Abstract
At-sea surveys facilitate the study of the distribution and abundance of marine birds along standardized transects, in relation to changes in the local environmental conditions and large-scale oceanographic forcing. We analyzed the form and the intensity of black-footed albatross (Phoebastria nigripes: BFAL) spatial dispersion off central California, using five years (2004–2008) of vessel-based surveys of seven replicated survey lines. We related BFAL patchiness to local, regional and basin-wide oceanographic variability using two complementary approaches: a hypothesis-based model and an exploratory analysis. The former tested the strength and sign of hypothesized BFAL responses to environmental variability, within a hierarchical atmosphere—ocean context. The latter explored BFAL cross-correlations with atmospheric / oceanographic variables. While albatross dispersion was not significantly explained by the hierarchical model, the exploratory analysis revealed that aggregations were influenced by static (latitude, depth) and dynamic (wind speed, upwelling) environmental variables. Moreover, the largest BFAL patches occurred along the survey lines with the highest densities, and in association with shallow banks. In turn, the highest BFAL densities occurred during periods of negative Pacific Decadal Oscillation index values and low atmospheric pressure. The exploratory analyses suggest that BFAL dispersion is influenced by basin-wide, regional-scale and local environmental variability. Furthermore, the hypothesis-based model highlights that BFAL do not respond to oceanographic variability in a hierarchical fashion. Instead, their distributions shift more strongly in response to large-scale ocean—atmosphere forcing. Thus, interpreting local changes in BFAL abundance and dispersion requires considering diverse environmental forcing operating at multiple scales.
Highlights
Terrestrial and marine ecologists have long quantified the spatial dispersion of organisms, defined by the changes in their occurrence and abundance across space, to gain insights into the ecological factors influencing species patchiness and structuring biological communities [1,2,3,4]
We studied BFAL dispersion within the productive continental shelf / slope system of central California over two breeding seasons using five years (2004– 2008) of standardized vessel-based surveys collected by the ACCESS (Applied California Current Ecosystem Studies) partnership
Because non-breeding BFAL can travel across the north Pacific Ocean, covering 1000s of km in a matter of days [61], our finding suggest that local BFAL dispersion off central California is influenced by broad-scale ocean-atmosphere variability
Summary
Terrestrial and marine ecologists have long quantified the spatial dispersion of organisms, defined by the changes in their occurrence and abundance across space, to gain insights into the ecological factors influencing species patchiness and structuring biological communities [1,2,3,4]. Researchers have quantified the intensity (degree of aggregation) and the form (patch sizes) of spatial dispersion, to investigate the ecological processes influencing species distribution and abundance patterns [5,6,7,8]. Replicated surveys over seasons and years have facilitated the study of changing seabird dispersion during periods of contrasting oceanographic conditions and prey availability [7, 8, 14, 15]. These spatially-explicit perspectives of aggregation and predator-prey associations are being used to monitor changing marine ecosystems [16, 17]
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