8 Jan 1989 - Bottlenose Dolphins

A day of catching up with bits and pieces. Nature watching is not entirely neglected as we spend much of the afternoon watch­ing a pod of dolphins in the mouth of the est­uary.

There are about a dozen of them, large and dark with a prominent, slightly backward bent, dorsal fin. Mostly they swim near the surface with only the dorsal fin and back showing, but sometimes they dive, showing tails above the surface as they go down, and usually staying sub­merged for several minutes. Often they blow water spouts when they returne to the surface.
Occasionally one breaches, leaping out of the water and falling back in with a splash, show­ing off a pale belly. There are two sizes, perhaps adults and youngsters.

They stay in the estuary for at least sev­eral hours, moving to and fro between the estuary mouth and the western end of Setubal (the industrial port area), but prefer­ring to be on the far side, where the water is deeper. They seem to ignore boats of all sizes, but when a large con­tainer vessel gets in the way of our observations, the dolphins seem to disappear.

This evening we watch the dolphins again; they are sometimes in one group, sometimes in two and threes. Again, mostly all we see is the back and dorsal fin as they swim around and between the boats. As the light fades they came much closer than before and we are able to identify them as bottle-nosed dolphins.

Dolphins are said to have inhabited the estuary since ancient times, although the first written report of them was in 1863. A study done between 1981 and 1986 (ref: dos Dantos & Lacerda) showed that there was a more-or-less stable group of about forty bottle-nosed dolphins ranging along the coast between Sines and the Tagus estuary, and going up to 40 km upstream in the Sado Estuary. They could be seen at any time of the year, and at any time of the day or tide. Twenty-six of these animals could be recognised individually from scars and the shape of their dorsal fins.

The dolphins used the estuary for feeding, resting, travelling and probably general social interaction. Much of the behaviour observed during the study was similar to what we were seeing, except that it was often ob­served from a boat closer to the animals. The most common activity pattern was a compromise between travelling and feeding. The animals showed surfacing sequences which involved two or three short submersions (less than fifteen seconds) followed by a longer, deeper one of up to three minutes. Sometimes they travelled quite fast in one direction, sometimes they stayed in one area for some time if there was food there.

Their preferred foods were cuttlefish and mullet, but a wide variety of other species was taken when the opportunity arose. The mullet were likely to be chased, thrown up and caught in mid air, or, in shallow water, thrown onto the beach when the dolphins would partly strand themselves to get hold of the prey. Cuttlefish were caught and neatly broken in half with the cuttlebone half being rejected.

Another Swede staying in the campsite tells us that until about three years ago (ie the end of the above study) it was not uncom­mon to see lots of dolphins in the estuary (he thought two or three hundred!). Then the industry which grew up at the western end of the docks polluted the water to such an extent that, according to a local fisher­man, there had been thousands of dead fish float­ing on the water, or washed up on the beach. The fisherman blamed the EEC.

No fish meant no dolphins, and the local fishing industry was also in danger. Local laws were passed to control, or at least reduce, the pollution, and now the fish and the dolphins are back, though not yet in their previous numbers. Dolphins deserted the nearby Tagus estuary in the 1960s, and that estuary is thought by some to be so polluted that the dolphins will never return there.