Abstract
Large-scale line-transect surveys of cetaceans in European Atlantic waters (known as SCANS) began in 1994, at that time covering the North Sea, parts of the Celtic Sea and adjacent waters (Hammond et al., 2002) with the primary goal to obtain the first comprehensive abundance estimate of harbour porpoise to contextualize bycatch estimates. These regional coordinated survey efforts continued in 2005 with SCANS-II, expanding the survey to all shelf waters (Hammond et al., 2013), and in 2007, extending to offshore waters (CODA, 2009). Subsequent SCANS surveys covering a much-expanded area followed in 2016 (Hammond et al., 2021) and 2022, with the ObSERVE project covering Irish waters in 2016 (Rogan et al., 2018) and 2022 (Giralt Paradell et al., 2024). By now, these surveys have yielded a time series of data on regularly occurring cetacean species, thus facilitating investigation of changes in distribution and abundance at an ecologically appropriate large spatial (European Atlantic) and temporal (almost three decades) scales for long-lived mobile species. These surveys have set high standards for data collection and analysis.
The SCANS surveys aim to provide critical information on population abundance and trend of cetacean species for statutory reporting and assessment needs across the NE Atlantic region, supporting EU Member States in reporting on Favourable Conservation Status (FCS) under the Habitats Directive (HD) and Good Environmental Status (GES) under the Marine Strategy Framework Directive (MSFD, Article 17) or national equivalent for non-EU Member States. The SCANS surveys also facilitate delivery of coherent and consistent marine mammal assessments under the Regional Sea Conventions OSPAR and HELCOM.
The SCANS projects have been implemented at approximately decadal intervals during the summer months. However, to inform relevant reporting and ecosystem assessments, the frequency was changed to six years, which is the reason why the fourth iteration of SCANS was conducted in the summer of 2022. SCANS-IV covered a 1.7 million km2 study area stretching from the Strait of Gibraltar to southern Norway, providing robust abundance estimates and trends for regularly occurring cetacean species. The conventional design-based estimates of abundance are presented in Gilles et al. (2023), which are critical to inform assessments of the impact of anthropogenic activities, such as fisheries (e.g. by-catch) and offshore industries, especially renewable energy.
In the context of marine spatial planning, the outputs of habitat-based density models or species distribution models (SDMs) are increasingly used for marine management and conservation applications, including the assessment of potential impacts from a wide range of anthropogenic activities (Hammond et al., 2013; Gilles et al., 2016, Lacey et al., 2022, Pigeault et al., 2024a). The effectiveness of SDMs as conservation management tools is attributed to their capacity to predict spatial and temporal changes in species distribution patterns. Consequently, another major aim of SCANS-IV, and all preceding surveys, was to supply information on summer distribution by modelling the data in relation to environmentally-linked spatial features to generate density surface maps.
In this report, we summarise the development and results of predictive habitat-based models of cetacean density for seven species and two species groups using survey effort and sighting data from the SCANS-IV surveys conducted during summer 2022.