Who are the Lisbon Dolphins?

Dolphins had been frequently sighted in the Tagus river for 50 days now. It has been incredible the day by day frequency that common dolphins are seen so close to the city centre.

‘Core group’ in 2020

During this time it is clear that all these sightings were just a visit, as the groups quickly return to the Sea after a meal in the river. These are in agreement with the data from past years, only with the higher frequency.
There are many reasons that could explain this phenomenon and most likely it will not be just one. We keep a firm position on the food source hypothesis as the heaviest factor, published on the last article. However some pertinent hypothesis had been put on publications from others, namely:

  • The improvement of water quality, which could very well explain an increase over the past decade, but unlikely to explain a large increase since the past year
  • The dolphins in the river are just a reflection of an increase in the numbers of the overall population of dolphins in the Portuguese coast

The last one is puzzling and on our last tour we managed to put some light on this subject. Through photo-ID we managed to positively match at least half of the 21 identifiable individuals from the last sighting. These individuals were all sighted together in July 2019, just 1 hour worth of swim from the mouth of the river. We can conclude that these dolphins are locals that had been around for few time.

‘Core group’ in 2019

It can not be implied that it is always the same group visiting the Tagus river, but data points towards that at least that a ‘core group’ of individuals is on the large majority of the sightings in the Tagus river in 2020. It is to be noticed that all of these ‘core group’ individuals were sighted together in 2019.

DD12 is one of the most sighted individuals, as the wound in the beak is easily recognisable

One month of dolphins

No guests to see the dolphins but still we record every sighting we can. Since May 24th, common dolphins had been sighted in the Tagus river on a regular basis and very close to the city centre.

On the 3rd week in a row we managed to identify an old acquaintance, the individual DC10 is a very conspicuous common dolphin, due to the missing tip of the dorsal fin (likely cut by fishing lines).
‘He’ (likely a male) was first recorded in 2018 and had been re-sighted every year since then.
We wander why this regularity, environmental factors are unlikely, as water temperature, tides, currents had been shifting all over the entire period. The stronger factors are the ones basic for life, like food. Fishermen gave us a clue, it seems that there is a lot of sardines and mackerel inside the river and these come to be the favourite fish of common dolphins.
Estuaries are among the most productive natural habitats in the world, so it makes sense for large schools of these fish to settle inside or close to the Tagus river, as Tagus has the widest estuary in Europe.
The next questions are to determine if indeed the amount of fish found last month inside the river is significantly higher than past years and if so why. Can the current pandemic played an important rule in the fishing industry? Can these impacts allow the fish stock to grow? It happened before, after BP oil spill in the Gulf of Mexico back in 2010.