Thomas R. Carruthers

Erratum: Simulating spatial dynamics to evaluate methods of deriving abundance indices for tropical tunas

Relative abundance indices derived from nominal catch-per-unit-effort (CPUE) data are a principle source of information for the majority of stock assessments. A particular problem with formulating such abundance indices for pelagic species such as tuna is the interpretation of CPUE data from fleets that have changed distribution over time. In this research, spatial population dynamics are simulated to test the historical pattern of fishing effort as a basis for making inferences about relative abundance. A number of age-structured, spatially disaggregated population dynamics models are described for both Atlantic yellowfin tuna (Thunnus albacares) and bigeye tuna (Thunnus obesus) to account for uncertainty in spatial distribution and movement. These models are used to evaluate the reliability of standardization methods and a commonly applied model selection criterion, Akaike’s information criterion (AIC). The simulations demonstrate the pitfalls of aggregating CPUE data over spatial areas and highlight th...

An Evaluation of Rebuilding Policies for U.S. Fisheries.

Rebuilding depleted fish populations is a priority of modern fisheries management. In the U.S., strong statutory mandates extend to both the goals and process by which stocks are to be rebuilt. However, the National Standard Guidelines that govern the implementation of the Magnuson-Stevens Fishery Conservation and Management Act may change to increase flexibility in rebuilding requirements. In this study we evaluate performance of the status quo approach to fish stock rebuilding in the United States against 3 alternatives that have been proposed to improve rebuilding outcomes. These alternatives either simplify the analytical requirements of rebuilding analyses or apply ‘best practices’ in fisheries management, thereby avoiding the need for rebuilding analyses altogether. We use a Management Strategy Evaluation framework to evaluate rebuilding options across 6 fish life history types and 5 possible real-world fishery scenarios that include options for stock assessment quality, multiple fleets, and the degree to which the stocks are overfished at the start of the analysis. We show that the status quo rebuilding plan and a harvest control rule that reduces harvest rates at low stock size generally achieve the best rebuilding outcomes across all life-history types and fishery scenarios. Both approaches constrain fishing in the short term, but achieve high catches in the medium and long term as stocks rebuild to productive levels. These results support a growing body of literature that indicates that efforts to end overfishing early pay off in the medium- to long-term with higher cumulative catches than the alternative.

The Data‐Limited Methods Toolkit (DLMtool): An R package for informing management of data‐limited populations

Management strategy evaluation (MSE) is a riskbased approach increasingly used in the management of exploited populations that accounts for uncertainties in population and exploitation dynamics (Bunnefeld, Hoshino, & MilnerGulland, 2011; Butterworth & Punt, 1999; Punt, Butterworth, de Moor, De Oliveira, & Haddon, 2016). MSE involves the simulation of the exploited system—the operating model—that encompasses plausible hypotheses for population, exploitation, observation, and management implementation dynamics. MSE can be used to test management procedures (MPs—a model or algorithm providing management recommendations from data) and fixed management policies (e.g., a constant harvest level) over a projected time period accounting for feedback with the simulated system represented by the operating model. Over 90% of fish populations are data limited: there are insufficient data to conduct a conventional population assessment (Costello et al., 2012) and in most cases there is considerable uncertainty over population status and trajectory. In order to meet national and international guidelines for sustainability it is necessary to demonstrate that MPs that are proposed for management are robust to such uncertainties. MSE offers a powerful tool for informing management of datalimited populations: even if the performance of an MP cannot be established explicitly through a population assessment, performance may be evaluated implicitly by simulation. For example, a size limit or spatial closure that does not itself inform population status may consistently achieve management objectives over a wide range of simulated conditions. The MSE approach has been used to evaluate a range of datalimited MPs (Carruthers et al., 2014, 2015). The modelling framework was formalized in the DLMtool (Carruthers & Hordyk, 2018) (‘the package’) with the overarching aim to use the MSE in support of transparent and rigorous decisionmaking in datalimited fisheries. The package is intended for fishery scientists providing strategic advice to fishery managers. Given a fully specified operating model and explicit management performance objectives, the package can evaluate the performance of alternative management approaches, identify the most effective fishery control types (e.g., catch limits, Received: 30 April 2018 | Accepted: 20 August 2018 DOI: 10.1111/2041-210X.13081