Vulnerability to fishing in reef fishes that aggregate

Many species of coral reef fish form aggregations at predictable times and locations for the purpose of spawning. Targeted fishing of spawning aggregations can result in loss of aggregations, rapid population depletion and fishery collapses. However, there are also examples of spawning aggregation fisheries that have persisted for decades, including some that lack robust conservation or management measures. That spawning aggregation fisheries vary in their vulnerability to overfishing is often overlooked and can result in narrow recommendations for protection of spawning sites using marine reserves. To enable a broader range of societal objectives and management interventions to be considered, improved understanding of the risks posed by spawning aggregation fishing is therefore required.\ud \ud This thesis integrated the roles of biology, fish and fisher behaviour, and the socioeconomic and cultural context of fisheries to disentangle causes of vulnerability to overfishing in coral reef spawning aggregation fisheries. I aimed to develop a bivariate, indicator-based risk assessment of spawning aggregations that could be applied in the data-poor management context of most of these fisheries. A bivariate approach is required since the risks posed by fishing of spawning aggregations depend on intrinsic and extrinsic components of the system. Life history traits and spawning behaviour determine the productivity of fish populations and their inherent (i.e. intrinsic) capacity to recover from depletion. Predictable increases in population density that are associated with spawning aggregation behaviour influence the potential for development of a targeted fishery. However, the exposure of populations to spawning aggregation fishing will be determined by extrinsic (i.e. fishery) factors that influence catchability and fishing effort at spawning sites. These factors include aspects of fisher knowledge, accessibility, technology, markets and management. Through four data chapters (Chapters 2 to 5), I explore mechanisms by which many of these intrinsic and extrinsic factors influence vulnerability to overfishing in these fisheries. The findings of these data chapters are used to develop robust indicators for a risk assessment framework, which is applied to a global selection of spawning aggregation fisheries and its predictive ability examined (Chapter 6).\ud \ud Spawning aggregations are ephemeral events and fishers must first acquire knowledge on their location and timing for a targeted fishery to develop. In my first data chapter, interview-based surveys were employed to examine how socioeconomic and cultural context leads to variation in knowledge and targeting at two case study locations (Ahus, Karkar islands) in Papua New Guinea (Chapter 2). While variable access rights among fishers and genders to reef habitats were sources of wide heterogeneity in knowledge at Ahus, knowledge was more homogenous at Karkar and influenced solely by gears used by fishers. However, the exposure of aggregations to fishing depended primarily on socioeconomic drivers of fishing effort (dependency on fishing, markets, management). While knowledge was equivalent for a few species (e.g. Lutjanus gibbus) across locations, highly efficient fishing techniques have emerged to exploit aggregating behaviour at Ahus. This finding demonstrates that, while knowledge clearly acts as a precursor, socioeconomic drivers determine how and to what extent aggregations are targeted.\ud \ud Once fishers have acquired knowledge on reproductive behaviour, the development of a targeted fishery will depend on the level of population density changes associated with spawning aggregation formation and whether or not those changes translate into increases in catchability (i.e. exhibit density-dependence). Chapter 3 tests assumptions regarding density-dependent catchability in a trap fishery for rabbitfish (Siganus sutor) in Seychelles and models their management implications in terms of marine reserve outcomes for spawning stock biomass-per-recruit and yield-per-recruit. Though spawning aggregation formation involves nine to thirteen fold increases in population density, catch rates (a proxy for catchability) exhibited weak density-dependence, being only fourfold greater in spawning habitat than catch rates in non-spawning habitat. Catch rates in spawning habitat were also highly variable across the spawning season and generally only peaked when currents were strong. Since spawning behaviour incurs lower than expected increases in catchability, marine reserves implemented in non-spawning areas would be more beneficial than those protecting spawning sites in terms of maintaining yield-per-recruit while rebuilding spawning stock biomass-per-recruit. These findings have implications for both fisheries management and ecological resilience, since Siganus sutor is a key macroalgal browser in Seychelles where a high proportion of reefs have transited to macroalgal-dominated states in the wake of mass coral bleaching.