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

All OECMs showed mean pH values smaller than the global mean of 8.1, with mid-century future values consistently lower than the baseline values (Fig 3B). Notably, the Parent Bank Sponge Conservation Area showed the lowest pH values across all time frames. This pattern is exacerbated by the end of the 21st century (S3 Fig). Notably, the Parent Bank Sponge Conser- vation Area showed a pH value a magnitude lower than the global mean under SSP5-85 (pH = 7.1). Overlap of OECMs and spawning habitats under future bottom environment changes By mid-century, eight out of the eleven OECMS overlapped with potential spawning habitat of at least one of the focus species (Fig 4). Four OECMs overlapped with all three focus species: The Central and Eastern Gulf of St. Lawrence Conservation Areas, the Slope of Magdalen Shal- lows and the South- East of Anticosti Island Conservation Areas. Interestingly, the Central and Eastern Gulf of St. Lawrence Conservation Areas are the two deepest areas. OECMs that were not overlapping with any potential spawning habitat are the shallowest areas, which is in line with the reproductive behaviour of all three focus species. However, these areas, and poten- tially the three focus species, were impacted by climate-driven environmental changes with the magnitude of changes varying by depth, SSP and projection time frame (Fig 4 and S4–S6 Figs). Notably, mid-century bottom temperature under both SSPs increased between 25–50% across all OECMs, except for Beauge´ Bank (Fig 4). Under SSP5-85, the South-East of Anticosti

Island as well as the Eastern and Central Gulf of St. Lawrence stood out, as they overlapped with all three focus species and are projected to experience simultaneous changes in bottom temperature (� 50% increase), bottom oxygen concentration (� -25% decrease) and bottom pH (� 1% decrease). Interestingly, Beauge´ Bank showed the smallest changes in all bottom variables in mean oxygen concentration under both SSPs; however, this area did not overlap with any of the proposed spawning sites of the three focus species. Mean pH levels decreased by mid-century across all OECMs, with Beauge´ Bank showing the largest decrease, followed by the Central Gulf of St. Lawrence, which overlaps with potential spawning habitat of all three focus species, and the East of Anticosti Island, which overlapped with potential redfish spawn- ing habitat (Fig 4). By the end of the 21st century, projected changes in the three bottom vari- ables were exacerbated in their direction of change across all OECMs, challenging the long- term provisioning of BCBs under intensifying climate change (S5 and S6 Figs). Discussion We assessed indirect biodiversity conservation benefits of selected OECMs in the Gulf of St. Lawrence using an integrated assessment approach. Our analysis showed that eight out of the eleven OECMs currently overlap with potential Atlantic halibut, Greenland halibut, or red- fish spawning habitat, indicating a life-history benefit for these species. OECMs that are not overlapping with any potential spawning habitat are the shallowest areas, which are, however,

also important for early life stages of Atlantic halibut [60]. All OECMs are projected to be impacted by future, climate-driven changes in bottom temperature, bottom oxygen oxygen concentration below 60 mmol m3, the threshold for a marine ecosystem to be considered hypoxic [20]. Dashed black line indicates today’s average ocean pH [59]. Boxplots: upper and lower hinges correspond to the first and third quartiles; upper/lower whiskers extend to the highest/lowest value within 1.5 times the interquartile range; horizontal lines within boxes correspond to the median; diamonds represent the mean; outlier dots represent data beyond the end of the whiskers. Baseline and end-century changes are shown in S3 Fig. https://doi.org/10.1371/journal.pone.0316754.g003 PLOS ONE Indirect biodiversity conservation in the Gulf of St. Lawrence PLOS ONE | https://doi.org/10.1371/journal.pone.0316754 January 9, 2025 9 / 20