A new chain

Seabirds’ feeding pattern linked to fishing boats  

By Diya Das
Published: Monday 15 March 2010

image‘If you cannot fight them, join them’. Call them opportunists but seabirds do not mind adhering to this saying. Ever since humans had to go disrupt the balance of the marine ecosystem by overfishing, the marine food web lost its usual threads. An interesting chain was recently studied—the feeding pattern of seabirds linked to fisheries.

Seabirds are free-ranging which means they hunt for food in the open sea. They leave their breeding grounds for two days at a time, covering 10 to 1,000 km at every trip.
But there is no guarantee the birds go back home satiated as the food consists of highly mobile creatures like small fish and squids.

With the increase in fishing activities, the birds have had an option of scavenging on the waste of fishing boats that fish for deep sea fish during the day. As compared to natural prey that may or may not be available, this is a more attractive alternative. The discards are available at fixed locations and are a reliable and ready food source.

Frederic Bartumeus, researcher at the Department of Ecology and Evolutionary Biology of the Princeton University in usa, studied the movements of the Mediterranean Balearic shearwaters (Puffinus mauretanicus) and Cory’s shearwaters (Calonectris diomedea). Information on their flight pattern was obtained by satellite tracking during 1999-2005. Data showed birds gather around the fishing areas on days when trawlers are active. Since trawlers do not operate on weekends and holidays, the scientist could draw definite contrasts in the birds’ movement patterns.

“They do seem t depend on the extra resource to improve their breeding success. One explanation is that borrowing from the boats is more efficient energetically because they have to spend less time looking for food,” said Bartumeus.

“Some studies show a direct link between discards and the birds’ reproductive success,” said Ivan Ramirez, the European marine coordinator of Bird Life International, a global conservation organization. “Discarding must not be justified for its effects on the birds’ breeding as it is a symptom of unsustainable fishing practices,” said Euann Dunn, senior marine policy officer at Royal Society for the Protection of birds, a UK non-profit.

Trawlers alter the birds’ usual movement pattern. “But the birds won’t forget their natural search strategy if the boats disappear. They adapt easily,” Bartemeus said. Young birds may have to free-range owing to the ferocious competition at the boats. In the long run, this might create differences in the breeding abilities of a population.

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