The past week had an unprecedented amount of dolphins sighted in the river Tagus. A total of 5 sightings were recorded on video and shared throughout social media, 4 of them were in consecutive days.
All sightings were of small groups of common dolphins, corroborating the previous studies which indicated that common dolphins are the most sighted species in the Tagus. This contrasts with all the other rivers, which have mostly bottlenose dolphin sightings. This could be related to the depth preferences of these species, common dolphins prefer depths of at least 20m while bottlenose dolphins are perfectly comfortable in waters 2m deep. Most rivers do not get deeper than 10m, making them unsuitable for common dolphins to visit. However the impressive depth of the Tagus river with depths up to 60m, delivers enough depth for the common dolphins to visit while hunting schools of fish.
Only 2 of the sightings were in the usual area where we find them in our dolphin watching tours, close to the mouth of the river.
Two others were closer to the 25th of April bridge, one downriver and the other upriver from the bridge.
And on Sunday the last sighting was further upriver, inside the estuary, very close to the shore at Parque das Nações, the “New Lisbon” 25 km from the Sea. Impressive indeed but not close to a record, in 2010 this species were photographed 48 km upriver!!
The reasons why so many sightings condensed within a week remain unknown. Most likely numerous factors contributed to it, namely regular schools of fish entering the river probably for spawning, as feeding was the main activity seen in the videos.