How catch stop endangers many seabirds

Jena ecologist involved in study on the effects of trapping stops

Fishermen prey on more than they realize. Because their catch also influences the feeding behavior of the seabirds. For example, the great skua - also known as the skua - has prepared itself to "pick up" part of its food directly from the fishing vessels. Fish that are too small or unusable are thrown back into the sea as "bycatch". These fish "served" by humans have been identified by the skuas as easy prey and have become an integral part of their menu. "Skuas are generalists, they eat almost everything," explains Simone Pfeiffer from the Institute for Ecology at the University of Jena. The great skuas have adapted their feeding behavior to long-term fishing practice in such a way that "self-caught" fish and smaller seabirds only supplement the food from the bycatch.

Simone Pfeiffer - Institute for Ecology at the University of Jena

However, if the amount of human catches - and therefore bycatch - changes, this will also affect the seabird populations. This has now been proven in an international long-term study, which was carried out under the leadership of Prof. Dr. Robert W. Furness from the University of Glasgow (UK) ran. The results can be found in the February 19th issue of the internationally renowned journal "Nature".

When bycatch is reduced, smaller seabirds die

The starting point for the article was the proposal of the International Council for the Exploration of the Sea (ICES) of 20 October 2003 to temporarily stop fishing for cod in the North Sea. With its decision, the Council naturally wanted to protect the fish population and thus make further fishing possible in the future. But the team of scientists was able to prove, based on long-term data collections, that such an absolute fishing ban will have drastic effects on the population of some seabird species. Because the big birds, like the skuas, then switch to other food sources like smaller seabirds. "If the skuas' diet contains only 5% more seabirds, this means that around 2.000 kittiwakes have to die and are then eaten by the skuas," explains co-author Simone Pfeiffer using an example. "A sea bird community can therefore be disturbed by fishery management," says the 29-year-old from Jena. The absolute protection of the fish would therefore lead to the decline of other animal species. To protect birds - to which z. B. also fulmar, puffin and guillemot belong - not to be deliberately on the menu of the great skuas, the fishing quotas should only be lowered slowly. "However, it is not possible to control the size of the population by changing the catch quota," adds Pfeiffer, since too many factors have an influence. The "Nature" study can, however, show a direct relationship between bycatch availability and use by Skuas.

The world's largest and best-studied Skua population

The basis for this is the long-term observations that British and foreign researchers carried out on the island of Foula from 1986 to 2002. About 2.500 pairs of Skua breed on the westernmost island of the Shetland Islands. This makes it the largest Skua population in the world. "There are only a few populations of seabirds that have been studied so well," says Simone Pfeiffer. Thanks to the good contacts of the group leader Dr. Hans-Ulrich Peter, examined the influence of environmental factors on the breeding success of Skuas for three months as part of her diploma thesis in 1998. Part of the work was a collection and analysis of the Skuas' "wools". This is the undigested part of the food that the bird spits out on a regular basis. With the help of the vaults, Pfeiffer was able to analyze the composition of the Skua food. These data flowed into the current Nature article and were put in relation to bycatch by the team of scientists.

"Only thanks to such long-term studies of population dynamics can the far-reaching relationships in the marine food web and thus the direct connection to humans as users of the fish population be deciphered," Pfeiffer pleads for the continuation of these intensive projects. The Jena native is currently involved in one of these. She is doing her PhD in the group of Dr. Peter on the impact of human activities in Antarctica. And here, too, the thesis is that human influence goes much further than we are usually aware of.

Source: Jena [Uni Jena]