Influences on spatial patterns of Gulf of Maine harbor porpoises

Abstract

Background: inter-annual spatial and temporal differences in the sighting rates of harbor porpoises in the Gulf of Maine/Bay of Fundy region have been documented since 1970. This paper explores a possible reason for these differences. Methods: using data collected in 1991 and 1992, harbor porpoise densities adjusted for sea state were related to environmental factors (surface temperature, water depth, density index of prey species, and spatial location) by inspecting contour maps of kriged values of each factor and by fitting generalized additive models (GAMs) to all the factors simultaneously. Results: on a large spatial scale, high density aggregations of harbor porpoises were located in the same general regions during both years. However, on a smaller spatial scale, exact location and magnitude of the aggregations were correlated with the small scale distributions of environmental factors. High densities of harbor porpoises were associated with waters that had surface temperatures of 10–13.5°C, contained fish densities of 1.5–11 fish caught per minute of trawling, and were 30–70 fathoms deep. Conclusions: inter-annual changes in surface temperature and fish density may be a reason for changes in the distribution and abundance of harbor porpoises.