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

OECMs; however, both positive and negative unintended consequences of area-based protec- tion are common [80–82]; warranting a wider assessment focus. For example, to other, less economically valuable species that also play a role in a resilient ecosystem functioning and structure, and consequently also for long-term biodiversity conservation (i.e., winter skate (Leucoraja ocellata) and capelin (Mallotus villosus) [13, 83]. Uncertainty in the projections—the climate change projections for bottom temperature, bot- tom oxygen concentration, and bottom pH were obtained from Bio-ORACLE v. 3.0 [58]. [58] assessed the reliability and accuracy of the Bio-ORACLE v. 3.0 dataset through cross-validation against data from the Global Ocean Data Analysis Project (GLODAP). GLODAP provides in- situ observations for two of our focus variables (Data for pH cross-validation was not avail- able); however, their spatial coverage does not extent across the entire Gulf of St. Lawrence, potentially influencing the accuracy in the region [58]. For the areas in the Gulf covered by GLODAP observations, the accuracy of the average difference between the downscaled ensem- ble projections and in situ observations were relatively low for temperature (0.141 ˚C) com- pared to oxygen concentration (5.222 mmol m3) [58]. Hence, oxygen concentration was generally overestimated for the Gulf, potentially leading to a somewhat smaller future impact on BCBs in our analysis. Conclusion Our study emphasizes the critical role that recently established OECMs in the Gulf of

St. Lawrence currently play in providing indirect biodiversity conservation benefits (BCBs) for key commercially exploited species Atlantic halibut, Greenland halibut, and redfish. While current OECMs are effectively protecting crucial benthic habitats and seem to be contributing to the life-history success of these species, their long-term effectiveness is increasingly uncer- tain due to projected climate-driven changes in bottom temperature, oxygen concentration, and pH levels. These environmental shifts threaten the continued provision of indirect BCBs, potentially leading to habitat degradation, which could undermine the overall conservation goals of these areas. Our findings highlight the need for adaptive OECM management and policy to ensure that these areas continue to fulfill their biodiversity conservation objectives in the long-term. This includes the integration of climate-informed ecosystem indicators and regular non-invasive ecosystem monitoring to assess continuing provision of direct and indirect BCBs. Addition- ally, broadening the focus beyond economically important species to include a wider array of biodiversity is central for sustaining ecosystem functioning and long-term conservation out- comes. Addressing these challenges is essential for the OECMs in the Gulf of St. Lawrence and beyond this region to remain effective contributors to marine biodiversity conservation in the face of accelerating climate change. Supporting information S1 Fig. Spawning likelihood of Atlantic halibut (Hippoglossus hippoglossus) in the Gulf of

St. Lawrence. Range of Atlantic halibut spawning likelihood within the existing OECMs in the Gulf of St. Lawrence, as calculated by [49]. Numbers represent the OECM ID in Table 1. (TIF) S2 Fig. Bottom temperatures in the Gulf of St. Lawrence and preferred temperature ranges for three demersal fish species. (A) Baseline (2010–2019) and projected (2090–99, under SSP2-45, SSP5-85) bottom temperature range across the Gulf of. St. Lawrence. Boxplots: upper and lower hinges correspond to the first and third quartiles; upper/lower whiskers extend to PLOS ONE Indirect biodiversity conservation in the Gulf of St. Lawrence PLOS ONE | https://doi.org/10.1371/journal.pone.0316754 January 9, 2025 14 / 20