The times they are a changin’: Temporal patterns in small cetacean abundance in the northeast Atlantic

Conserving marine species effectively requires spatially and temporally explicit knowledge of their abundance and distribution for assessing potential impacts (e.g., from fishery bycatch, anthropogenic sound, ship strikes) over different spatial and temporal scales. We used aerial surveys and distance sampling to estimate abundance of seven frequently encountered small cetacean species in the northeast Atlantic and examine seasonal and interannual variations. Five surveys were carried out over the Irish EEZ in summer 2016, 2021 and 2022 and winter 2016–2017 and 2022–2023. Seasonal and annual variations in abundance were seen across species. Harbour porpoise showed a steady decrease in abundance, with estimates ranging from 38,260 (CV: 23.7) individuals in summer 2016 to 6,604 (CV: 40.8) in winter 2022–2023. Similarly, bottlenose dolphin numbers ranged from an unprecedented 212,646 (CV: 15.5) individuals in the winter of 2016–2017 to 11,328 (CV: 42.4) in summer 2021. In contrast, common dolphins increased from 13,192 (CV: 75.2) individuals in winter 2016–2017 to 594,293 (CV: 28.2) in summer 2021. Other species, such as Risso’s, Atlantic white-sided, white-beaked and striped dolphins were seen more sporadically and in lower numbers. The recent decline in harbour porpoise abundance is concerning and merits further investigation. Results provide the first measure of seasonal and interannual variability in abundance, possibly reflecting large scale movement patterns and population change and allow for the seasonal assessment of potential anthropogenic impacts.

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