Acoustic Assessment of Populations of Common Dolphin off the West Wales Coast, With Perspectives from Satellite Infrared Imagery

Abstract

Passive acoustic monitoring of common dolphins was conducted across a survey area of 1000 n.m.2 (nautical miles) off the west Wales coast during the months September, October, November and December 1995. Dolphin presence and distribution was assessed by a system of percentage acoustic contact. Results were compared with those from a seismic/dolphin survey during the same months of the previous year, and across the same survey area. Distributions of common dolphins within the survey area showed similar trends for both years, with a marked decrease in dolphin contacts between September and October of both years. These observations suggest offshore migration of common dolphin populations at the time of year. Sea surface temperature distribution across the entire region was visualized during two survey periods using infrared satellite imagery. It is hypothesized from these observations that offshore migration of common dolphins coincides with a break-up of the Celtic Sea Front, a distinct oceanographic feature which crosses the survey area.