Responses of bottlenose dolphins (tursiops truncatus) to construction and demolition of coastal marine structures

Construction and demolition activities are com-monplace in offshore and coastal waters, in habitats that are important feeding and breeding grounds for marine mammals. In Sarasota Bay, Florida, the construction of a large fixed-span bridge was com-pleted in July 2003, followed by two in-air explo-sions and a final underwater explosion to demolish thepre-existing drawbridge. Boat-based surveys were conducted to compare distribution of bottle-nose dolphin (Tursiops truncatus) sightings during bridge construction and demolition to historical sighting records. Additionally, underwater sound pressure levels were monitored at six listening sta-tions to the north and south of the bridge. Dolphin density in the vicinity of the bridge was signifi-cantly higher after construction was completed than during construction. The fewbottlenose dol-phins that used the waters in the general vicinity of the bridge during construction did not appear to avoid the bridge, suggesting that some bottlenose dolphins may still have preferred the habitat around the bridge despite the construction and demolition noise. During the underwater detonation, the small sample of observed bottlenose dolphins decreased nearest neighbor distance, increased group size, and exhibited heading changes. The underwater explosion, which was contained by a steel coffer-cell, was quieter under water than were both in-air explosions, also measured under water. Based on these results, in-air explosions occurring close to water level (< 5 m) should be considered for their potential to effect marine mammals. These explo-sions and persistent noise associated with con-struction and/or demolition operations could have contributed to the change in density of the bottle-nose dolphins observed near the bridge.

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Author Douglas Nowacek Marine Science and Conservation