Occurrence, site-fidelity and abundance of bottlenose dolphins (Tursiops truncatus) in Broadhaven bay, northwest Ireland during long-term construction of a gas pipeline

Abstract

During the construction of a pipeline from an offshore gas field in northwest Ireland, shore-based watches and opportunistic boat-based photo-identification surveys for bottlenose dolphins Tursiops truncatus were undertaken. Seven years of data were used to investigate the influence of Construction-Related Activity (CRA) on the occurrence of dolphins in the bay where the pipeline made landfall. CRA varied substantially over the seven years and included dredging, pipe-laying, and rock armouring as well as seismic surveys. While individual activities could not be examined in isolation, there was no evidence of aggregated CRA or season influencing occurrence, despite a greater number of CRA days during summer months. There was weak support for the influence of year, but lower occurrence rates did not correspond to years with more CRA days, or vice versa. The uncorrected (marked animals, only) and corrected (accounting for unmarked individuals) annual abundance estimates with the lowest coefficient of variation were 67 (95% CI = 62–84; CV = 0.08) and 140 (95% CI = 106–174; CV = 0.12) in 2010, which was comparable to estimates from a neighbouring region on the west coast of Ireland. Of the well-marked individuals approximately 50% were re-sighted in two or more years and re-sightings within years were as high as 70%. Overall, there was little evidence to suggest that the extended period of CRA resulted in a short- or long-term reduction in the occurrence of dolphins within the bay. However, given the relatively coarse spatial and temporal scale of the CRA data it was not possible to assess fine-scale movements, such as redistribution within the bay in response to CRA within the immediate area. To better inform management and mitigation practices, greater emphasis should be placed on designing robust monitoring programmes that focus beyond fine-scale displacement as the sole indicator of an impact.