Protected high-value reefs and movement pathways improve conservation of reef sharks

New research from Gerardo Martín and colleagues looks at how we might support shark species if protected areas are multi-use and still open to fishing. The key is focusing on reefs preferred by sharks and developing our knowledge of shark movement pathways. Here the authors share their work and look at how we can improve species conservation while still supporting communities that depend on coral reefs.

There is a suite of sharks that spends much of their time at coral reefs. These shark species face challenges from fishing pressure and degradation of coral reefs, with declines reported for many of them, at least in some of the areas where they occur. Three of these species: grey reef shark (Carcharhinus amblyrhynchos), silvertip shark (Carcharhinus albimarginatus) and bull shark (Carcharhinus leucas), were the focus of our research on how to design effective multiple-use marine protected areas (those with areas open to fishing and areas that are closed). Each of the three shark species uses reefs to different levels: grey reef sharks are most attached to individual reefs for long periods, bull sharks move widely between individual reefs every few days; and silvertip sharks fall somewhere between these two extremes.

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Photo: Colin Simpfendorfer

To investigate marine protected area design, we developed computational models based on movement data from these three species that were collected on the Great Barrier Reef, Australia. The models followed the fate of individual sharks over the course of their life to determine their chances of surviving, reproducing and other metrics relevant to assessing conservation success. The models also incorporated information on the protection status of individual reefs, level of fishing at reefs open to fishing, habitat quality of each reef (we know some reefs are more favoured than others by sharks) and distances between individual reefs.

The results of our model simulations showed that conservation outcomes could be improved by closing more reefs to fishing, but there was a point beyond which the benefits of adding more protected areas were minimal. Importantly, the optimal level of protection for each species was achieved with fewer protected areas when those reefs favoured by sharks were protected. Improved conservation outcomes could also be achieved if movement pathways between frequently used reefs were protected, especially for those species that moved between reefs the most (i.e. bull shark and silvertip shark). At present there is limited knowledge on movement pathways for reef sharks, so further research will be required to identify suitable areas for protection.

The model results also showed that the three species would benefit differently from using multiple-use marine protected areas. The more a species moves between reefs, the more it is exposed to fishing, and hence the lower the value of protection provided by protected reefs. Thus grey reef sharks benefited more than silvertip sharks, which benefited more than bull sharks, from equivalent marine protected area designs. These patterns of protection were also reflected in demographic effects, with the more mobile species estimated to have shortened life span and produce few offspring than the less mobile species.

Globally, reef sharks face a range of challenges from human activities that degrade coral reefs. The results of our work demonstrate that the use of multiple-use marine protected areas can improve conservation outcomes for this group of species, while still enabling communities to reap the benefits from these systems that support food security and livelihoods. The role that habitat quality played in improving conservation outcomes points to the fact that we need to better understand what makes reefs more favourable to sharks, something we are yet to fully understand. New approaches to managing reefs that may also prove valuable include implementing buffer zones and movement pathways that protect sharks when they move between reefs.

The full article, Estimating marine protected area network benefits for reef sharks, is free to read for a limited time in Journal of Applied Ecology.

 

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