Responses of Bottlenose Dolphins to Construction and Demolition of Underwater Structures
By Kara Buckstaff, MSc and Janet Gannon, MSNR

Evaluating the effects of anthropogenic noise on marine mammals is a rapidly-developing field of scientific inquiry.  The goal of this project, funded by the Disney Wildlife Conservation Fund, NOAA Fisheries, and Earthwatch Institute, was to expand our understanding of the suite of threats facing bottlenose dolphins and Florida manatees in coastal waters by investigating a source of potential mortality, injury, or disturbance that has received relatively little research attention to date – marine construction and demolition.  In Sarasota Bay the construction of a fixed-span bridge was completed in July 2003, followed by two in-air explosions and a final underwater explosion to demolish the pre-existing Ringling Bridge.  Before, during, and after these events occurred boat-based dolphin surveys and aerial manatee surveys were conducted to compare distribution of sightings during bridge construction/demolition to historical sightings records.  Additionally, underwater sound levels were recorded at 6 listening stations within a small study area designated to the north and south of the Ringling drawbridge..

In comparing distance from the construction site before, during, and after construction, we found a significant difference in distribution of both bottlenose dolphins and manatees in the post-construction stage.  This may indicate that long-term exposure to construction/demoliton noise may cause shifts in habitat use, and may in fact have more impact than acute noises during construction.  Observations made during the underwater detonation at distances of 0.73 km and 1.83 km from the explosion site indicated that dolphins do exhibit behavioral responses.  While it was not possible to quantify their reactions, observable changes in at-the-surface behaviors were evident.  Responses were not assessed for manatees during the underwater explosion as there were no manatees present.  Source-level estimates of 160 dB re 1μPa-m and 176 dB re 1 μPa-m were calculated for the in-air explosions and 147 dB re 1 μPa-m was calculated for the underwater explosion.  Both in-air explosions were louder underwater than the underwater explosion that was contained by a steel coffercell.  A permit that required a Marine Species Watch Program within a designated danger zone of 1000 ft was implemented for the underwater explosion, however this was not recommended for the in-air explosions.  Based on these findings, in-air explosions occurring close to water level (< 15 ft altitude) should be considered for the potential to affect marine life as well.