Co-occurrence and aggregation of marine birds and mammals in Monterey Bay, California, USA

Patterns of spatial and temporal co-occurrence or avoidance among different species of marine birds and mammals can provide insights into the degree to which these top predators compete for prey. I conducted at-sea surveys in nearshore waters of Monterey Bay, California, USA, and used a randomization technique to assess co-occurrence patterns of marine birds and mammals in 1 km transect segments. As expected, strongest positive associations were among members of 3 different foraging guilds: pursuit-divers, surface-feeders, and plunge-divers. Within guilds, pursuit-divers exhibited marked avoidance of one another (negative co-occurrence), although surface-feeders often co-occurred with each other. The study was conducted during 2 yr, 1999 and 2000; during 2000, when predator abundance was greater and prey abundance may have been decreased, pursuit-divers exhibited more avoidance of one another than in 1999. These data suggest that competition reduces foraging ability for pursuit-diving species, making it more profitable for them to disperse more widely, whereas surface-feeding species (primarily gulls) benefit from flock foraging under most conditions. Larger animals tended to frequently co-occur, while the smallest member of each guild tended to avoid other species, indicating competitive exclusion of smaller predators.