Distribution patterns of small cetaceans in Galician waters

Abstract

Cetacean sightings are reported from opportunistic deployment of observers on fishing boats during 1998–1999 in Galician waters (north-west Spain), a region of high biodiversity, intensive fishing activity and an important cetacean habitat. During 111 trips, a total track length of 8128 km and estimated area of approximately 9840 km2 was surveyed, including both inshore and offshore waters. The most frequently sighted species were common dolphins (Delphinus delphis) and bottlenose dolphins (Tursiops truncatus). Taking account of biases in the survey data, the former species was most commonly sighted in deeper offshore waters (>200 m depth) with the highest sightings rate in the second quarter of the year. Bottlenose dolphins were seen mainly in inshore waters to the south of Galicia but also in offshore waters further north. Sightings rates were higher when the observers' boats were engaged in fishing than when travelling. Published dietary data suggest that this may simply indicate that dolphins tend to congregate in fishing areas where their main prey occur. Several different treatments of the data were used to derive relative abundance indices (taking account of observation biases and of spatial variation in survey coverage). These calculations suggest that there may be around 7000–10,000 common dolphins and 600–1000 bottlenose dolphins in Galician waters, although dedicated surveys are needed to produce robust estimates. Other species seen during surveys included the long-finned pilot whale (Globicephala melas), harbour porpoise (Phocoena phocoena), Risso's dolphin (Grampus griseus) and fin whale (Balaenoptera physalus). Incidental observations from boats and land-based surveys supported the high relative abundance of common dolphins and, in inshore waters, of bottlenose dolphins, as well as providing additional records of porpoises and Risso's dolphins.