Assessing indirect biodiversity conservation benefits of fisheries closures in the Gulf of St. Lawrence, Canada

provide various benefits, i.e., enhanced feeding opportunities, protection from predators, spawning and nursery areas, spawning aggregation sites, and attachment points for fish eggs [9, 10]. Whether the current BCBs will prevail under future climate change conditions is uncer- tain. The OECMs that overlap with the potential spawning cites presently have preferred temperature ranges for all three adult and non-spawning fish species and are above the hyp- oxia threshold [20]. However, the preferred temperature ranges for adult spawners are at least an order of magnitude smaller than for non-spawners (Fig 2B), rendering their spawn- ing behaviour vulnerable to climate warming. Already, record-breaking bottom tempera- tures in the Laurentian Channel have been documented [18], with a Gulf-wide average of 6.7 ˚C, and 7.7˚C in the Cabot Strait. Alongside, average oxygen concentration in the Gulf of St. Lawrence has shown a decreasing trend over the last decade, especially in the deeper waters of the Gulf (i.e., the Laurentian Channel), having decreased from ~100–150 μM/kg in 1990s to * 75–100 μM/kg in the early 2020s [19]. As the measurements from the 2020s are largely from samples between 200–250 m, average oxygen concentration may be even lower in depths beyond 250 m, as shown in our baseline model data of deeper OECMs i.e., Central and Eastern Gulf of St. Lawrence Conservation Areas, North of Bennet Bank, and Slope of Magdalen Shallows. Further, all OECMs under all time frames are showing signs of ocean acidification, indicat-

ing future implications for indirect BCBs, not only through impacts on habitat forming species but also through synergistic effects of changing bottom temperature and oxygen concentration on the life-history of our selected demersal fish species [43]. While current research suggests that cold-water corals, such as Lophelia pertusa [22, 61] and sponges, such as Porifera spp. [62, 63], can still grow under ocean acidification, the overall habitat can undergo significant structural loss providing less complexity and support for biodiversity [22]. This could also impact the continuing provision of spawning habitat and nursery habitats the OECMs in the Gulf of St. Lawrence for redfish [52, 64], Atlantic halibut [49], and Greenland halibut [50, 51]. As bottom temperature increases simultaneously with decreasing oxygen concentration and pH, species will face an increasingly insufficient oxygen supply. This is due to higher metabo- lism demands with rising water temperature, eventually negatively impacting growth and reproduction, among others [43, 65]. Acidification acts as another stressor on a specie’s metab- olism, overall, leading to an reduced resilience to environment changes and long-term survival [43, 66]. Consequently, our results highlight potential losses of indirect BCBs in terms of changing habitat conditions that could affect continued spawning aggregations, larval survival, and growth [17, 67, 68]. However, indirect BCBs could also be challenged by shifting distribu- tions of the three focus species in response to climate-driven environmental changes. We

observed the large climate-impacts on the deeper waters of the Laurentian Channel, which begets the question where the redfish, Atlantic halibut and Greenland halibut of the Gulf will find refuge, aggregate for successful spawning and larval development with intensifying cli- mate change—deeper and colder waters as climate-refuges are not available in the Gulf in the future. However, Atlantic halibut and Greenland halibut have already been observed to shift towards northern latitudes in response to warming waters in the Northwest Atlantic [15, 41, 67]; redfish abundance in the Gulf of St. Lawrence, on the other hand, has been increasing significantly in the past decade [17]. However, their habitat within the Gulf of St. Lawrence is projected to become unavailable by the end of the 21st century, due to unfa- vourable changes in, among others, bottom temperature and bottom oxygen concentration [15, 69]. Whether redfish in the Gulf will also shift their distribution north remains to be seen. PLOS ONE Indirect biodiversity conservation in the Gulf of St. Lawrence PLOS ONE | https://doi.org/10.1371/journal.pone.0316754 January 9, 2025 11 / 20