within sanctuaries. Restoration efforts using high-relief reefs were predicted to allow recovery within a shorter period of time than low-relief reefs. Models such as ours, that allow for feedbacks between population and habitat dynamics, can be effective tools for guiding management and restoration of autogenic ecosystem engineers.

Trends in Abundance Indices of Fishes in Maryland’s Coastal Bays During 1972–2009

Maryland’s coastal bays provide habitat for juveniles of many commercially and recreationally important species of shellfish and finfish. Since 1972, the Maryland Department of Natural Resources has conducted the Maryland Coastal Bays Trawl and Seine Survey to monitor the populations of key species. The survey has undergone substantial spatial and methodological changes affecting the interpretation of simple indices of abundance. We developed generalized linear models to standardize the indices of abundance of five commonly caught fish species: Atlantic menhaden Brevoortia tyrannus, weakfish Cynoscion regalis, spot Leiostomus xanthurus, bay anchovy Anchoa mitchilli, and summer flounder Paralichthys dentatus. Density declined significantly since 1972 for menhaden, bay anchovy, and spot in at least one region within the coastal bays. The northern bays had significantly higher densities than the southern bays for all species. Changes in abundance indices of the five species examined were not related to sea grass coverage, temperature, salinity, nitrogen-to-phosphorus ratios, and other habitat variables but were likely a result of stock-wide recruitment processes.