The structure of pelagic seabird assemblages in the African sector of the Southern Ocean
This study investigated the potential usefulness of aerial seabirds as indicators of the distribution of peculiar oceanic biotopes and prey populations in the African sector of the Southern ocean. The hypothesis examined was that the distribution and abundance of seabirds are non-random and predictable with respect to the availability of prey at the sea-surface. The distribution of seabirds was correlated with prey density, in cases where the appropriate information was available. More often than not, however, data on prey distributions were unavailable, so that seabird distribution was correlated with the abiotic indicators of hydrodynamic processes which order the distribution of potential prey. The distribution of seabirds was determined by means of shipboard observations. The trophic structure of seabird assemblages was assessed according to the diversity, biomass and abundance of 35 seabird species according to four principal diet-classes. The ecological structure of seabird populations was defined in terms of relationships between the trophic structure of seabird assemblages and Southern Ocean oceanography, meteorology and biogeography. The predictability of seabird distribution and abundance with respect to environmental variation was assessed using linear and nonlinear regression procedures. A deterministic model was developed in which analytical techniques are standardized and which can be applied to other ocean areas. There are predictable associations between the trophic structure of seabird assemblages and the structure of the surface of the sea. Assemblages of putative prey feature squid and fish in low latitudes, and plankton predominate in higher latitudes. The ecological structure of seabird populations reflects this trend on a macro-scale level. On a meso-scale level, the abundance of seabirds by diet-class correlates with the relative abundance of putative prey. Moreover, seabird abundance and biomass correlate positively with abiotic indicators of frontal zones and sea-surface mixing, where prey diversity and abundance are greatest. Regressions of seabird abundance with sea-surface temperature explain important aspects of the variation in the ecological structure of selected seabird populations. This study showed that it may be possible to track the distribution of prey stocks through concurrent observations of seabirds and hydrography. Consequently, the descriptive model developed in this study contributes towards the objective of circumventing the need for direct sampling of prey stocks. The predictability of seabird distribution and abundance previously obtained has been surpassed by the results of this study through progressive reduction of the spatio-temporal scale of data collection and analysis, and by treating biotic-abiotic relationships with non-linear regression models where appropriate. The study concludes that sufficient advances have been made to justify further research into the use of selected species of aerial seabirds as indicators of prey distribution and abundance at sea.
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