Effects of oceanographic variation on marbled murrelet diet and habitat selection

I investigated two questions concerning Marbled Murrelet (Alcidae: Brachyramphus marmoratus) diet and at-sea habitat selection in the California Current System. First, how are prey choice and availability affected by oceanographic variation, breeding status and human fishing pressure? Second, how do murrelets select marine habitat in this highly dynamic upwelling system? Dietary questions were addressed using stable isotopes to track the trophic structure of historic and modern murrelet diets and the food webs they rely upon. Hypotheses regarding habitat selection were tested with predictive models based on at-sea surveys.; Many seabirds in temperate and arctic ecosystems initiate breeding and have higher reproductive success when oceanographic conditions prompt an abundant and less diverse prey base. Murrelets also followed this pattern, feeding on a lower trophic level during non-El Niño years when fecundity was higher and lower trophic levels prey was more abundant. Murrelets with indicators of active reproduction also fed on a lower trophic level. Females, regardless of breeding status, fed on a lower pre-breeding trophic level, while only breeding males switched to a lower pre-breeding trophic level. Thus, pre-breeding diet appears to be a function of the interaction between prey availability due to oceanographic variation and individual characteristics of breeding status and sex.; Modern murrelet trophic levels were compared with the years 1895–1911 using isotope ratios in feathers from museum specimens. Dietary variation was explained by era (historic versus modern) and PDO state (cool versus warm water). These data suggest that both overfishing and variation in ocean temperatures interact to explain modern murrelet diet.; I also investigated daily and annual variation in marine habitat selection of Marbled Murrelets at several scales. I predicted that prey-aggregating mechanisms should be more important during low upwelling conditions when cool, productive water is more limited, and murrelets should forage closer to nesting habitat when prey availability was high. These predictions were supported by the at-sea survey data. Selection was scale dependent with selection for prey patches (fish schools) and fronts occurring within larger scale selection for SST or distance from nesting flyways. Similar selection patterns also occurred at smaller spatial and longer temporal scales.