Dolphins & Shutdowns
Dolphins (family Delphinidae) are among the most widespread and commonly encountered cetaceans, occurring throughout the world’s oceans. The majority of dolphin species are classified by the National Marine Fisheries Service (NMFS) within the high-frequency cetacean hearing group, which has a generalized hearing range of 150 Hz to 160 kHz. By comparison, low-frequency cetaceans (baleen whales) are estimated to hear sounds from approximately 7 Hz to 36+ kHz.
Many dolphin species spend a substantial portion of their time in the upper water column, although dive behavior varies among species and habitats.
Some policies have proposed requiring geophysical survey operations to shut down, ramp down, and delay ramp-up when dolphins are observed within a designated exclusion zone. These measures can substantially affect survey efficiency and operational continuity, while their corresponding conservation benefit is often unclear. Dolphin hearing characteristics and habitat use are important considerations when evaluating potential interactions between dolphins and geophysical survey operations.
The majority of the acoustic energy in a seismic pulse is concentrated at frequencies below approximately 200 Hz. Dolphins are classified by NMFS as high-frequency hearing specialists, with hearing that is most sensitive at frequencies substantially higher than those that contain most of the energy in a seismic pulse. As a result, dolphins are generally less sensitive to the low-frequency components that contain most of the energy in seismic pulses.
In addition, geophysical survey sources are designed to direct most of their acoustic energy downward toward the seafloor, where it is used for geological imaging. Sound radiated horizontally and into the near-surface layer is substantially lower because it does not contribute to survey objectives. Consequently, sound exposure in the upper portion of the water column – where many dolphin species spend a considerable amount of their time – is reduced relative to areas directly beneath the survey source.
In areas with high densities of dolphins, such as the U.S. Gulf of America, shutdown requirements for species that frequently exhibit bow-riding behavior – actively approaching and interacting with vessels – could result in frequent operational interruptions. Implementing dolphin shutdown requirements would substantially increase the number of shutdowns and ramp-up delay, leading to higher acquisition costs and extended survey durations while maintaining data quality and survey efficiency.
Because the expected conservation benefit to dolphins is limited, such measures may inadvertently prolong the overall duration of survey operations. Longer survey durations can increase the time that survey sound is present in the environment, potentially extending the period during which other marine mammals may be exposed to geophysical survey activities.
EnerGeo Alliance does not support dolphin shutdown requirements and encourages exemptions for all dolphin species, regardless of whether an animal is attempting to bow-ride. Such exemptions are supported by the best available science, which has not demonstrated biologically significant adverse effects of geophysical survey operations on dolphin populations and provides no evidence that dolphin shutdown requirements improve population-level conservation outcomes.
