Assessing the Importance of Frontal Zones on the Distribution of Upper Trophic Level Predators off Cape Hatteras

Abstract

Effective conservation of upper trophic level marine predators requires a comprehensive understanding of their distributions and of the underlying biological and physical processes that drive these distributions. We investigated the spatial distributions of marine mammals and seabirds off Cape Hatteras, NC, in relation to positions of the shelf break and Hatteras Front system. We conducted transect surveys with synoptic, fine-scale oceanographic sampling in August 2004, and derived the daily position of the Hatteras Front from temperature, salinity, and pressure data collected by a scanfish. To account for the correlated and autocorrelated nature of the environmental data, we assessed the influence of the Front on species distributions using a suite of Mantel’s tests. Pure partial Mantel’s results show that marine mammal distribution over all survey days was influenced by salinity. Results of the daily Mantel’s tests show that no one variable consistently influenced the distribution of marine mammals. Pure partial Mantel’s results show that seabird distribution over all survey days was influenced by depth, distance from shelf break, fluorescence, and space. The significance of space indicates that another variable or variables with spatial structure influenced the distribution of seabirds but were not tested. Results of the daily Mantel’s tests show that different combinations of environmental variables influenced the distribution of seabirds on different days. However, one variable consistently influenced seabird distribution – fluorescence. These findings enable consideration of spatially explicit approaches to the conservation of marine mammals and seabirds and other upper trophic level predators in this region. Keywords: marine mammal distribution; seabird distribution; spatial analysis; Hatteras Front