Environmental Determinants of Pelagic Seabird Distribution in the African Sector of the Southern Ocean

Abstract

The distribution of birds at sea in the Southern Ocean south of Africa relates to the environmental parameters at regional scale: barometric pressure, salinity, air temperature,water temperature, wind strength and weather. Multivariate regression statistics are used to assess the usefulness of modelling seabird abundance on weather and sea-surface conditions indicated by these variables. The birds' distribution appears most strongly correlated with sea- surface temperature. A variety of possible combinations of cause-effect interac- tions among the physical parameters (barometric pressure, temperature and wind) revealed low linear correlations between seabird distribution and weather. Linear models explain a consistently small proportion of the spatial variation in seabird distribution and abundance in relation to sea-surface structure. Seabird abundance and sea-surface temperature exhibit strong curvilinear correlations across frontal regions. Thus, the use of abiotic environmental features as predic- tors of seabird distribution requires non-linear analysis, which suggests that: (1) pelagic seabirds are distributed randomly at sea, or (2) their distribution is non- random as a consequence of a combination of biogeographical history, food requirements, breeding period and locale, and physical environmental features. This hypothesis can only be tested with a complex stochastic model, and we lack sufficient data on the distribution of hydrological events which control prey availability to aerial seabirds; this information is necessary for the construction of a model which explains greater than 50% of the variation in the distribution of seabirds. At present, meaningful patterns in the distribution of Southern Ocean seabirds relate to spatio-temporal separation by seabird diet-classes and the distribution of thermal fronts.