Marine heatwave exposure is changing unevenly across Scottish aquaculture sites

Abstract

Introduction Marine heatwaves (MHWs) are an increasing climate hazard for coastal aquaculture, yet regional assessments may not resolve how exposure varies across the locations where farms operate. Methods Here, we analysed daily ~5-km OSTIA sea surface temperature data from 1990 to 2025 at the locations of 361 active Scottish finfish and shellfish farms, represented by 218 unique grid cells, and compared satellite temperatures with in situ records from eight farms. We used principal component analysis and hierarchical clustering to describe long-term MHW exposure, and mixed-effects models to assess temporal changes. Results Two broad exposure profiles were identified. Cluster 1 was characterised by higher mean, maximum and cumulative intensity, whereas differences in event duration and frequency between clusters were smaller. Temporal trends, however, differed more strongly. MHW frequency increased at similar rates in both clusters, while event duration, cumulative intensity and severity increased more rapidly in Cluster 2. Between 1990 -2019 and 2020 -2025, estimated event duration increased by 44.6% in Cluster 1 and 89.9% in Cluster 2, while cumulative intensity increased by 50.0% and approximately 127.5%, respectively (cluster × period, p < 0.001). The estimated probability of a Severe or Extreme event rose from 2.2% to 26.2% in Cluster 2, compared with 1.8% to 6.1% in Cluster 1 (cluster × period, p < 0.001). Mean event intensity changed comparatively little. Discussion These findings show that MHW exposure across Scottish aquaculture is multidimensional, spatially uneven and changing at different rates through time. The resulting hazard -exposure landscape helps bridge regional MHW assessments and individual-farm monitoring, providing a foundation for more targeted monitoring and future assessment of aquaculture vulnerability and climate risk.