Our coastal dolphins, the Bottlenose dolphins (Tursiops truncatus) and Short-beaked common dolphins (Delphinus delphis) were sighted during all three tours today. Both these species prefer the safer, coastal waters of the island but will venture further out if a good hunting opportunity presents itself. We’ve had several sightings of common dolphins hunting further offshore, at around 4 nautical miles, where the dolphins were likely gorging on planktivorous fish. At this distance there must be a beneficial current bringing nutrients for plankton to flourish, attracting more planktivorous fish and their predators.
Bottlenose dolphins also move offshore to predate but also to de-predate. The latter refers to an activity where the dolphins don’t act as hunters but rather as scavengers. In Madeira, they often approach fishing boats reeling in their long-lines and scavenge on their prized catch, the infamous Black scabbard fish (A.carbo). Under natural circumstances the two species would never come into contact with one another, since scabbards usually occur beyond 800m into the deep, a depth beyond the reach of the dolphins. The problems associated with this interaction include the possibility of the dolphins getting entangled in the lines and the fishermen losing their catch, meaning they would have to lower their long lines again into the fragile ecosystem that is the deep sea.
We witnessed such a depredation event today but are glad to confront our guests with such situations, since they simply represent realities.
By Paula Thake
Sightings of the day
09:30 Bottlenose dolphins
13:30 Bottlenose dolphins
16:00 Short-beaked common dolphins