18.02.2021 – An exchange

Our morning tour on the Stenella was spent with a dispersed group of Bottlenose dolphins (Tursiops truncatus) that were travelling through the waters of Madalena do Mar, one of the islands hotspots for this species. Despite being amongst one of the smartest and most adaptive of all ocean dwellers, these dolphins often display behavioural modifications that can well be linked to the problems we cause in our oceans. 

Here in Madeira, Bottlenose dolphins are often seen associating with fishing boats, particularly with vessels pulling up Black scabbard (A.carbo) long-lines and often even steal the catch from the fisherman. The scabbard is a deep-sea fish that occurs in depths below 800m, far beyond the deepest documented dive of 550m for the species, making encounters between the two under natural circumstances highly unlikely to say the least. Scabbards are also notorious for the elevated levels of heavy metals in their tissue which can not be metabolised and will accumulate in the tissue of dolphins. Some heavy metals may be of natural geological origin, such as nickel or zinc but chemical pollution in several regions has increased their concentration to concerning levels, particularly in the deep sea.

Today the Bottlenose dolphins we encountered were also seen close to a fishing vessel before suddenly darting to another very different source of marine protein for humanity; the aquaculture farms in front of Calheta. These farmed enclosures often contribute to a significant change in the coastal ecology of an area, with the smell and noise created by the trapped fish in the cages attracting several hungry predators including large fish, sharks, turtles and even cetaceans! Not long ago, our team even saw a large baleen whale and her calf circling dangerously close to the nets and today the Bottlenose dolphins darted through the area. Farming these fish, in this case Gilt-headed breams (Sparus aurata) a species that does not naturally occur in the region, often involves the use of antibiotics which may also spread into the coastal ecosystem surrounding the fish farm. 

Every relationship is an exchange and requires a certain balance so that it remains mutually beneficial. We have long tipped this balance in our oceans but, now that we are aware of it, we can finally make an effort to restore this necessary equilibrium. Harvesting our oceans more intelligently and minimising the amount of chemicals seeping into our oceans may help improve our relationship to one of the worlds most important and ancient ecosystems.

By Paula Thake

Sightings of the day

Stenella

10:00 Bottlenose dolphins

18.02.21