Evaluating the Effectiveness of Fish Stock Rebuilding Plans in the United States (2013)Ocean Studies Board
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Fish stock rebuilding plans have proven successful in reducing fishing pressure on many overfished stocks, and stock sizes have generally increased. However, in some cases fisheries have not rebuilt as quickly as the plans projected, due to factors such as overestimation of the size of stocks and incidental catch by fisheries targeting other species. Even when fishing is reduced appropriately for the actual stock size, the rate at which rebuilding occurs will depend on ecological and other environmental conditions. Because of all these factors, it is difficult to make accurate predictions of the time it will take stocks to rebuild. Therefore, rebuilding plans that focus more on meeting selected fishing mortality targets than on adhering to strict schedules for achieving rebuilding may be more robust to assessment uncertainties, environmental variability, and the effect of ecological interactions. Click here to view an interactive chart that shows how fishing limits are helping rebuild depleted fish stocks in U.S. waters.
- The concept of Maximum Sustainable Yield (MSY) can be useful in establishing population size and harvest rates that a population can sustain. Rebuilding plans based on MSY have proven successful for many stocks: of the 85 stocks declared overfished or approaching an overfished state between 1997 and 2012, 41 are no longer classified as overfished (of these, 11 are rebuilding and 30 are rebuilt). However, a further 41 stocks have not rebuilt yet, and are still classified as overfished. The committee identified several challenges associated with current rebuilding plans.
- Current rebuilding plans use a static metric of MSY, which in theory represents average conditions. In reality, ecosystems are dynamic and as a consequence MSY varies with factors such as changes in environmental conditions and ecological interactions. Generating reliable estimates of MSY depends on having extensive information about the biology of the species and its abundance in the years before it was overfished. This wealth of information is only available for a relatively few stocks, hence there is considerable uncertainty in the MSY estimates for most stocks.
- Scientists rely on a variety of sampling and statistical methods to estimate stock size, and to project future trends. The accuracy of the population estimates and the projections depends on the quality and amount of data available, and the ability of models to reproduce the primary determinants of the species’ population growth. As more data are collected and new models and assessment methods become available, past estimates of the status of fish stocks can change substantially.
- Climate, environmental conditions, and ecosystem interactions can have significant effects on the rate at which a stock rebuilds. Although these factors affect the time it takes for a stock to rebuild, it is not currently possible to incorporate them into models to improve projections to a degree of accuracy that is useful for management.
- When one stock within a mixed-stock fishery is declared overfished, reductions in fishing required by the rebuilding plans affect all the stocks in the fishery, leading to a loss of yield and income.
- Current rebuilding plans rely on a prescriptive approach, which has had demonstrated successes in identifying and rebuilding some fish stocks. However, the plans’ focus on achieving rebuilding targets within set timeframes forces reliance on forecasts and estimates of MSY-based reference points, which often carry a high level of uncertainty. Rebuilding plans that focus on meeting selected fishing mortality targets may be more effective than a plan with an exact time period for rebuilding. The report’s authoring committee made several suggestions for alternative management strategies for rebuilding fish stocks.
- Delaying reductions in annual catch until the stock biomass has declined to overfished status means that fishery managers must then make immediate and substantial decreases in fishing. Alternative management actions, such as harvest control rules that promptly but gradually reduce fishing as estimated stock size falls would result in a lower likelihood of a stock becoming overfished.
- Fishing mortality reference points seem to be more robust to uncertainty than biomass reference points, both in the context of rebuilding and more generally. Estimates of biomass are expected to change, and hopefully improve, as stocks rebuild -- but predicting these changes is difficult as it requires predicting average future recruitment of juvenile fish to the adult population. In contrast, setting reference points based on fishing mortality requires comparatively less information.
- Most fisheries select for the largest fish, and by doing so reduce the average age of the fished population. A rebuilding strategy based on maintaining reduced fishing mortality for an extended period—longer than the mean generation time of the stock—would help restore the stock’s age structure and would be more robust to changing environmental conditions than strategies that require rebuilding to pre-specified biomass targets by a given time limit.
- The Magnuson-Stevens Act specifies that stocks designated as overfished must rebuild within a maximum time period (usually 10 years). If rebuilding is slower on average than expected when the plan was adopted -- for example, due to unfavorable environmental conditions that affect recruitment of juveniles -- ever more severe controls on fishing must be applied to try to keep rebuilding "on schedule," often with significant socio-economic impacts to the fishing industry and community. Maintaining fishing at some constant level below the catch limit may help achieve rebuilding goals on a schedule that accommodates natural variability in stock productivity.
- Analytical assessments are not available for many of the stocks classified as overfished because there are not enough data and information to build the required models and to estimate MSY reference points. Without these data, catch limits are difficult to establish. In these cases, empirical rebuilding strategies that rely on input controls to reduce fishing -- for example, reductions in fishing operations, or closing fishing areas -- may be more effective and defensible than strategies based on annual catch limits and biomass targets.
- It is clear that reducing fishing to meet rebuilding targets can have severe social and economic impacts on fishing communities and the fishing industry. However, socio-economic information is not readily available to evaluate the broader and long-term impacts of rebuilding plans. Retrospective reviews of the socio-economic impacts of rebuilding plans would help in refining rebuilding plans and objectives, improving the consequences of future plans.