A Review of the Effects of Seismic Surveys on Marine Mammals

Abstract

This review highlights significant gaps in our knowledge of the effects of seismic air gun noise on marine mammals. Although the characteristics of the seismic signal at different ranges and depths and at higher frequencies are poorly understood, and there are often insufficient data to identify the appropriate acoustic propagation models to apply in particular conditions, these uncertainties are modest compared with those associated with biological factors. Potential biological effects of air gun noise include physical/physiological effects, behavioral disruption, and indirect effects associated with altered prey availability. Physical/physiological effects could include hearing threshold shifts and auditory damage as well as non-auditory disruption, and can be directly caused by sound exposure or the result of behavioral changes in response to sounds, e.g. recent observations suggesting that exposure to loud noise may result in decompression sickness. Direct information on the extent to which seismic pulses could damage hearing are difficult to obtain and as a consequence the impacts on hearing remain poorly known. Behavioral data have been collected for a few species in a limited range of conditions. Responses, including startle and fright, avoidance, and changes in behavior and vocalization patterns, have been observed in baleen whales, odontocetes, and pinnipeds and in some case these have occurred at ranges of tens or hundreds of kilometers. However, behavioral observations are typically variable, some findings are contradictory, and the biological significance of these effects has not been measured. Where feeding, orientation, hazard avoidance, migration or social behavior are altered, it is possible that populations could be adversely affected. There may also be serious long-term consequences due to chronic exposure, and sound could affect marine mammals indirectly by changing the accessibility of their prey species. A precautionary approach to management and regulation must be recommended. While such large degrees of uncertainty remain, this may result in restrictions to operational practices but these could be relaxed if key uncertainties are clarified by appropriate research.

Sightings of dolphins during seismic surveys on the coast of Bahia state, Brazil

“…Although relatively uncommon, incidental catches of small cetaceans in long-lines have been recorded in northeastern Brazil (Siciliano, 1994). Even though some authors reported negative effects of seismic activities on dolphins (Finneran et al, 2002;Gordon et al, 2003), our results do not allow further comments about this topic. …”

Echolocation-based foraging by harbor porpoises and sperm whales, including effects of noise and acoustic propagation

“…This range is usually estimated from the maximum allowable exposure level using the backcalculated, broadband, on-axis source level of the airgun array. Most allowable range estimates also assume a geometric spreading transmission loss or a range-independent acoustic model with an omnidirectional sound source (Barlow and Gentry, 2004;Gordon et al, 2004). Since most airgun array source levels are calculated only on-axis and for frequencies below 250 Hz (Gausland, 2000), the range estimation described above does not account for the full frequency range produced by the array or the directionality of the array [although some regulatory approaches include a correction for array beampattern effects (NMFS, 2003)].…”

“…Even so, many research papers (e.g., Mohl et al, 2003; Au et al, 2004; Madsen et al, 2004; Mooney et al, 2004) and management protocols (e.g., National Marine Fisheries Service, 2003; Barlow and Gentry, 2004;Gordon et al, 2004) continue to use highly simplified descriptions of acoustic propagation without verifying their validity (either to simplify calculations, because they wish to present results relevant to a generic rather than a particular location, or because data on the environmental characteristics (required for more detailed models) was unavailable). In some cases, as detailed in Chapters 3-4 of this thesis, failing to model acoustic propagation realistically can lead to large errors in estimates of animal detection ranges or noise exposure levels.…”

“…They are generally deployed in towed arrays for geophysical exploration . Airgun arrays have very high source levels and there is a concern that exposing sperm whales and other marine mammals to airgun noise may have adverse impacts on their behavior (Gordon et al 2003).…”

Diversity of cetaceans as tool in monitoring environmental impacts of seismic surveys

“…Some studies have demonstrated that noise emitted by seismic survey air-guns causes differing effects on different species of cetaceans due to: hearing sensitivity, behavior, habituation or desensitization, age, sex, location of exposure, and proximity to a shoreline (e.g. , Evans et al 1993, Gordon et al 2004, Madsen et al 2002. Recently, a study off Scotland has suggested that the absence of certain whale species from one sea area surveyed in 1998, where they might be expected to have been found, may have resulted from the incidence of seismic survey work there, although other potentially confounding variables are also noted (MacLeod et al, in press).…”