I had the pleasure of accompanying many lovely guests on both our tours today who were all eager to learn as much as possible about our favourite ocean inhabitants. During our morning tour on the Stenella this morning, our spotter managed to track down a group of Short-finned pilot whales (Globicephala macrorhynchus) that were dashing eastward in a rather dispersed formation.
Our guests were most impressed by the diving behaviour of these large dolphins, who engage in deep sprint dives to hunt for mesopelagic fish and squid. Although an air-breathing mammal hunting prey that is able to breathe underwater may seem to be in the disadvantage, it is anything but that! Before diving, the animals almost completely empty their lungs so that their bodies can be compressed by the immense pressures in the deep and, from that moment on, rely on the abundant oxygen reserves in their muscles to power them through their foraging dives. Deep-sea animals tend to be rather lethargic in their movements due to the low levels in oxygen. The powerful energy cetaceans receive from their oxygen storages as well as their acute sonar and intelligent hunting techniques gives them several advantages during their hunts in the dark ocean. Such deep dives are, however, quite strenuous and usually call for some resting time at the surface which was observed in some of the pilot whales this morning.
Bottlenose dolphins (Tursiops truncatus) around Madeira almost always dive to hunt in the waters around the island and are rarely observed hunting at the surface. While they cannot dive as deep as the pilot whales, dives of up to 550m have been documented for these charismatic dolphins which is more than enough for them to score some deep-sea prey.
Atlantic spotted dolphins (Stenella frontalis) around Madeira also appreciate the deeper waters of the island, often travelling at the surface of waters as deep as 1300m. Although the spotted dolphins are often seen hunting Atlantic mackerel (Scomber scombrus), they have also been reported to hunt prey in deeper areas of the water column including squid.
Before they dive, all cetaceans must take several deep breaths and move lethargically, to lower their heart rate and decrease the oxygen uptake of the body as they descend to hunt. Many of these techniques may be acquired through social learning and require a lot of practice, but the result is nothing short of impressive and I think my guests on todays tours will definitely agree!
By Paula Thake
Sightings of the day
14:30 Atlantic spotted dolphins, Bottlenose dolphins
10:00 Bottlenose dolphins, Short-finned pilot whales