Effects of fish movement assumptions on the design of a marine protected area to protect an overfished stock.

Jorge Cornejo-Donoso,Bjorn Birnir,Steven D Gaines,Baldvin Einarsson

doi:10.1371/journal.pone.0186309

Abstract

Marine Protected Areas (MPA) are important management tools shown to protect marine organisms, restore biomass, and increase fisheries yields. While MPAs have been successful in meeting these goals for many relatively sedentary species, highly mobile organisms may get few benefits from this type of spatial protection due to their frequent movement outside the protected area. The use of a large MPA can compensate for extensive movement, but testing this empirically is challenging, as it requires both large areas and sufficient time series to draw conclusions. To overcome this limitation, MPA models have been used to identify designs and predict potential outcomes, but these simulations are highly sensitive to the assumptions describing the organism’s movements. Due to recent improvements in computational simulations, it is now possible to include very complex movement assumptions in MPA models (e.g. Individual Based Model). These have renewed interest in MPA simulations, which implicitly assume that increasing the detail in fish movement overcomes the sensitivity to the movement assumptions. Nevertheless, a systematic comparison of the designs and outcomes obtained under different movement assumptions has not been done. In this paper, we use an individual based model, interconnected to population and fishing fleet models, to explore the value of increasing the detail of the movement assumptions using four scenarios of increasing behavioral complexity: a) random, diffusive movement, b) aggregations, c) aggregations that respond to environmental forcing (e.g. sea surface temperature), and d) aggregations that respond to environmental forcing and are transported by currents. We then compare these models to determine how the assumptions affect MPA design, and therefore the effective protection of the stocks. Our results show that the optimal MPA size to maximize fisheries benefits increases as movement complexity increases from ~10% for the diffusive assumption to ~30% when full environment forcing was used. We also found that in cases of limited understanding of the movement dynamics of a species, simplified assumptions can be used to provide a guide for the minimum MPA size needed to effectively protect the stock. However, using oversimplified assumptions can produce suboptimal designs and lead to a density underestimation of ca. 30%; therefore, the main value of detailed movement dynamics is to provide more reliable MPA design and predicted outcomes. Large MPAs can be effective in recovering overfished stocks, protect pelagic fish and provide significant increases in fisheries yields. Our models provide a means to empirically test this spatial management tool, which theoretical evidence consistently suggests as an effective alternative to managing highly mobile pelagic stocks.

Highlights

Marine Protected Areas (MPAs) are a spatial management tool commonly used to restore and protect populations of marine organisms
The differences in the optimal MPA size that maximize the fishery yields were modest across movement scenarios at equilibrium (Fig 4C), the trend was that larger MPAs were required as movement complexity increased; the maximum fisheries yields were obtained
The complexity involved in the movement of organisms within marine environments makes their spatial management challenging, and this has sparked a fruitful debate about the viability and effectiveness of such approaches [43]

Summary

Introduction

Marine Protected Areas (MPAs) are a spatial management tool commonly used to restore and protect populations of marine organisms. To be successful for both conservation and fisheries goals, MPA designs must adequately address the consequences of species movement, including swimming behavior of adults and dispersal of larvae [4,5]. Theoretical studies suggest that a well-designed MPA can provide comparable benefits to those obtained with perfect management of the catch or they can even exceeded under the right conditions [3]. Hasting and Botsford [6] showed that for species with sedentary adults and larval dispersal, the optimal MPA size can offer identical stock protection and yields to those provided by the optimal fishing mortality rate. Several other authors have corroborated this result, concluding that most of the fisheries benefits of MPAs are obtained when adults have medium to low annual movement, and these benefits are reduced as the movement rate increases [1,2,7]. Gaines et al [4] indicates that one of the main requirements for an effective MPA is an area size proportional to the movement rate of the organisms, suggesting that MPAs can be effective in protecting highly mobile organisms if designed with larger areas that exclude extractive activities

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Conclusion