Reducing fishing impacts on species of conservation concern at multiple scales

Numerous human activities directly and indirectly threaten marine biodiversity and ecosystems, but fishing is a primary threat directly driving the decline of many marine species. Advancing fishing technologies have enabled humans to exploit nearly every corner of the ocean and expand into increasingly deep, remote, and previously unexplored areas. Fishing disrupts the integrity of marine ecosystems in many different ways, including damage to benthic habitats by fishing gear, alteration of fish community structure, changes to species' behavior, selection for less genetically advantageous traits, and disruption of trophic webs. But perhaps the most obvious impact of fishing is simply that it removes vast amounts of biodiversity from the ocean, whether species are targeted or caught incidentally. Protecting species from fishing impacts is a monumental task. To prevent marine biodiversity loss and ensure the future viability of marine ecosystems and the billions of people that rely on them, marine conservation efforts must work in tandem with dedicated fisheries management...

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The perverse impacts of fisheries on marine species is a vast topic, and there are numerous research gaps that, if addressed, would help deliver effective fisheries management and conservation solutions. Through this thesis, in eight chapters, I explore and help address gaps in our understanding of how to manage overfishing impacts on biodiversity at different geographic and regulatory scales. First, I map the political distributions of marine biodiversity, including many fished species, and find that marine biodiversity is far more transboundary than terrestrial biodiversity, with the vast majority (over 90%) of species’ distributions spanning an international border and over 50% of species occurring in more than ten jurisdictions (Chapter 2, Roberson et al. [in review]). Second, I provide a baseline assessment of the conservation status of widely exploited seafood species and find that 92 threatened fish and invertebrate species are reported in global catch records, with many wealthy nations driving both catch and international trade of threatened seafood (Chapter 3, Roberson et al. 2020). Third, in Chapters 4-6, I focus on tuna fisheries in an important and understudied region, the Indian Ocean. I show how an outdated categorization of fishing sectors allows the industrial-scale gillnet fisheries to operate essentially without monitoring or regulation (Chapter 4, Roberson et al. 2019). I present a case study of cetaceans' susceptibility to capture in tuna gillnet fisheries, and demonstrate a method that provides more mathematically robust estimates of risk using expert judgment in data-poor contexts (Chapter 5, Roberson, Hobday and Wilcox [in prep]). I then use this new method to provide the first spatially-explicit risk assessment of catch susceptibility of cetaceans, sea turtles, and elasmobranchs in Indian Ocean tuna fisheries, and find that—as anecdotes and reports suggest—gillnets likely pose a serious threat to many threatened megafauna species, and all three gear types likely interact with a much wider range of species than available records show (Chapter 6, Roberson et al. [in prep]). Finally, I explore fishing impacts at the level of individual vessels, and show that there are significant variations in threatened species bycatch among skippers within five Commonwealth fisheries, which suggests that an alternative framing of management questions could improve the environmental performance of fisheries (Chapter 7, Roberson and Wilcox [in prep]). Context-appropriate innovations in fisheries management are instrumental in reducing overfishing impacts on marine biodiversity. Considerable barriers remain to actually implementing effective management solutions, but this work provides baseline information and tools for management in different contexts. If we are serious about protecting the ocean and our fisheries, we need a portfolio of management actions at many different scales, from high-level national and international policies all the way down to changes in the behavior of the fishers themselves.