Responses of Bottlenose Dolphins to Construction and Demolition of Underwater Structures

Open Access Report 2006

Mote Marine Laboratory

Of all of the anthropogenic noise sources in the marine environment, construction and demolition noise and their effects have received perhaps the least attention, and yet they are commonplace activities in many offshore and coastal waters. In the Gulf of Mexico, for example, thousands of oil production platforms have been constructed since the mid 1900′ s, and more than 100 of the older rigs are being removed each year (Klima et aI. , 1988). Coastal development involves the construction of bridges, causeways, piers, and other structures at the water’s edge. Further development leads to the replacement of existing structures and removal of these older structures. These activities occur within the ranges of a variety of marine mammals, but little work has been done to evaluate their impacts on the animals. Though the precautionary principle would dictate otherwise, marine mammals are given little regulatory protection from potential adverse impacts from construction/demolition activities. There are currently no provisions under the federal Marine Mammal Protection Act of 1972 requiring review and permitting of coastal construction or demolition projects. Evidence indicates that construction and similar activities, such as drilling, impact marine mammals in several ways. Early work with bowhead whales (Balaena mysticetus) indicates that they avoid areas of heavy industrial activity. Richardson et aI. (1985) examined distribution patterns of whales exposed to oil and gas exploration and production in the Beaufort Sea relative to historical records, and noted shifts in habitat usage. In another study, bowhead whales were distributed farther from a drilling rig than they would be under a random scenario (Schick and Urban 2000). At a construction site in waters off western Hong Kong, groups of Indo-Pacific humpback dolphins (Sousa chinensis) doubled their swim speed during periods of active pile driving, however, abundance estimates did not change significantly (Wiirsig et aI. 2000). Todd et al. (1996) examined the distribution, resighting rate, and residency of humpback whales (Megaptera novaeangliae) in Newfoundland waters before, during, and after exposure to underwater explosions and did not notice marked behavioral reactions to the detonations. Importantly though, the rate of entrapment in acoustically enhanced fishing nets increased at the onset of underwater drilling activity and sequences of explosions, suggesting a decline in orientation ability. Finneran et al. (2000) measured the auditory and behavioral responses of two dolphins and a beluga whale to simulated underwater explosions. None of the animals showed threshold shifts to even the loudest stimulus (500 kg explosive at 1.7 km range), but all showed behavioral responses during the experiment (Finneran et al. 2000). In Sarasota Bay, Florida, the construction of a large fixed-span bridge and subsequent demolition of the existing drawbridge allowed us the opportunity to investigate the potential effects of marine building on coastal bottlenose dolphins (Tursiops truncatus). In the absence of data clearly indicating no jeopardy, or data clearly showing effects to distance from the activity, it is important for biologists and regulatory agencies to develop a better understanding of the potential adverse effects of such wide-scale activities on these animals. One approach would be to follow the model of Richardson et al. (1985), taking advantage of opportunistic “experiments” to examine the distribution and behavior of dolphins before, during, and after construction or demolition activities. Extensive long-term databases exist that can quantitatively define dolphins’ patterns of habitat usage in Sarasota Bay before construction (Scott et al. 1990; Wells 1991, 2003), and can be used for comparisons with similar data collected during and after demolition. Photographic identification surveys for dolphins have been conducted for two weeks each month through these waters for the last decade. We continued these surveys and intensified data collection efforts relative to the bridge project schedule, and related behavior and distribution patterns to the noise generated by the construction and demolition activities. Research Objectives To develop a better understanding of the potential effects of bridge construction and demolition on bottlenose dolphins, in order to aid regulatory agencies in protecting these animals by: 1) documenting distributions of dolphins relative to an area of marine construction or demolition and comparing distributions to historical distributions; 2) describing sound levels of construction/demolition; 3) during the acute (explosive) phase, relating dolphin behavior to the explosion and associated sound levels at the animals.

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