Scale-dependent variability in seabird abundance

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Abstract
Patchiness of co-occurring species of seabirds in an anisotropically structured physical regime was investigated by making continuous counts of birds along 4 transects in the Benguela upwelling region, in the southeastern Atlantic Ocean. Birds were highly aggregated at spatial scales ranging from 0.3 to 23 km. This variability could not be modeled by standard statistical distributions, including negative binomial and gamma distributions. Patchiness was scale-dependent, as indicated by significant changes in moment ratio with change in frame size (measurement interval). Patchiness was not isotropic (independent of transect orientation). Extensive aggregation was observed in 3 out of 5 abundnant species along an east-west transect, but was not found in any of these 5 species along a north-south transect. Extensive aggregation was observed in 4 of the same 5 species when the east-west transect was repeated on the following day. Patch scale along the east-west ranged from 6.8 to 20.5 km on the first passage, and from 11.5 to 23 km on the second passage. Variation in patch size among species along the same transect indicates that patch scale in seabirds depends on species-specific responses to the pelagic environment.