An autumn aggregation of fin (Balaenoptera physalus) and blue whales (B. musculus) in the Porcupine Seabight, southwest of Ireland

During a 16 week geophysical survey over the Porcupine Seabight (PSB) southwest of Ireland in July to October 2013, marine mammal observers logged 9382 km of effort. Balaenopterid whales comprised some 60% of a total of 373 cetacean sighting events (s), with a cumulative count (n) of 392 whales. Fin whales (Balaenoptera physalus) were especially abundant (s=111, n=209) and the number of blue whales (B. musculus) seen (s=12, n=16) exceeded the total previously reported from Irish waters, but 43% of balaenopterid sightings (s=98, n=172) were not identified to species level. Data for all balaenopterid whales were pooled and generalised additive models applied to identify environmental variables that predicted whale density and to estimate abundance and the spatial distribution of density. Depth range and chlorophyll-a concentration were significant predictors of whale presence, and depth and sea floor rugosity were significant predictors of group size. There appeared to be an influx of whales in September and October and the predicted abundance peaked in October with an estimate of 138 (95% CI 121-151) whales. Analysis of the direction of movement of whales showed no significant bias in any one direction. Feeding behaviour was observed in both whale species and circumstantial evidence suggested that they were aggregating to exploit seasonally abundant northern krill (Meganyctiphanes norvegica). Chasing behaviour observed among fin whales was interpreted as evidence that this aggregation also provided opportunities for social interaction related to their reproductive cycle. The PSB may provide a link between the high latitude summer feeding habitats of krill-feeding whales and a chain of highly productive habitats in the Eastern Boundary Upwelling Ecosystems and we suggest that whales may migrate southwards in autumn along this eastern route to the northwest African upwelling zones, where productivity peaks in winter.