Cetaceans may be found in all the oceans of the world, but some individual species are not universally distributed because of their specialised diet. For instance, the great Baleen Whales do not migrate from one hemisphere to the other, but remain in the high latitudes where their food is usually found and have adapted to the seasonal availability of this food. During times of plenty, each one can be taking anything up to 4 tons of krill a day. This is sufficient to tide them over periods of scarcity which may last for several months.
The Toothed whales, the odontoceti, generally do not have this limitation as they are the true hunters of the cetacean world and are distributed over a much wider area. The Killer Whale, or to give him his proper name Orca, can be found anywhere in the oceans between the northern and southern ice caps.
It is only in a few instances that this rule does not apply, namely the species of dolphin that live in fresh water. They are to be found in such rivers as the Amazon, Indus and Ganges and represent a specialised form, as they rely more on their sonar than other dolphins and in some cases have become totally blind.
Dolphins, such as the Common Dolphin, may be found virtually anywhere and are especially prevalent in the Mediterranean. These are the ones that have been known to man for thousands of years. Indeed the story of how the poet Arion was rescued from pirates 600 years before Christ, shows how long man and dolphins have had an affinity for each other.
The Straits of Gibraltar are one of the great natural land bridges of the world and are famous for the biannual migration of bird life. What is generally not so realised, is that there is a continual passage of cetaceans passing between the Mediterranean and the Atlantic. It is certain that the Straits are monitored by NATO listening devices. It would be of invaluable assistance to marine biologists to know what record, if any, is kept of cetacean movements which are much noisier than those sounds created by submarines.
It is only in comparatively recent geological times that all this marine movement has become possible. Twelve million years ago, Africa slowly bumped into Spain and a low mountain ridge was formed between Tangier and Tarifa. The Mediterranean became a vast lake eventually drying up, leaving vast deposits of salt behind. Six million years later, Africa started to retreat from Europe and a breach was created in the mountain dam previously formed. The largest waterfall the world has ever known was created. For a hundred years, the Atlantic thundered over a 3000 ft drop, yet again filling the Mediterranean basin, thus allowing the reintroduction of marine life into this area.
Because the Straits are only 13 miles wide at their narrowest point, the concentration of cetaceans is very high, especially so towards the middle. It is about 3 miles offshore that you find the larger examples, such as the Sperm Whale. For some reason most sightings show that they are heading in a westerly direction. The prevailing surface current is easterly and these animals may be just stemming the current flow and not in fact travelling out of the Straits at that particular time.
From prominent headlands, with patience and binoculars, it is possible to see the smaller cetaceans closer inshore. In the Bay of Gibraltar, Common and Striped dolphins are always present.