The potential effects of sounds from seismic exploration on the distribution of cetaceans in the Northern Gulf of Mexico

Abstract

The goal of this study is to determine the possible effects of seismic exploration sounds on distribution of cetaceans in the Northern Gulf of Mexico. The study consisted of ten Gulfier research cruises, surveying the Northern Gulf of Mexico from 1992 to 1997. Cetacean sighting rate (sightings per hour) were examined in relation to the low frequency sounds originating from oil exploration. Recordings of seismic pulses were collected via a towed passive hydrophone array, and grouped according to the signal-to- noise ratio (SNR) into levels of 12 do above ambient. Visual observations of cetaceans utilized 'big eye' binoculars to identify animals. Five hydrographic features were determined by analyzing sea surface height measurements obtained from satellite altimeter and by analysis of shipboard hydrographic measurements. The five defined hydrographic regions were: cold core rings (CCR), warm core rings (WCR), ring peripheries (PER), confluence zones (CON), and other (OTHER). Over 514 hours of recordings were analyzed, with seismic exploration pulses present 21% of the overall time, with a high of 49% in the last cruise. The sighting rates for Stenella attenuate were significantly higher in the > 12 dB above ambient region of "OTHER'' (x²=10.26, p=0.005); this was the only significant according to hydrographic feature for the overall sightings (x²= 8.46, p=0.07), and for Physeter microcephalus (x²=12.13,p=0.016). Sighting rates were significantly higher in the CCR and the PER for these groups. It appears that cetacean distributions are related to environmental conditions and not to sound intensity level of seismic exploration. The CCR often occur on the continental margin of the mouth of the Mississippi River, an area of great oil exploitation. Tolerance of noise by cetaceans in this important cetacean habitat may impact these findings. Displacement of cetaceans away from immediate source of sound into regions of moderate sound intensity may also influence our ability to observe a change in cetacean distribution associated with seismic exploration.