The new EU landing obligation requires fishermen to land all quota-regulated fish, including undersized fish. The bycatch consisting of juvenile fish, crabs and other creatures have always formed a rich buffet for the ship-following seagulls. The new policy may therefore have a negative effect on seabird populations. To find out how discards are used by birds and how many remain for benthic communities, BENTHIS researchers Jochen Depestele and Pascal Laffargue did an experiment.
In the stormy month of November 2013, the BENTHIS crew joined a cruise on board the French research vessel ‘Thalassa’ in the Bay of Biscay. After every haul, the researchers prepared a standard meal from the catch. Often groups of more than 100 hungry feathered friends were following the ship. The portions contained a mix of 75 roundfish and 50 individuals of cephalopods, Norway lobster or boarfish. Depestele and Laffargue fed the birds experimentally by throwing the discards into the water, and then recorded which birds fancied which prey.
To extrapolate their findings to the whole of the bay of Biscay, they studied the discard composition for 6 different types of fisheries and combined this with distributions of seabirds taken from aerial counts and analogous experiments in the North Sea. The result: a model that predicts what part of the discards serve as food for birds, depending on the type of fisheries, time of the year, attraction of birds to fishing vessels, and even on the competition between birds. Depestele: “Large gulls for example are three times less efficient in catching prey when gannets are around”.
Although the model study showed a lot of variation in time and space, some general findings emerge. Depestele: “In total, over 500 million individuals of fish and other species were discarded in the Bay of Biscay during a period of 1 year. We found that about 25% of these discards is consumed by seabirds, while 75% sinks to the seafloor. The birds’ main preys are roundfish and cephalopods. Flatfish and benthic species do not tend to float, and are therefore quickly out of reach of the birds.”
The study suggests that on a large spatial scale, reductions in discards may significantly affect the food availability of seabird communities. For benthic communities, the impact is different. Depestele hypothesizes that benthic communities are only locally subsidized by the extra food. The discard ban is therefore likely to have significant ecosystem effects on scavenging birds at large scale and on scavenging benthic communities at local scales.