\ud \ud Fish and fisher behaviour at multispecies spawning sites are highly dynamic and are likely to cause vulnerability to overfishing to vary across aggregations or between species. Chapter 4 examines these dynamics at a multi-species grouper spawning site in Papua New Guinea. Despite similar aggregation sizes, Epinephelus polyphekadion was 8-fold more vulnerable to capture than E. fuscoguttatus. Catch rates of both species were unrelated to the size of their respective aggregations. However, E. polyphekadion catch rates declined as the density of fishing effort increased at the site, with gear saturation identified as the likely mechanism for this effect. E. fuscoguttatus catch rates were negatively related to the size of aggregations formed by its congener, stemming from the superior competitiveness and therefore higher selectivity of the gear for E. polyphekadion. The findings have implications for gear-based management, which must be based on understanding of gear selectivity for aggregating species, and fishing effort controls, which must consider the potential for effort-dependent patterns in catchability.\ud \ud Critically, changes in catchability that typically occur with spawning aggregation formation also offer an opportunity for rebuilding populations and sustaining fisheries. In Chapter 5, a per-recruit model was used to compare the long-term effects of marine reserves protecting either spawning or normal residence areas for S. sutor and E. fuscoguttatus populations in Seychelles, allowing for contrasts between catchability, life history traits and sexual modes. Normal residence reserves were more effective at improving both the spawning stock biomass-per-recruit and yield-per-recruit of S. sutor. By contrast, the protection of spawning sites is preferable for maintaining spawning stock biomass-per-recruit and normalising the sex ratio in the protogynous grouper E. fuscoguttatus. Neither spawning reserves nor normal residence reserves improved E. fuscoguttatus yield-per-recruit. Analyses revealed that relative reserve effects are more sensitive to changes in catchability than to life history traits. Because changes in catchability occurring with S. sutor aggregation formation are small, compared to many other aggregation-forming populations, this species suffered higher fishing mortality in normal residence areas than at spawning sites. Consequently, normal residence reserves will be more beneficial than spawning reserves. Marine reserves protecting spawning sites are not always the most effective tool for balancing conservation and exploitation objectives and should be weighed against other management options.\ud \ud A major challenge for management stems from the data-poor context of many spawning aggregation fisheries, which typically lack information on productivity, catch and exploitation status. To address this gap, the thesis culminated in the design of an indicator-based tool for assessing risks of overfishing in these fisheries, using the findings of earlier chapters to inform indicator development (Chapter 6). A bivariate risk assessment tool was selected, composed of an axis of intrinsic vulnerability comprising indicators of species life history and spawning behaviour, and an axis of extrinsic vulnerability comprising indicators of fisher knowledge, technology, accessibility, markets and management. Using interviews with fishery experts to score indicators, the tool was examined for a global selection of spawning aggregation fisheries and its predictive ability tested. The tool performed moderately in predicting fishery status. The risk of overfishing varied extensively, though many assessed fisheries scored as high risk in terms of intrinsic vulnerability and low risk for extrinsic vulnerability. Key intrinsic risk factors were species productivity and the type of spawning aggregation formed. A key driver of extrinsic vulnerability was management context, while the status of many fisheries was stable or improving due to management intervention. Successful management interventions included gear, fishing effort and catch controls, as well as measures that restrict access to spawning aggregations (e.g. marine reserves). The findings highlight the importance of considering fisheries management objectives in addition to conservation, which requires that these socio-ecologically complex fisheries are better integrated in fisheries management planning.\ud \ud The discourse surrounding spawning aggregation fisheries has typically focused on the challenges they pose for population persistence, rarely recognising that not all fisheries will be at high risk of overfishing and that aggregating behaviour also presents opportunities for rebuilding populations and sustaining fisheries. This thesis provides insights for a broader approach to addressing these fisheries, identifying important causes of vulnerability and demonstrating their implications for assessment and management. Aggregating behaviour typically predisposes populations to density-dependent catchability, but fishing pressure will ultimately be determined by catchability and the socioeconomic drivers of fishing effort, which constitute levers for management intervention. Though management targets or limits for spawning aggregation fisheries must be precautionary and adaptive given the largely unknown indirect (i.e. non-lethal) impacts of fishing, fisheries management objectives should not be discounted in favour of conservation in all cases