Significance of nocturnal purse seine fisheries for seabirds: a case study off the Ebro Delta (NW Mediterranean)
Seabirds are known to make extensive use of fishery waste, a phenomenon that has been particularly well studied in relation to demersal fisheries, especially when operating during daylight hours. Contrarily, very little is known about the importance of predominantly nocturnal fisheries in providing feeding opportunities to seabirds. We considered the particular case of purse seining for small pelagic fish, which takes place basically at night, and assessed the significance of this fishing practice for seabirds off the Ebro Delta (NW Mediterranean). Fieldwork was conducted on board commercial purse seiners (nocturnal activity) in 1997–1998 and was complemented with observations performed on board both commercial bottom trawlers (diurnal activity; 1997– 1998) and a research vessel (1999–2000). The purse seine fleet targets small clupeoids, which are attracted and concentrated by the light of a powerful lamp, and then captured using an encircling net. Purse seiners frequently changed fishing area, took very variable catches, and were strongly influenced by the weather. This made the fishery unattractive to most seabird species, which primarily attended purse seiners during the discarding process. This process took place on the way back to port after dawn (i.e. with daylight) during daylight and was quite irregular, thus attracting lower numbers of seabirds than did trawlers. The threatened Audouin’s gull Larus audouinii was the only species attending purse seiners regularly at night, capturing live fish concentrated at the sea surface during the hauling process. This is in accordance with the specialisation of this gull in the capture of epipelagic fish at night, which seems facilitated by purse seiners. Indeed, the vessels would favour the direct capture of fish (illumination of the sea surface, concentration of the fish), as well as the location of the shoals (light signalling). The purse seining fishery was especially important for Audouin’s gull during trawling moratoria (when trawling discards were not available) and in the non-breeding season. A simple bioenergetic model estimated that individual Audouin’s gulls could obtain a mean of 669 kJ haul–1, which would represent far more than half of the daily energy requirements of breeding birds. Provided that Audouin’s gulls feeding in a given area could attend more than one haul in a short time, birds attending purse seiners at night could easily meet their energy requirements. However, there was strong variability in our estimate (from 0 to 1659 kJ bird–1 haul–1, 95% CI), and feeding at purse seiners would only sometimes be profitable for Audouin’s gulls. Purse seine fisheries could be of importance for other nocturnal seabirds in other regions such as the SE Pacific, and this deserves further research. It is important to note that purse seiners could also be detrimental for many seabirds, through direct competition and eventual depletion of fish stocks.