By Gaëlle Courcoux and Bintou Bonkoungou
From gannets to seagulls, puffins to penguins, all seabirds suffer the same drop in birth rates when the supply of fish drops to less than a third of maximum capacity. That’s the result from an international study (1) on the relationships between predators and prey in seven ecosystems around the world, published in the magazine Science and coordinated by Philippe Cury, an IRD researcher.
Based on nearly 450 cumulative years of observation, the research team compared the growth in fish supplies and the reproductive patterns (2) of 14 species of coastal birds. These birds mainly feed on sardines, anchovies, herring and prawns, all of which are victims of over fishing. Below the critical level of one third of the fish biomass, the birds — and the stability of the entire ecosystem — come under threat.
These studies also provide a reference level for the sustainable management of fisheries, so as to safeguard the bird population, which is often imperilled, and so as to maintain the healthiness of marine environments.
From the Arctic to Antarctica and from the Atlantic to the Pacific, when the supply of fish is reduced, seabirds stop reproducing. Previous studies had uncovered the relationships between the availability of food to birds and their reproductive rates, but this new international study (1) has just made a monumental discovery. Coordinated by Philippe Cury, an IRD researcher, and published in the magazine Science, the study identifies the level of a critical supply of fish below which the stability of the bird population is endangered.
A third’s the limit
Gannets, terns, puffins, seagulls and penguins: taking all the species together, if fish supplies drop below a third of their maximum size, then the number of baby birds born drops precipitously. Everywhere in the world. Once the amount of their prey falls below sufficiency, the seabirds fail to reproduce (2). Yet above this level, their reproduction rate doesn’t increase. Abundance in the food supply didn’t produce the expected result as other factors limit reproduction, such as the fact that nesting areas fill up quickly.
By demonstrating such a singular phenomenon, these studies validated empirically — i.e. from data rather than from models — that ecosystems, over the long term, obey similar laws. Until these studies were carried out, the basic principle of living ecologies remained theoretical. Now, for the first time, scientists have achieved a model of predator and prey behaviour based on actual observation in marine environments.
Almost 450 years of data
The research team compared almost 450 years’ worth of data from all over the world to see how the supply of fish correlated with the reproduction of seabirds. To do so, the scientists concentrated on 14 species of birds in seven ecosystems around the globe. The species selected feed mainly on sardines, anchovies, herrings and other small coastline fish that fishermen regularly catch and whose populations are under pressure. Each of the ecosystems was studied for periods ranging from 15 to 40 years; and the study showed that it generally takes 13 years of data to form an accurate notion of what the maximum fish supply is in a given ecosystem.
This is the first time that so much data on the relationships between predators and prey have been brought together over such a long period of time. The scientists stressed cooperation between specialists from the north and the south made it possible to bring together such a vast quantity of information. Scores of research specialists have devoted countless years, sometimes their whole career, to these studies.
Over fishing can kill ecosystems
This study makes it plain that over fishing endangers the survival of higher-level predators such as birds. In fact, they are in direct competition with human fishermen: both groups consume about 80 million tonnes of fish per year. Small bait which are used to make meal and oils in fish farming — such as sardines, anchovies, herring, smelt and capelin — make up 30 % of today’s catch worldwide. As global demand rises, these findings make it possible, at last, to achieve a standard against which to measure the sustainable management of fish, so as to sustain populations of seabirds, over the long term.
Seabird populations are one of the best ways to judge the health of marine ecosystems — which the European Commission, for instance, wants to be able to monitor — and are one of the easiest measurement barometers to use. These bird species are among the most endangered, owing to the lack of food but also to climate change and the destruction of coastal habitats where, once again, they are in competition with humans for space along the water’s edge.
In order to understand the dynamics of ecosystems, it is essential to determine the key relationships between predators and prey. International authorities today have few tools by which to gauge and restrict fishing. The limit of one third of the fish population set forth by this study may not be a hard number in all cases, but it can certainly serve as a guideline for fishery management policies.
(1) These studies were carried out jointly with scientists from St Andrews and Glasgow Universities in the United Kingdom, from Ifremer, from the Norwegian Institute for Nature Research, the Department of Environmental Affairs and the University of Cape Town in South Africa, the British Antarctic Survey, the University of Stockholm in Sweden, the University of British Columbia in Canada, the Alaska Science Center and the Farallon Institute for Advanced Ecosystem Research in the United States and the Ministry of Fisheries and Marine Resources of Namibia.
(2) Reproductive success is the ability of individuals of a species to reproduce, measured by the number of baby birds that survive.