11.07.2019 – Phantom bottom

This morning the conditions were good closer to the coast with wind picking up further out at sea. Our Ribeira Brava scanned the ocean up to Ribeira Brava before meeting a school of Striped dolphins (Stenella coeruleoalba), an evasive but beautiful oceanic dolphin species while our zodiac additionally managed to track down Atlantic Spotted dolphins (Stenella frontalis) and Short-finned pilot whales (Globicephala macrorhynchus) further east. For the afternoon tour, the Ribeira Brava scanned the same ocean area and got lucky at the end with a small group of Bottlenose dolphins (Tursiops truncatus) surfing the waves of the choppy seas outside Calheta.

The long searches gave me ample time to discuss interesting things regarding whales and dolphins with the guests. One interesting question was: what do dolphins do at night? We consider night-time as a time of rest and, considering that the ocean is at its darkest at night, why shouldn’t dolphins use that time to rest also. Thing is we don’t actually know what they do at night but life must still go on in a hostile marine environment with no light and this is where the animals bio sonar comes into the game. Dolphins can freely hunt, socialise and navigate at night because of their acute acoustic abilities, allowing some species, like the Short-finned pilot whales, to even hunt during the night. This in turn brings up another question; why hunt at night if during the day it is so much easier?

Well, Pilot whales enjoy hunting squid who, along with a variety of other mesopelagic creatures, migrate to shallower areas of the water column at night to feed on the abundant plankton with less danger of predation. This  so-called Diel vertical migration is so extensive that the migrating organisms even form a layer in the water column that can be detected by sonar operators on ships. This distinct layer is known as the “deep-scattering layer” (DSL)  or “phantom bottom”, because it gives the impression of a false sea floor at shallower depths. 

On yesterdays Stenella tour our team found a little Warmings’s lanternfish (Ceratoscopelus warmingii) wrangling with death at the surface. This mesopelagic fish is considered a DSL-organism, remaining at depths of up to 1500m during the day and migrating to depths as shallow as 20m during the night. It seems this little fellow didn’t get down in time. The specimen was collected and transferred to the Marine Biology station in Funchal, granting scientists in Madeira a small glimpse into our phantom bottom.

By Paula Thake

Sightings of the day

Ribeira Brava

09:00 Striped dolphins

13:30 Bottlenose dolphins

Stenella

09:00 Atlantic Spotted dolphins, Short-finned pilot whales, Striped dolphins

15:30 Atlantic Spotted dolphins, Bottlenose dolphins