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

Lawrence Coral Conservation Area, Eastern Gulf of St Lawrence Coral Conservation Area, North of Bennett Bank Coral Conservation Area, Slope of Magdalen Shallows Coral Conserva- tion Area, and South-East of Anticosti Island Sponge Conservation Area) overlapped with pro- posed spawning habitat of Greenland halibut (Fig 1). The Central Gulf of St Lawrence Coral Conservation Area had the highest likelihood of harbouring Atlantic halibut spawning habitat with a range of 0.06–0.12 (on a scale of 0–1; Table 2), followed by the North of Bennett Bank Coral Conservation Area, the Slope of Magdalen Shallows Coral Conservation Area, and the Western Honguedo Strait Coral Conservation Area (Fig 1 and S1 Fig). The highest redfish larvae density (100–120 ind./m2) overlapped with the Central Gulf of St Lawrence Coral Con- servation Area, the Eastern Gulf of St Lawrence Coral Conservation Area, and the Slope of Magdalen Shallows Coral Conservation Area. The potential spawning habitats for all fish spe- cies overlapped with vulnerable benthic habitats for sponges and sea pens (Fig 1). Current and future bottom environment of the Gulf of St. Lawrence Mean bottom temperature across the Gulf of St. Lawrence from 2010–2019 (henceforth, referred to as baseline) was 4.33 ˚C; under the mid-century future projections (2050–2059), mean bottom temperatures were between 5.57 ˚C (SSP2-45) and 5.95 ˚C (SSP5-85) (Fig 2). The mean baseline and the mid-century temperatures under both SSPs lay within the preferred temperature ranges for non-spawning adult Atlantic halibut (-1.5–15 ˚C), redfish (3–8 ˚C),

and Greenland halibut (-1.9–9 ˚C). For spawning adults, the preferred temperature range is narrower compared to non-spawning adults. Mean baseline and future bottom temperature across the Gulf of St. Lawrence was within the preferred temperature of adult spawning redfish and Atlantic halibut; adult Greenland halibut is likely to face unfavourable spawning habitat conditions in the future (Fig 2). While mean temperature largely remained within preferred temperature ranges of the three focus species, the high variability of the projected temperature under both SSPs across the Gulf also indicates that these thresholds may be seasonally exceeded (Fig 2 and S2 Fig). This pattern was exacerbated with intensifying future climate change, as represented by the end-century changes under SSP5-85, where the projected mean bottom temperature is exceeding the preferred temperature for spawning adults across all three focus species (S2 Fig). The impact of changing bottom temperature on non-spawning and spawning adults, and by extension on the provision of long-term BCBs, may be exacerbated by changes in decreas- ing bottom oxygen concentrations and increasing pH (Fig 3). This becomes especially appar- ent when zooming into the individual OECMs (Fig 3B). For all OECMs, mean baseline bottom temperature was largely within the preferred temperature range for spawning adults of the three focus species (Fig 3B). Mid-century, projected temperature changes under both emissions scenarios remained only within this range for the East of Anticosti Island, Jacques-

Cartier Strait, and Parent Bank; all other OEMCs were projected to experience temperature increase exceeding the species-specific preferred spawning temperature (Fig 3B). This pattern is exacerbated by the end of the 21st century (S3 Fig). Mean bottom oxygen concentration across the OECMs was largely above the hypoxia threshold for the baseline and mid-century time periods (Fig 3B). Mid-century mean oxygen concentration was below the hypoxia threshold within the Eastern Honguedo Strait, Parent Bank, South-East of Anticosti Island, and Western Honguedo Strait, and the South-East of Anticosti Island Sponge Conservation Area (Fig 3B). By the end of the 21st century, these areas remain within hypoxic conditions, with the Central Gulf of St. Lawrence and North of Bennet Bank moving into hypoxia as well (S3 Fig). PLOS ONE Indirect biodiversity conservation in the Gulf of St. Lawrence PLOS ONE | https://doi.org/10.1371/journal.pone.0316754 January 9, 2025 6 / 20