09.03.2020 – Seismic surprise
Our tour today took place under beautiful sunny conditions and our traditional boat managed to track down a group of Bottlenose dolphins (Tursiops truncatus) and Sperm whales (Physeter macrocephalus). While the Bottlenose were travelling east, the Sperm whales were more preoccupied in diving into the abyss to feed. The matriarchal group that was engaging in these deep ventures was the first to be sighted by our company in four months and ironically are spotted once again after Women’s day weekend and after a small seismic shock hit Madeira.
I was already anticipating the question of whether the 5.3 magnitude earthquake that shook the island also affected cetacean populations nearby. At the water’s surface I would imagine things wouldn’t have been much different to the 7 second tremor on land; more or less like a bowl of water being shaken slightly making resting at the surface a little more uncomfortable for Loggerhead turtles (Caretta caretta). Earthquakes, however do also rock the deep sea and happen to produce one of the loudest natural sounds underwater which can be a terrible ordeal for acoustically sensitive animals like cetaceans.
A study conducted conducted on 54 Sperm whales in the aftermath of the terrible Kaikōura earthquake in New Zealand on November 14, 2016 has shown that these seismic events can indeed cause injuries, hearing damage, displacement as well as behavioural modifications. The large tremor hit a 7.8 magnitude on the Richter scale causing high energy currents to flush 850 tonnes of sediment into the underwater Kaikōura canyon, a popular feeding areas for the Sperm whale groups feeding in the area. In fact the affect was so severe that the associated behavioural changes, which included the animals vacating their usual feeding areas, were maintained for at least a year.
One could therefore assume that earthquakes are more likely to affect deep-diving species and that, although the earthquake that shook Madeira wasn’t as strong as the one recorded in New Zealand, it may have caused some changes in the deep which we simply aren’t aware of and this, of course, affects the cetaceans roaming these areas. Thankfully the earthquake didn’t stop us from enjoying a lovely sighting with these deep-diving giants, we can only hope that the event didn’t affect them too much. Understanding how wild populations respond to such occurrences is an important step in understanding their level of resilience in order to better ensure their conservation.
By Paula Thake
Sightings of the day
10:00 Bottlenose dolphins, Sperm whales, Loggerhead turtle