Quantifying food subsidies to the benthos due to discards: Task 4.4 Report on the quantification of discard flow to the seabed, relative to natural food sources
Depestele, J.; Rochet, M.-J.; Stienen, E.W.M.; Dorémus, G. (2014). Quantifying food subsidies to the benthos due to discards: Task 4.4 Report on the quantification of discard flow to the seabed, relative to natural food sources. Benthic Ecosystem Fisheries Impact Study (BENTHIS): [s.l.]. 33 pp.
Discards generated from marine fisheries have significantly affected species populations and community composition of seabirds through mechanisms such as competition, predator–prey interactions and nutrient transfer. Our understanding of the fate of discards beyond seabird scavenging is fragmented, but it is likely that similar mechanisms come into play. This study contributes in resolving this knowledge gap by quantifying the amount and composition of discards that become available to scavengers other than seabirds. The key was nevertheless found in revealing seabird discard consumption, as seabirds are likely the first in taking profit of fishery discards due to their high mobility. Their selective consumption determines the composition and amount of food remaining for others.The consumption of discards by scavenging seabirds was assessed for the French fishing fleet in the Bay of Biscay. Experimental sea trials were conducted to assess the proportion of discards consumed by foraging guild and discard type. Experimental discard consumption (EDC) was raised to fleet level by foraging guild using the total number of discards by discard type. The raising procedure accounted for the spatio-temporal variability of both foraging guilds and discards, by standardising both distributions to their lowest common resolution. Discards limited inferences in space, whilst the highest temporal resolution was determined by the biennial monitoring of foraging guilds. As EDC of roundfish by Large gulls and Gannets varied considerably, we investigated the main drivers. Consumption increased logarithmically with the number of ship followers for both guilds, but was greatly impaired by competitors for Large gulls. Competition between Large gulls and Gannets reduced the roundfish consumption by threefold, while other guilds such as Kittiwakes, Procellariids and Skuas had only a limited though significant influence. As Large gulls dominated during the first semester (April to September), the consumption of discards and notably roundfish was dictated by this foraging guild. The abundance of ship following Gannets was remarkably higher in the second semester (October to March). This change in flock composition and in numbers of ship followers implied an increase in the consumed proportion of roundfish of 27.9%. The total number of discards that became available to non-avian scavengers was however higher in the first semester, as more discards were produced during this period. Most discards comprised benthic invertebrates, but excluding this discard type revealed that over two thirds of the discards remained roundfish.
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