Seventy years of tunas, billfishes, and sharks as sentinels of global ocean health

Fishing activity is closely monitored to an increasing degree, but its effects on biodiversity have not received such attention. Using iconic and well-studied fish species such as tunas, billfishes, and sharks, we calculate a continuous Red List Index of yearly changes in extinction risk over 70 years to track progress toward global sustainability and biodiversity targets. We show that this well-established biodiversity indicator is highly sensitive and responsive to fishing mortality. After ~58 years of increasing risk of extinction, effective fisheries management has shifted the biodiversity loss curve for tunas and billfishes, whereas the curve continues to worsen for sharks, which are highly undermanaged. While populations of highly valuable commercial species are being rebuilt, the next management challenge is to halt and reverse the harm afflicted by these same fisheries to broad oceanic biodiversity. Description Conservation works Tuna and billfishes are large species that have long been targeted by fisheries, whereas sharks, which are also large fishes, have tended to be considered as by-catch or nontarget species. Juan-Jorda et al. used an approach that monitors yearly changes in the International Union for Conservation of Nature Red List status to estimate population status for these three groups (see the Perspective by Burgess and Becker). After almost three decades of decline, tuna and billfishes have begun to recover because of proactive fisheries management approaches. Sharks, however, which have received much less conservation attention, have continued to decline. These results both reinforce the value of conservation and management and emphasize the need for immediate implementation of these approaches for sharks. —SNV The IUCN Red List index tracks the recovery of tunas and billfishes as the status of undermanaged sharks continues to decline. INTRODUCTION Recent biodiversity assessments show unprecedented loss of species, ecosystems, and genetic diversity on land but it remains unclear how widespread such patterns may be in the oceans. There is an urgent need to develop surveillance indicators to track the health of ecosystems in the marine realm, including changing extinction risk of marine species. These will allow evaluation of progress toward achieving global goals and commitments established by the Convention of Biological Diversity (CBD) and Sustainable Development Goals (SDGs) to halt and reverse marine biodiversity loss. RATIONALE Highly monitored oceanic fisheries comprising iconic predatory tunas, billfishes, and sharks yield an opportunity to support the development of linked sets of pressure and ecological state indicators capable of measuring progress toward global biodiversity and sustainability targets. We derived a continuous Red List Index (RLI) based on International Union for Conservation of Nature (IUCN) Red List categories and criteria for tracking yearly changes in extinction risk of oceanic tunas, billfishes, and sharks over the past 70 years to assess the health of oceanic biodiversity. Furthermore, by assessing the sensitivity and responsiveness of the RLI (state indicator) to fishing mortality (pressure indicator) and assessing the alignment between the most recent Red List status and fishery exploitation status of tunas, billfishes, and shark populations, we offer decision-makers a robust set of linked pressure-state indicators for tracking biodiversity loss and recovery in oceanic ecosystems. RESULTS We find that since 1950, the global extinction risk of oceanic predatory fishes has continuously worsened as a result of rising and excessive fishing pressure, up until the late 2000s when management actions reduced fishing mortality, allowing for recovery of tunas and billfishes. However, sharks remain undermanaged and their extinction risk continues to rise. Our findings reveal a core problem and ongoing challenge in the management of oceanic multigear and multispecies fisheries. Whereas target species are increasingly sustainably managed to ensure maximum yields, the functionally important shark species being captured incidentally by the same fisheries continue to decline as a result of insufficient management actions. Furthermore, our study also connects annual changes in global extinction risk with changes in fishing mortality over the last 70 years, demonstrating how the global RLI trajectory of oceanic predatory fishes is highly sensitive and responsive to fishing mortality. CONCLUSION Although halting biodiversity loss by rebuilding highly valuable commercial tuna and billfish species has been achieved, the next challenge is to halt declines in shark species by setting clear biodiversity goals and targets as well as implementing science-based conservation and fishery management measures and international trade regulations. Unless an effective mitigation hierarchy of management actions to reduce shark mortality is urgently implemented (and adapted to the complexity of each fishery and shark species), their risk of extinction will continue to increase. Furthermore, we demonstrate a high alignment and complementarity between the current population-level Red List status and fishery exploitation status of tunas, billfishes, and sharks, when applied at the same scale. Although we do not propose that the RLI be used to manage fish populations, this strong alignment eliminates any technical barrier for use of the RLI by policy-makers for tracking CBD and SDG targets. Global Red List Index (RLI) of oceanic predatory fishes for tracking progress toward global biodiversity and sustainability targets. (A) The global population-level RLI (state indicator) closely tracks changes in fishing mortality (pressure indicator) for 52 oceanic tuna, billfish, and shark populations over the last 70 years, thus providing decision-makers with a linked set of pressure-state indicators for tracking the health of oceanic biodiversity. The population-level RLI was reversed in 2008 following a reduction in fishing mortality after implementation of fisheries management measures in tuna regional fisheries management organizations. The horizontal gray line denotes F/FMSY =1, FMSY being fishing mortality (F) which produces the maximum sustainable yield (MSY). (B) Global continuous species-level RLI of tunas, billfishes, and oceanic sharks (seven, six, and five species, respectively) tracking yearly changes in extinction risk over 70 years and the global episodic RLI of oceanic sharks and rays (21 and 10 species, respectively) estimated in 1980, 2005, and 2018. An RLI value of 1 indicates that a given taxa qualifies as least concern (that is, not expected to become extinct in the near future), whereas an RLI value of zero indicates that all taxa have gone extinct.