Close cooperation with fishermen can benefit dolphins. That’s according to a multi-year analysis of bottlenose dolphin behavior and survival (Tursiops truncatus gephyreus) who live near the southern Brazilian town of Laguna. There has been a special collaboration between the two mammal species for about a hundred years, with dolphins chasing schools of fish in the bay, after which the fish are caught by fishermen who cast their nets at exactly the right time.
It has now been established that the dolphins that cooperate with the fishermen have a 13 percent better chance of survival than dolphins that do not. This is evident from three hundred hours of detailed observation of this coordinated behavior (spread over three years) and from inventories of the dolphin population over a period of twelve years. The research was published this week in the PNAS.
The fact that the cooperating dolphins have a higher survival rate is also influenced by the fact that the non-cooperating dolphins more often swim outside the bay where they can get entangled and drown in (illegal) large nets of other fishermen at sea.
Artisan fishing
In Laguna it is a small-scale fishing method in a bay for migrating mullets, Mugil liza, a fish about 40 cm long that can be found all along the eastern coast of South America. The researchers call it ‘artisan fishing’. The fishermen stand in the water along a hundred-meter-wide beach and each cast their own net by hand.
Normally they throw out the net when they see fish. But adding the dolphins creates a fine coordination between the fishermen and the dolphin group – sometimes a dolphin alone, sometimes a group of twelve dolphins. When the dolphins see the fishermen standing in the water, they chase the schools of fish to them. The fishermen see this and wait without any movement, the nets ready for the cast. When dolphins are about four meters in front of the fishermen, one of the dolphins makes a ‘special move’ towards the fishermen, usually by briefly raising its back and dorsal fin, or sometimes by putting its head or tail on the water to hit. At that signal, the fishermen cast their nets right in front of the approaching fish.
The advantage for the fishermen is clear: with the dolphins nearby, they are 17 times more likely to catch fish in their nets under similar conditions, and they also catch four times as much fish as they would otherwise.
The total dolphin population consists of about fifty dolphins (the number varies per year). The fishermen recognize most individual dolphins by their dorsal fins, and differentiate them into ‘good’, ‘lazy’ and ‘bad’ dolphins. That classification corresponds to the dolphins that cooperate regularly (the ‘good’, about 40 percent), the occasionally cooperating (the ‘lazy’, about 20 percent) and the rarely or never cooperating (the ‘bad’, approx. 40 percent).
Throwing stones
It is clear that the dolphins themselves also catch fish during this cooperation, the fishermen usually feel that the dolphins pull one or two fish from their nets. A few fishermen try to prevent this ‘robbery’ by throwing stones, but most allow it.
The cooperation does decrease over the years, say the fishermen, and the researchers saw this reflected in their figures, closely related to the decrease in the schools of mullet, which are overfished by industrial fishers at sea. The researchers predict that cooperation will therefore disappear completely in about fifty years, unless the fishermen continue to work together despite the lower yields. The illegal nets outside the bay must also be combated more actively, in which the dolphins sometimes drown. Subsidies for ‘artisan fishermen’ could make a major contribution to conservation in the area, they write.
Incidentally, fishing with the help of dolphins has already been declared a protected cultural asset by the state in which Laguna is located. The throwing net spectacle with dolphins has also become a tourist attraction. Lots of fishermen also pride themselves on their dolphin-assisted fishing.
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