Behavioral reactions of harbor porpoise to pile-driving noise

Tougaard, Jakob; Kyhn, Line A.; Amundin, Mats; Wennerberg, Daniel; Bordin, Carolina

Advances in Experimental Medicine and Biology (2012)

Abstract

Pile driving of large steel monopiles in offshore waters has increased rapidly in recent years due to the expanding development of offshore wind energy. In particular, Phocoena phocoena (harbor porpoise) has been the focus of attention with respect to a possible negative impact. Impact pile driving, where a large steel monopile is driven 20-30 m into the seabed, is capable of generating very loud sound pressures, exceeding 230 dB re 1 μPa peak-peak in source levels and detectable at distances of tens of kilometers (Bailey et al. 2010). Such high sound pressures, coupled with the repetitive emission of sounds (1–2 strokes/s) at a high duty cycle (10%) gives the potential for exposing nearby animals to very high and potentially damaging sound exposure levels (Gordon et al. 2009). Besides the potential to inflict acute injury, the pile-driving noise has the potential to affect behavior of marine mammals over an even larger area.

Editor's Notes

Tougaard et al 2012 serves as a reanalysis of how we think about the implications of disturbance to harbor porpoises in response to pile-driving noise. Much of regulation of pile-driving has focused primarily on reducing risk of acute injury to individuals, such as damage to auditory systems. Mitigating this risk has been achieved primarily through the use of deterrent devices (pingers and seal scarers) and the ramping up of piling activity to allow animals the opportunity to vacate the exposure zone before starting the full-power pile-driving procedure. However, there is a large discrepancy between the size of the zone of acute injury and the zone of behavioral response. This article asks if protection against acute injury alone is sufficient to protect a population. The authors conclude that the level of behavioral impact that should be tolerated is heavily dependent on the status of the population. The more at-risk or endangered a population is, the fewer behavioral impacts can be tolerated. On the contrary, if the status of a species or population is deemed to be of little concern, then the focus of mitigation should fall on those of acute injury rather than behavioral effects. The assessment of how noise can impact a population is quite difficult though as many impacts are only manifested later in the animal’s life as a result of reduced fecundity or increased mortality. The authors highlight the need for a thorough understanding of the links between behavior and population parameters of the study species in these assessments. Quantifying impacts in terms of habitat exclusion alone can also be problematic as a stressor, such as noise, can also impact a population through both direct and indirect effects such as reduced feeding time, fewer mating opportunities, and reduced time for nursing. This means that the lack of a reduction of abundance in a zone is not necessarily evidence of no effect. The article concludes with the discussion of the results of several studies focused on the behavioral effects of pile-driving noise in harbor porpoises. The first studies focused on using passive acoustic monitoring to estimate the zone of behavioral effects (here the reduction of acoustic activity) of porpoises. These studies found noticeable effects at distances of up to 25km from the pile-driving site, however the reduction of acoustic activity could be a result of a decrease of abundance within the zone or a change in behavior, meaning that porpoises could still be present in the zone, but were altering their acoustic behavior as a result of the noise. This question was addressed using a controlled-exposure experiment (CEE) that used underwater speakers to play pile-driving sounds for porpoises at a location where porpoises could be tracked visually using a theodolite. This study found that even with lower source levels (140 dB for the CEE versus 230 dB for actual pile-driving procedures) porpoises responded similarly to what had been found for porpoises studied at real pile-driving sites and vacated the impact area. This will allow for a better quantification of the impact zones for future pile-driving activities.

Editor's notes provide additional information about a library entry. They are written by the library's editor and are not part of the original article.

Species

Phocoena phocoena

Porpoise Research Library