Sea sparing, sea sharing, and production at least cost to the planet

Fisheries are the most direct threat to marine biodiversity. However, fisheries are also vital for food security, with over 3.2 billion people relying on fish for a significant part of their diet. With growing international support for protecting 30% of the planet by 2030, no-take marine protected areas (MPAs) that exclude fishing from certain areas entirely have emerged as a popular tool for conserving biodiversity and ensuring fisheries sustainability. But some argue that the benefits of no-take MPAs may be limited in seascapes that are already ‘well-regulated’ by other forms of fisheries management, particularly if they only displace fishing effort to other areas for no net biodiversity gain. In this thesis, I aimed to evaluate the conservation benefits of no-take MPAs in such seascapes through the creation of a sea sparing and sharing framework. With it, I posed and attempted to answer the following question: for any sustainable level of fishery harvest, what proportion of a seascape – if any – ought to be spared from fishing to maximise seascape-wide biodiversity? By fixing catch targets across management scenarios, this framing ensured that the biodiversity impacts of fishing effort displacement from no-take MPAs would not be overlooked. In Chapter 2, I outline the framework and adapt it to a modelled archetypal trawl fishery using empirically informed estimates of species’ sensitivities to fishing. I find that a strategy with no-take MPAs is best for biodiversity when avoiding the local extinction of sensitive species is a priority, but that a strategy without no-take MPAs is best for the abundance of more resilient species and reducing fishing effort. I also extend these findings to crustacean trawl fisheries globally, where I find that biodiversity in 72% of fisheries could potentially benefit from no-take MPAs. In Chapter 3, I develop a more complex model that also includes differences in species’ ranges and the impacts of fishing-induced habitat damage. When habitat damage is absent, I find that no-take MPAs provide a net biodiversity benefit so long as there are significant range overlaps between fishing grounds and bycatch species. When habitat damage is instead present, I find that no-take MPAs generally benefit biodiversity regardless of range overlap, but that this is diminished if the carrying capacities of deliberately fished species also decline due to damage. However, as in Chapter 2, no-take MPAs still increase the total fishing effort required to reach catch targets in the seascape. Finally, in Chapter 4, I integrate the insights from Chapters 2 and 3 with previous work from agriculture and forestry to present a general theory of how conservation operates within production systems. Through it I argue that almost all conservation interventions in production systems act upon just three domains: area-use allocation, the direct impacts of production on biodiversity, and supply and demand. I also outline how these domains interact with one another, and in so doing show how many otherwise well-intentioned conservation interventions can actually risk increasing biodiversity loss by causing knock-on effects in other domains. Additionally, in the Chapter 4 Supplementary Material, I illustrate how these domains and their interactions can be modelled to identify ideal conservation actions using the historic case of the endangered Australian sea lion. In all, I find that no-take MPAs are generally able to deliver biodiversity benefits in ‘well-regulated’ fisheries when accounting for the impacts of fishing effort displacement, but that this can change where seascapes lack sensitive species or fishing-induced habitat damage. No-take MPAs also always increase the total fishing effort required to maintain fixed catch targets. The best strategy in a given seascape is thus likely to depend on its ecology and the management priorities of its stakeholders, although no-take MPAs are likely to be useful if biodiversity conservation is deemed important. When considering conservation in production systems more broadly, I identify how just three domains – area-use allocation, production’s direct impacts, and supply and demand – characterise the conservation interventions available to managers, and show how these can interact to limit the effectiveness of certain actions. The latter in particular suggests that many single-domain conservation interventions may be far less beneficial than commonly supposed.

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