Understanding marine foodweb processes: an ecosystem approach to sustainable sandeel fisheries in the North Sea: IMPRESS final report

Chapter 0, Summary for non-specialists seabirds were apparently sampling from the same sandeel population since in neither year were marked discontinuities found between the length/date relationships for the individual predator species. Seabirds at-seaThe offshore seabird community comprised at least 37 different seabird species.Within the principal study area, the most species were Common Guillemot (49%), Atlantic Puffin (21%), Black-legged Kittiwake (12%), Razorbill (11%), and Northern Gannet (6%).Clear-cut, generalised seabird distribution patterns were derived, with Gannets far-ranging, Guillemots and Kittiwakes highly concentrated in the western (nearshore) half of the study area at large, and with Shag concentrated in inshore waters (not well covered in these surveys).Seabird densities and species richness declined and species composition altered with increasing distance to the UK coast.The study area, almost entirely within 90 km from land, was part of the areas with the highest mean top-predator densities and diversity of the northwest North Sea.In biomass terms, the seabird community was dominated by Guillemot, Puffin, Razorbill, Gannets and Kittiwakes (98%).For all seabirds within the study area, the energy requirement per unit area was estimated at nearly 120 000 kJ day -1 km -2 . Seabird foraging distributionFrom a combination of logger data (VHF telemetry, compass loggers, satellite transmitters and GPS loggers) and ship-based offshore surveys, a much more complete picture of foraging trips in seabirds was obtained than ever before.In Shags, three main foraging areas could be identified: inshore, west and north of the colony, and close to the island.Guillemots were foraging much closer to the colony than expected from at-sea surveys, with the furthest distance from the colony estimated at 25.3 km.Kittiwakes used a wide variety of foraging areas, from inshore coastal areas in St Andrews Bay area out to the Wee Bankie/Marr Bank region (foraging range 37.0 16.3km; range 13.6 -83.5 km).Northern Gannet ranged from West Norway to NE Scotland, into the north central North Sea and the southern North Sea.The mean foraging range was between 165 and 190km, mean trip duration being approximately 24 hours.Northern Gannets foraging within the study area acquired 59% of their energy over the "Banks".All Shags were foraging in "Freshwater Influenced" waters.Kittiwakes acquired 53% of their energy on the "Banks", and a further 27% in Freshwater Influenced waters.Guillemots acquired 48% of the energy on the Banks, and 24% in Freshwater Influenced Waters.A combination of all data suggests that for these three species the Bank Areas are the most important foraging grounds (49% of the energy acquired) and that Mixed Waters near the coast were relatively unimportant (8%). Seabird foraging behaviour and flock formationMost birds that were recorded as feeding participated in feeding flocks and most these flocks comprised several species (multispecies feeding associations, MSFAs).Three major types of MSFAs were recognised: (1) flocks associated with fishing vessels, (2) marine mammal-induced feeding flocks, and (3) 'natural' feeding flocks (seabirds, sometimes with marine mammals, jointly exploiting a shared resource).In the study area at large, in 12 out of 18 common species recorded, at least 80% of the feeding individuals were joining some sort of MSFA.A notable exception was the Shag (23%).81% of the feeding Guillemots were found in 'natural' MSFAs.Northern Gannets were commonly represented near fishing vessels (43%), with cetaceans (19%), and in 'natural' MSFAs (21%).69% of the Kittiwakes were feeding in 'natural' MSFAs.Aerial species, such as surface feeding gulls, had a greater tendency to participate in 'natural' MSFAs than expected on the basis of their overall abundance at sea.MSFAs formed around marine mammals occurred primarily in stratified offshore waters, while cetaceans within c. 80km from the coast were largely ignored.Feeding Gannets were mostly deep plunge-diving or scooping.Scooping was entirely associated with MSFAs and sandeel feeding.Feeding Kittiwakes were dipping or shallow plunge-diving.Guillemots were observed pursuit diving from the surface mostly. Seabird foraging depth and time activity budgetsIn Shags, the foraging depth was between 21-30, with the mean dive depth being greater in 2003 than in 2002.In Guillemots, the foraging depth varied considerably.Depth distributions peaked between 41-60 m in 2001 and 2002, but in 2003 59% of dives were <20 m deep.In Gannets, dive depth did not differ between years (about 3m).Time activity budgets were obtained for all four species during chick rearing, using a combination of bird-borne data loggers.In Shags, the proportion of time spent foraging was greater in 2002, when less time was spent on land.There was no difference in the number of trips by Guillemots made per day between seasons, but in 2003, they spent more time flying and diving and less time resting on the sea compared with 2002.Kittiwakes made more trips per day in 2003 than in 2002 or 2001, and spent more time in foraging flight.On feeding trips, Gannets spent 0.3%, 43%, and 57% of time respectively diving, flying and resting/swimming on the surface of the water.From gastric temperature loggers, used to measure prey capture rates, it appeared that Shags were successful on 10% of dives and in 55-65% of feeding bouts.In Guillemot, success rates per dive (33%) were much higher. Seabirds foraging habitat characteristicsSurface feeding seabirds require processes that bring prey to the sea surface and horizontal frontal systems were predicted to provide such opportunities.From at-sea surveys, it was demonstrated that foraging Kittiwakes utilised a broad zone off the Scottish east coast, with peak densities at the shallow-sea front (the seaward side of the mixed coastal zone).This front appeared to form an outer barrier for breeding Kittiwakes, and foraging occurred throughout the area between colony and front.Guillemots were essentially highly widespread in a broad zone along the coast, with relatively low concentrations in mixed coastal waters and beyond the shallow sea front in offshore stratified waters.Isle of May breeding birds utilised only a small part of the principal study area, mainly to the west of the Wee Bankie/Marr Bank complex.The bulk of the (adult)

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