Fishing vessels in the North Sea return a large portion of their catch to the sea, a practice that is referred to as discarding. In the Southern North Sea, most fishing activities consist in bottom trawling. Their discards are mainly composed of benthic invertebrates and undersized, nonmarketable fish. Discarded fish are mostly Sole (Solea solea), Plaice (Pleuronectes platessa), Dab (Limanda limanda), Whiting (Merlangius merlangus) and other Gadoids. Part of the discards are consumed by scavenging birds, mainly Herring Gulls (Larus argentatus), Lesser Black-backed Gulls (Larus fuscus), Northern Gannets (Morus bassanus), Northern Fulmars (Fulmarus glacialis), Black-legged Kittiwakes (Rissa tridactyla), Great black-backed Gulls (Larus marinus), Common Gulls (Larus canus), and Common Terns (Sterna hirundo). During the last century, local populations of these birds have extensively grown due to an ever-increasing fishing activity and production of discards. In recent years, discarding has come to a turning point, and enforcement of new policies on fishery waste will soon lead to a sharp decrease in the production of discards. This study investigated how scavenging birds exploit discards, and how this activity determines the quality and quantity of discarded items that return to the seafloor. The proportion of consumed discards, selectivity in their consumption, competition and food robbery between scavengers were assessed, together with temporal and spatial variation of these parameters. Two types of experiment were performed: single-item and standardized multi-item discard experiments. Single-item experiments consisted in throwing one item at a time and observing its fate. In multi-item experiments, a discard fraction of known composition is thrown over a fixed period of time, and each captured item is registered. Single-item experiments took place from April to August 2011, one day per month, at 2.5, 5, 10 and 20 km off the Belgian Coast. Multi-item experiments took place in the vicinity of the Thames Estuary, in December 2011, February and April 2012. Round fish was preferred by birds to any kind of flatfish, but selectivity disappeared during the chick-rearing period. Herring Gulls appeared behind the vessel most frequently at closer distances to the shore, while Lesser Black-backed Gulls appeared at greater distances. Adult birds outcompeted immatures, which were more reliant on food robbery outside the chick-rearing period. Larger birds tend to outcompete smaller species, but observed trends were not consistent. Two tentative General Linear Models were constructed, explaining variation in the proportion of consumed discards during multiitem experiments in December 2011, on the basis of abundance of large gulls, daylight conditions, presence of Gannets, and abundance of small gulls. Possible applications of modeling discard consumption as a function of local scavenging seabird community composition are as well discussed. |
