BioEd Online

Sea birds have developed an unhealthy habit in snacking on the fish waste tossed overboard by fishermen. Relying on this 'junk food' means the birds aren't getting the balanced diet they need to adequately feed their young, according to new research.

Fishermen often gut fish while still on board the ship, taking the valuable parts and dumping the rest at sea. Included in this waste are the carcasses of some whole fish and species caught by accident. In total, about 7.3 million tonnes of waste are discarded each year around the world. This waste has long been assumed to be harmless since it is just organic material. Some have even argued that the additional food is helpful to sea birds feeding on it.

Unfortunately, the waste is not as benign as originally thought.

Previous research has shown that for South African Cape gannets, for example, this 'junk' has about half the calorific value of birds' natural prey. Now David Grémillet at the CNRS — the French national scientific research body — in Montpellier, France, and his colleagues have shown that this has a negative impact on young chicks being fed by malnourished parents.

Fishy diet

Over the course of a year, Grémillet and his team analysed the regurgitations of 444 South African Cape gannets as the birds returned from hunting excursions at sea. From May to September, the season for fishermen in the area, waste — often in big chunks — comprised more than 70% of the gannet diet.

Cape gannets normally feed on small fatty fish like anchovies and sardines. But fishery waste in South Africa comes primarily from the body organs of hake, a lean fish that lives at the bottom of the sea, far beyond the reach of the diving gannets. While hake organs are suitable for adult gannet survival, Grémillet says that they lack the fat content that young gannets need while growing. The researchers estimate the chicks are getting approximately 29% less energy than they need during the rearing period.

Lack of calories might not be the only problem. "I wonder to what extent [the waste] contains bones sufficiently small to be consumed," says avian nutritionist Shana Lavin of the Lincoln Park Zoo in Chicago, Illinois. The bones in anchovies and sardines are likely to be an important source of calcium for growing birds. If the bones in the waste are too big for them to eat, calcium deficiency might be an issue, she explains.

Food for the kids

The team also tracked 14 adult Cape gannets using data loggers that monitored the depths to which they were diving when hunting. The loggers revealed that gannets with young to feed were diving more often, and thus working harder, than birds without chicks in the nest. Dives are necessary to catch live fish, but not to scavenge floating fish waste.

Despite all this extra diving, live prey catches remained just 20% or less of the birds' diet. This is probably because anchovy and sardine numbers have plummeted in the area due to the combined effects of over-fishing and climate change, says Grémillet. The lack of high-calorie food in the parents' diets could be causing a crash in chick survival; breeding success dropped from 0.42 chicks per nest in 2004–2005 to 0.02 chicks per nest in 2005–2006, compared to 0.69 in the 1980s. The gannet population has dropped by 50% over 50 years, says Grémillet.

Keeping up with the sardines

The anchovies and sardines have moved to the eastern shores of South Africa as climate has warmed. The gannets, however, have not budged. This could be because gannets tend to stick to nest sites year after year. The existence of readily available food, in the form of waste, probably reduces any incentive to move on.

"In the North Sea where fish waste has also had a negative impact, it is recommended that the waste be brought back to shore to be processed as fish meal. We would encourage the fishermen in South Africa to do the same," says Grémillet.

It remains to be seen whether all sea birds suffer from eating fishery waste. "Gannets are highly specialised feeders that need high fat foods; more generalist sea birds could have far fewer problems with waste," says avian biologist Stephen Votier at the University of Plymouth, UK. "We need to study more species to assess the global situation."