Abalones at risk: A global Red List assessment of Haliotis in a changing climate

While this partnership between aquaculture and wild fisheries can appear promising, espe- cially in the face of climate driven stressors, greater support and funding by foundations and governments is still needed. Despite this exponential increase in abalone aquaculture, the decline in wild populations persists while success to reverse it can only be described as modest with current levels of support. Even with sustainable fisheries, failures in policing and enforce- ment can rapidly reverse gains where there is aggressive and widespread poaching, especially from organised criminal gangs. It may be the case that effective visible protection of large wild abalone assemblages will be more successful and less costly than relying on restocking with aquaculture produced abalones [99]. Recent work has demonstrated the importance of sus- tainable fishing, conservative fishing quotas and periodic fishery closures. While protecting larger females within MPAs that can serve as climate refugia, bolstering resilience to climate stressors will require close collaboration between fishers, fishery managers and scientists [100]. Elevated sea-surface temperatures are giving rise to a multiplicity of immediate threats through loss of algae and habitat, expansion of pathogens and transition of marine ecosystems. Acidification of the oceans, resulting from the current levels of absorption of CO2 from the atmosphere, will have a deleterious effect not only on the development of larval shells but also on the crustose coralline algae (CCA) on which larval settlement depends [14, 101, 102]. How-

ever, it should be noted that while CCA may have reduced growth and survival during acidifi- cation, it has been recorded that elevated pCO2 in upwelling acclimated waters maintained settlement cues for H. rufescens after prolonged acidification exposure [103]. Furthermore, low oxygen conditions in a warming ocean will also impact growth and survival of abalones [104]. Evidence shows that over future decades, abalones, in common with most, if not all calcareous marine taxa, will become increasingly vulnerable to such changes in ocean chemistry. In addi- tion to the impact on calcification, fertilisation, embryonic larval development and settlement may all be compromised [105] allowing for less surplus productivity for fisheries. We know the environment for abalones will in future be warmer and less oxygenated as MHWs are predicted to increase in frequency, intensity and duration [106]. Anomalous heat- ing at the ocean surface is driven by changing climate and further influenced by El Niño- Southern Oscillation (ENSO). ENSO events result in the episodic warming and cooling of the tropical Pacific, and although this effect is centred on the eastern Pacific, its influence can be observed across the whole of the Pacific, Atlantic and Indian Oceans [107]. During the austral summer of 2010/11 an extreme MHW affecting 2000 km of Western Australia coastline resulted in 99% mortality of Roe’s abalone (H. roei) [108]. Such MHWs not only cause mortal- ity of sedentary taxa such as abalones, but the algae on which they survive. Bull kelp (Nereocyc-

tis luetkeana), the principal diet of the red abalone (H. rufescens), is usually common off the northern California coast but declined by more than 90% following the 2014–2015 MHW [109]. This decline in kelp was aggravated by an explosion in population of the purple sea urchin (Strongylocentrotus purpuratus) along with the extirpation from sea star wasting syn- drome of the sunflower star (Pycnopodia helianthoides) [110], an important urchin predator in kelp forest ecosystems [21]. This resulted in 80% mortality of abalones in Northern California and the closure in 2018 of the valuable, popular and culturally important recreational red aba- lone fishery. Similarly, low oxygen events are impacting abalones in Baja California, Mexico [111]. Unless the combustion of fossil fuels that is driving temperature and acidification is arrested, the long-term future of abalones together with all other marine molluscs remains uncertain. We will need to use all the tools at our disposal including restoration aquaculture, reductions in fishing pressure, MPAs [112], favourable microclimates [113] and genetically more resilient families [114] to support abalones given that we know how vulnerable these molluscs are to climate stressors. They are truly oceanic “canaries in the coalmine”. PLOS ONE Abalones at risk: A global Red List assessment of Haliotis PLOS ONE | https://doi.org/10.1371/journal.pone.0309384 December 23, 2024 19 / 25