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Other than their large size, their mottled blue skin colour is unique among whales. Depending on light conditions, blue whales may appear light grey in colour. They are more robust than fin whales with a broad, rounded head and a tiny dorsal fin. Their baleen plates are navy in colour.
Up to 12 m vertical blows are produced when surfacing. The length of their back is obvious when surfacing as they slowly roll forward revealing their dorsal fin. They rarely breach and regularly tail fluke.
Sightings and acoustic detections in recent years have shown they occur during the summer and autumn months offshore along the continental shelf edge, in waters of around 700m depth. One Irish whale was matched between the Azores and Iceland using photographs of its mottled skin.