title: Effects of tidal currents on seabird foraging behaviour and diet in the North Sea
Creators: name: Daunt, F.
name: Wanless, S.
name: Wilson, L.
name: Peters, G.
name: Grémillet, D.
name: Scott, B.
name: Sharples, J.
name: Ross, O.
name: Greenstreet, S.
name: Fraser, H.
Publisher: name: ASC 2002 - N - Theme session
Year: 2024
Subjects: Ecosystem observation, processes and dynamics

description: No abstracts are to be cited without prior reference to the author.In the marine environment, the relationship between the abundance of shoaling fish and their availability to seabird predators remains poorly understood. Hydrography is a key driver of the spatial and temporal distribution of primary production and associated higher trophic levels, and thus plays an important role in predator-prey dynamics in the marine ecosystem (Hunt et al. 1999). In coastal temperate regions, variations in physical ocean structure are caused by the interaction of bathymetry, seasonal and diurnal cycles of solar irradiation, seasonal wind patterns and monthly and daily cycles of the tide (Mann &amp; Lazier 1996). Tidal currents are among the most significant physical forces operating in these regions, and a number of studies have shown that the interaction between tidal forces and bathymetry plays a key role in the distribution of fish in the water column, and thus their availability to top predators (e.g. Brown &amp; Gaskin 1988; Hunt et al. 1998; Irons 1998, Zamon 2001; Mendes et al. 2002). We investigated the effects of bimonthly and twice-daily cycles of tidal currents, collected at offshore moorings, with the foraging ecology of black-legged kittiwakes Rissa tridactyla and common guillemots Uria aalge, two seabird predators with highly contrasting foraging techniques. The black-legged kittiwake is a surface feeder, whereas the common guillemot obtains food by diving. Tidal effects on fish behaviour are likely to impact very differently on these two species. Black-legged kittiwakes are predicted to rely on tidal conditions that drive fish prey to the surface. These conditions will only occur under certain tidal regimes, in conjunction with wind and bathymetric effects (Mann &amp; Lazier 1996). Further, species and/or age claseses of fish are predicted to be affected by hydrographic conditions in different ways, due to their differing ability to swim against currents (Hunt et al. 1999). Therefore, we obtained food samples fomr black-legged kittiwakes throughout the breeding season and examined diet composition in relation to the biweekly tidal cycle. Common guillemots are able to exploit prey through the water column, and are not therefore under the same spatial constraints as the black-legged kittiwake. Common guillemot foraging depth may reflect the impact of tidal currents on prey distribution in the water column. Therefore, we attached electronic depth loggers to breeding common guillemots to test whether foraging depth was related to the twice daily cycle of the tide. Our study area was in the western North Sea in proximity to the seabird breeding colony on the Isle of May, south-east Scotland. We present data from two contrasting years, 2001 and 2002. We found effects of tidal phase on diet and foraging depth at different temporal and spatial scales, and discuss how tidals currents may influence the availability of fish prey to seabirds.