(photo credit: .)
Extensive commercial fishing trawlers endanger the common bottlenose dolphins in the Mediterranean, by both poaching their food supply and catching them in their nets, according to a new study carried out at an Israeli university.
The study, by researchers at the University of Haifa, also illustrates for the first time the characteristics of the dolphins inhabiting the sea region off the Mediterranean coast of Israel. This dolphin population is stable and at any given time numbers about 350. Of these, the researchers are personally familiar with 150 dolphins on a first-name basis as they can identify them by the dorsal fin, the dolphin’s “fingerprint.”
The study, carried out by Dr. Aviad Scheinin and supervised by Prof. Ehud Spanier and Dr. Dan Kerem, of the university’s Department of Maritime Civilizations, examined the competition between the two top predators along Israel's Mediterranean coast: the common bottlenose dolphins and commercial fishing boats called ‘bottom trawlers.’
Both the dolphins and the trawlers, which drag a large fishing net close to the sea floor, trap codfish, red mullet and sole, three popular commercial types of fish.
According to data from the Agriculture Ministry’s Department of Fisheries, the fish population on the sea floor along the coast dropped between 1949 and 2006, indicating that the number of fish taken from the sea by commercial trawling is larger than the amount that nature provides.
To determine whether or not the decline in the supply of fish living near the sea floor directly affects the dolphins, the researchers examined the contents of the stomachs of 26 dolphins that died and landed on the beach, or that had been caught by mistake. They also examined the behavior of living dolphins by carrying out 232 marine surveys along the central coast of Israel.
The dolphins’ stomachs contained mainly different kinds of fish from those targeted by the trawlers, suggesting that they perhaps do not compete directly with the trawlers. However, the behavior of the living dolphins paints a very different picture.
According to Scheinin, the chance of observing a school of dolphins near a trawler is 10 times higher than in the open sea. He explains that this is because the trawler serves as a “feeding station” for the dolphins. While they are not able to feed from the fish caught in the nets, they are able to catch other types of fish that swim around the trawler.
“The problem is that this type of fishing endangers the dolphins. Eight dolphins die each year off the coast of Israel on average, and of those, four die after having been mistakenly caught in trawling nets,” says Scheinin.
He believes that since numerous studies have proven the high intelligence of the dolphin, the sea mammals are probably aware of the danger posed by the trawlers, but approach them nonetheless due to their need for food, which is more plentiful around the boats.
His conclusion is reinforced by the behavior of suckling female dolphins that require larger quantities of food. Despite the risk for the younger and much less experienced dolphins that swim by their side, all the suckling dolphins were observed significantly more frequently around the trawlers, indicating that they could not obtain enough food in other places.
When taken together with the observation that dolphins off the coast of
Israel spend most of their time in search of food while those in other
parts of the world are far busier with social activities, it seems
clear that they are coping with a deficiency in food resources.
“There is a stable dolphin population off the shores of Israel, and any
resolution concerning the sea must also consider the dolphins. So as to
preserve this population we must declare extensive marine nature
reserves, so as to regulate fishing and bring an end to sea pollution.
Regrettably, we are not considerate enough of the dolphins,” concludes