Long-Term Fluctuations in Cetacean Occurrence Derived From Whale-Watching Platforms Off Southwest Gran Canaria:...

Long-term variability in cetacean detection patterns in oceanic archipelagos remains poorly documented, limiting our understanding of temporal dynamics and their relevance for marine conservation and management. Here, we analysed an 18-year dataset (2004-2025) comprising 9275 cetacean detections collected from whale-watching platforms off south-western Gran Canaria (Canary Islands, NE Atlantic). Twenty cetacean species were detected, with odontocetes dominating the records, marked seasonal variability and recurrent detection of mother-calf groups of Stenella frontalis and Tursiops truncatus in coastal waters. Species-specific models revealed contrasting temporal trajectories among taxa, while direct relationships between monthly detections and sea surface temperature or chlorophyll-a concentration were limited after accounting for temporal variability and sampling effort. Effort-standardised sightings per unit effort (SPUE) showed substantial temporal variability, including increased detection rates during 2013-2019, a marked reduction during the COVID-19 period, and variable detection rates thereafter, reflecting the combined influence of sampling disruption and natural variability in cetacean detection patterns. Spatial analyses showed overlap between areas of recurrent cetacean detections and existing Special Areas of Conservation (SACs), while additional areas of frequent detections were identified outside current protected boundaries. Our results demonstrate the value of long-term whale-watching datasets as complementary sources of information for assessing cetacean temporal and spatial variability. Although SPUE provides a cost-effective index of effort-standardised detection patterns, further validation with independent monitoring approaches is required before it can be incorporated as a robust ecological indicator.