Dolphins & Shutdowns
Dolphins are some of the most common cetaceans, and are found in all oceans and many rivers globally. The majority of these species are considered “mid-frequency cetaceans” in accordance with 2018 technical guidance issued by NMFS, with a hearing range of 150 Hz to 160 kHz. (For comparison, the hearing range of low frequency cetaceans – baleen whales – is believed to be approximately 7 Hz to 35 kHz.) While some dolphins are capable of deep dives, delphinids spend the majority of their time close to the surface. For example, one study tracking dive depths across two populations of common bottlenose dolphins showed that the average dive depth ranged from one to 12 meters, with very few deeper dives recorded (Fahlman et al. 2018).
Some policies have proposed that seismic survey operations stop or ramp down for dolphins, and that ramp-up be delayed if dolphins are present within the exclusion zone. These dolphin shutdown proposals, as mitigation measures, broadly and substantially impact seismic operations but have little corresponding environmental benefit. The majority of the energy in a seismic survey signal is below approximately 200 Hz. As mid-frequency hearing specialists, dolphins are only able to detect a small portion of the energy contained in the low-frequency impulsive sounds emitted by seismic operations. Additionally, seismic survey arrays are configured to direct the majority of the sound energy downward into the substrate because energy to the sides of the array is not useful for geological imaging. This means that sound in the upper few meters of the water column – where dolphins spend the majority of their time – is limited.
In areas of high-density dolphin populations, such as the U.S. Gulf of Mexico, shutdown requirements for a species that frequently exhibits bow-riding behavior – actively approaching and interacting with vessels – could bring all seismic survey activity to a halt. Implementing dolphin-shutdowns would significantly and unnecessarily increase the number of shutdowns and delays in ramp-ups, leading to increased costs for acquisition and much longer survey times to adequately ensure the quality and integrity of the survey data. With no environmental benefit to the target species, dolphin shutdowns prolong the duration of the survey, unnecessarily increasing the potential exposure of other marine mammals to seismic survey operations.
The IAGC does not support dolphin shutdowns and encourages exemptions for all dolphin species regardless of whether a dolphin is attempting to bow-ride. Such an exemption is well-supported by the best available science, which show that seismic surveys do not have any meaningful adverse effects on dolphin species.
- Proposed Incidental Harassment Authorizations for the Incidental Taking of Marine Mammals During Geophysical Surveys in the Atlantic Ocean, 21 July 2017 – IAGC comments
- Revised Application for Marine Mammal Incidental Take Regulations for Geophysical Surveys in the Gulf of Mexico, 23 January 2017 – IAGC comments
- BOEM Gulf of Mexico Draft PEIS, 29 November 2016 – IAGC comments
- Fundamentals of Sound in the Marine Environment
- Sound & Marine Seismic Surveys, Acoustics Today by Dr. Robert Gisiner
- NMFS Marine Mammal Acoustic Technical Guidance
- Fahlman et al. (2018), Modeling Tissue and Blood Gas Kinetics in Coastal and Offshore Common Bottlenose Dolphins, Tursiops truncatus