Activity: Other › Types of other (prices, external and other activities) - (Co)promotor masterthesis
Bottom-trawling is one of the main fishing activities in the North Sea and an important source of discards, mainly consisting of undersized and non marketable fish that fishermen throw back to the sea. Discarding impacts the marine ecosystem function, structure and biodiversity, as well as the structure of seabird communities. Discarded fish in the North Sea have become an important food source for several seabird species in the past decades, fostering an increase in their populations. The reform of the European Common Fisheries Policy will introduce a partial ban on discarding fish. To understand the consequences that this ban might have on local populations of scavenging seabirds, we have studied the factors that influence their consumption of discards. Standardized multi-item discard experiments were performed during five campaigns in the Southern North Sea between December 2011 and April 2013 aboard RV Belgica. Three types of standardized discard fractions, each containing different amounts and types of fish, were discarded to analyze the variations in composition of the flock of scavengers and in the proportions of fish consumed by birds. The most abundant scavengers were the Black-legged Kittiwake (Rissa tridactyla), Common Gull (Larus canus), Lesser Black-backed Gull (Larus fuscus), Great Black-backed Gull (Larus marinus) and Herring Gull (Larus argentatus) . Round fish was the most consumed type of discard. Sole (Solea solea) and Plaice (Pleuronectes platessa) were consumed in low proportions. A success index (SI) was calculated as the number of fish consumed by species in relation to its abundance, or by the whole flock in relation to its size . SI for the flock was higher in experiments where Sole and round fish were discarded together than for experiments where only flatfish was discarded. A clear decrease in SI with increasing numbers of seabirds was found for the first type of experiments. Black-legged kittiwakes were the most successful species (SI = 1,10±1,33), along with gannets (SI = 0,96±1,35), but these were also the least recurrent species. Generalized Linear Models were constructed to predict the consumption of fish in different types of discard fractions. The number of large gulls in the flock was found to be the main predictor of discards consumption. The calculation of a scavenging index for the Belgian part of the North Sea (BPNS) is hereby presented. This can be used as a tool to detect those areas where scavenging seabirds exploit discards most intensely. Combined with the estimations of amounts and types of discards produced in each area, and models of the consumption of discards by scavenging seabirds, the scavenging index could in the future be used to estimate the quantity of discards consumed by seabirds on a geographical basis.