Applying tree analysis to assess combinations of Ecosystem-Based Fisheries Management actions in Management Strategy Evaluation: a summary

Lucy McGinnis, 11 February 2021

[Editors note, we’re starting a new series where lab members write posts about eachothers’ research. Thanks Lucy for taking the reins on this first one!]

headshot of Amanda Hart, PhD candidate & first author of the publication.

Amanda Hart, PhD candidate

There is no one miracle cure for commercial fisheries. But last year, lab PhD candidate Amanda Hart published her first first author paper with PI Gavin Fay in Fisheries Research that considered how combinations of management actions that could make progress towards developing sustainable practices.

It starts with an ecosystem-scale perspective. In the past, fishery management strategies were built around individual species. However, as our understanding of the environmental, societal, and economic pressures that govern fisheries has grown, efforts have shifted toward Ecosystem-Based Fisheries Management (EBFM). EBFM takes into account the effects of inter-species interactions and abiotic pressures such as climate change and habitat availability, as well as fleet and gear interactions and the socio-economic drivers on the other end of the fishing line.

As fisheries around the world shift toward this approach, there is need to evaluate the performance of such strategies.Picture of the masthead for the publication in Fisheries Research

Amanda used Management Strategy Evaluation (MSE) to model scenarios and compare the efficacy of various management strategies on a ten species fishery characteristic of Georges Bank, USA, one of New England’s fishing hotspots. MSE simulates environmental management scenarios and identifies management procedures that have the greatest effects towards desired outcomes. MSE also can show how uncertainty associated with the fishery and management system influences outcomes.

Starting with historic biomass and catch information, they defined a management procedure to simulate that included three components: target fishing mortality, indicator-based harvest control rules, and catch ceilings. They conducted one thousand model simulations for each scenario (for a total of 29,000 simulations) over a thirty-year projection. The projections were then evaluated based on twelve performance metrics: frequency of species overfished, frequency of aggregate group collapse, piscivore catch, benthivore catch, planktivore catch, elasmobranch catch, frequency of system collapse, system biomass, system catch, biomass diversity, catch diversity, and catch revenue. This let them assess the effect of various management strategies on biological, catch, diversity, and economic objectives. Regression trees were used as a novel method of understanding how the different management actions drove the variability seen in the performance metrics obtained from the simulations.

Catch ceilings were consistently identified as the main drivers of variability in the results. By placing a limit on the total removals from the ecosystem (summed over all ten species), they achieved higher diversity, biomass, revenue, and lower frequencies of collapse. Optimal management performance corresponded with catch ceilings of 125-150 kt. Increasing the catch ceiling above this level did not substantially increase the system catch or revenue.

In fact, procedures that imposed high catch ceilings, or no catch ceiling at all, performed poorly, resulting in the highest frequency of aggregate collapse due to overfishing. Overfishing damages the structure of the ecosystem, resulting in negative long-term effects on its recovery and stability. That’s the trade-off between catch and biomass: the more fish you catch, the less you leave, which makes it difficult for stocks to replenish.

Without an optimal catch ceiling, there were still ways to improve management outcomes in the simulation results. Implementing ecosystem indicator-based harvest control rules and reduced target fishing mortality rates, even when there were fewer restrictions on overall catch, resulted in systems that performed better, which highlights the important role of these combined strategies in managing fisheries.

Amanda is a current NMFS-Sea Grant Population and Ecosystem Dynamics Fellow. We’re looking forward to learning more about the applications of MSE Amanda is conducting in her PhD research. Stay tuned!

Hart, A.R. and Fay, G., 2020. Applying tree analysis to assess combinations of Ecosystem-Based Fisheries Management actions in Management Strategy Evaluation. Fisheries Research, 225, p.105466.