Scale-dependent habitat selection by a nearshore seabird, the marbled murrelet, in a highly dynamic upwelling system

Abstract

We investigated daily and annual variation in the marine habitat selection of marbled murrelets Brachyramphus marmoratus (Alcidae) in the nearshore California Current System at scales of 10 to 100 km of coastline. We addressed 2 general questions: (1) how do murrelets select habitat while facing dramatic and often rapid variation in oceanographic conditions and prey availability; and (2) does selection vary between meso- (10 to 100 km) and fine- (<10 km) scales. Marine transects were performed during 1999 and 2000 to survey murrelets and prey-fish schools, and to collect data on physical oceanography. We predicted that habitat selection would vary with upwelling intensity and prey availability: prey-aggregating mechanisms should be more important under low upwelling scenarios when cool, productive water is more limited, and murrelets should forage closer to nesting habitat when prey availability is high. This was generally the case, as murrelets selected cooler locations when upwelling was low and locations closer to nesting habitat when upwelling was high. Interactions among variables were important; murrelet habitat selection for prey and fronts changed among different sea surface temperatures (SSTs), distance to nesting habitat, upwelling intensity, and overall prey availability. Scale-dependent selection for prey patches (fish schools) and fronts occurred within larger-scale selection for SST or distance from nesting habitat. Additional scale-dependent effects of prey availability were revealed by small-scale surveys in an area of high prey density. In this case, murrelets selected cool water (higher quality habitat) when prey availability was low and were associated with prey schools when prey availability was high. Additional large-scale surveys from 1996 to 2000 revealed that these patterns also manifested themselves on a longer temporal scale. Murrelets occurred farther from nesting flyways in years when spring upwelling was low and during the 1998 El Niño Southern Oscillation (ENSO) when local food webs were depressed and numerous other seabird species failed to reproduce.