Spatial and temporal distribution of cetaceans in the mid-Atlantic waters around the Azores

Abstract

Cetaceans living in offshore waters are under increasing pressure from anthropogenic activities. Yet, due to the lack of survey effort, relatively little is known about the demography or ecology of these populations. Spatial and temporal distribution of cetaceans in mid-Atlantic waters were investigated using a long term dataset collected from boat surveys and land-based observations around the Azores. From 1999 to 2009, 7307 cetacean schools were sighted during 271,717 km of survey effort. In 4944 h of land-based observations, 2,968 cetacean groups were detected. Twenty-four species were recorded: seven baleen whales, six beaked whales, eight dolphin species, Physeter macrocephalus, Kogia breviceps and Kogia sima. Overall, Delphinus delphis was the most frequently sighted species, but its encounter rate decreased in June–November, coinciding with the presence of Stenella frontalis in the region. Tursiops truncatus, P. macrocephalus and Grampus griseus were frequently encountered year-round, whereas large baleen whales showed a distinct peak in encounter rates in March–May. Mesoplodonts were fairly common and appear to be present throughout the year. These findings fill in a significant gap in the knowledge of cetaceans occurring in a poorly studied region of the North Atlantic, providing much needed data to inform management initiatives.