An exciting, comprehensive and important piece of work on underwater noise and its effects on marine mammals has just been published – Brandon Southall and colleagues have updated their 2007 publication in Aquatic Mammals, congratulations Brandon and team!
Perhaps the sound you hear is another groan among the offshore industry and those carrying out noise impact assessments; we’ve all just got used to the recent “NOAA criteria” proposed by the National Marine Fisheries Service (NMFS) in 2016 and its update in 2018. We’ve hardly had time to breathe and now there is another new set of noise exposure criteria to learn and understand!
But do not worry, here we will quickly get you used to the history of these and the new Southall et al. (2019) criteria!
But let’s start from the beginning (skip the next section if you know the background):
Back in 2007, Southall and his colleagues on the expert panel proposed their first noise exposure criteria. These criteria were designed as a tool to assess how much noise a marine mammal can be exposed to before it affects its hearing sensitivity (i.e. shifting up its hearing threshold) within certain frequency ranges. This shift could be either temporary (TTS) or permanent (PTS). The sound emitted by man-made sources may induce TTS or PTS in an animal in two ways: peak sound pressure levels (SPL peak) may cause damage to the inner ear (we come back to this further below), and the accumulated sound energy the animal is exposed to (cumulative sound exposure levels, SEL) over the entire duration of a discrete or repeated noise exposure has the potential to induce auditory damage if it exceeds distinct threshold levels. It was clear to the expert panel that the frequency content of the sound would play a role in causing damage when considering accumulated sound levels. Sound outside the hearing range of the animal would be unlikely to affect its hearing, while the sound energy within the hearing range could be harmful. Therefore, Southall et al. (2007) proposed frequency-weighting functions that removed the sound energy outside an animal’s hearing range before calculating the SEL value. As it was impossible to build individual hearing functions for each of the 130+ marine mammal species, they grouped species with similar hearing ranges into functional hearing groups:
- Low-frequency cetaceans (LF): which consisted of baleen whales such as humpback whales;
- Mid-frequency cetaceans (MF): which consisted of toothed whales except porpoises and river dolphins;
- High-frequency cetaceans (HF): which consisted of porpoise and river dolphins, and finally,
- Pinnipeds: which they further divided to under water (PW) or in air (PA) hearing animals, to account for their amphibious life style.
Southall and team then proposed weighted SEL-thresholds for each hearing group, at which the onset of TTS and PTS would be expected, based on the scientific knowledge available at that time.
Unfortunately, it gets more complicated (but why should life be easy 😉):
Exposure to loud, brief, transient sounds (impulsive sounds, such as explosions, airgun shots or pile strikes) is more damaging as it increases the hearing threshold faster than exposure to non-impulsive sound (such as from drilling and shipping), so less sound energy is needed to induce TTS or PTS. Therefore, a more conservative set of weighted SEL-thresholds was proposed for impulsive sound.
For impulsive sound, it is also important to consider the peak sound pressure levels (coming back to the SPLpeak criteria). SPL peak can elicit TTS or PTS regardless of its energy and frequency content, which lead to the proposal of an additional set of thresholds for impulsive noise (i.e. creating a “dual-criterion”): un-weighted SPL peak thresholds that need to be considered in addition to the weighted SEL thresholds.
A lot of research has been conducted since 2007. For example, new data from Lucke et al. (2009) suggested that high frequency cetaceans are much more sensitive to sound than estimated in Southall et al. 2007. In addition, research from Finneran (2010) indicated that more sound energy is needed to elicit TTS/PTS at frequencies an animal is less sensitive to than at frequencies the animal is more sensitive to (as reflected in their hearing curves, i.e. audiograms). This meant that Southall et al. (2007)’s weighting functions, which were flat over the hearing range of the functional groups, and thereby treating all frequencies equally regardless of the animal’s sensitivity to it, could be improved. James Finneran (who is part of the Southall et al publication team) presents in his 2016 technical report weighting curves that reflect the hearing abilities of animals more closely. These weighting curves, along with updated TTS/PTS thresholds, were based on estimated hearing parameters and audiograms, informed by the most recent research. Finneran also added sirenians (SI: manatees and dugongs) as a functional group, divided seals in water into phocid seals (earless) (PW) and otariids (eared seals) (OW), to which he added other marine carnivores such as walruses, sea otters and polar bears. The document did not include in-air hearing.
NMFS adopted the Finneran criteria (without including the SI group) in their 2016 publication, considering the most recent literature while focusing on US species. The criteria remain untouched in the 2018 revision, with only improvements and clarifications on the implementation of the guidance added to the document.
While the objectives of NMFS are to provide technical guidance for assessing the effects of underwater man-made sound on the hearing of marine mammal species under their jurisdiction, Brandon and his team’s aim is to provide an update of their 2007 work for the international community. The Southall et al. (2019) criteria are also based on the Finneran criteria, plus the most recent scientific knowledge and use the same weighting functions and TTS/PTS-thresholds as the NOAA criteria.
But what’s new then?
Firstly, they consider all marine mammal species (not just US species), both underwater and, for amphibious species, in air. While they adopt the functional species groups from Finneran 2016, they rename the toothed whale groups to acknowledge that their best hearing is at a higher frequency than their species group name suggests: mid frequency cetaceans (MF) are now called high frequency cetaceans (HF), and the previous high frequency cetaceans are now called very high frequency cetaceans (VHF). The toothed whale species have also been re-arranged within these two groups: VHF now includes true porpoises, most river dolphin species, pygmy/dwarf sperm whales and several oceanic dolphins, while the rest remain in the HF group. To the ‘other marine carnivores’ they added marine otters. They also highlight that in the future, the LF group may be split into very low frequency cetaceans (VLF) and LF, and the HF group into MF and HF once more data are available to support this separation. All in all: caution is needed not to mix the nomenclature of the different sets of criteria!
Secondly, back in 2007 Southall and colleagues defined sound to be impulsive or non-impulsive based on its characteristics at the sound source as a simplifying measure. However, sound from impulsive sources might lose its impulsive characteristics at greater ranges due to propagation effects and eventually become non-impulsive. Therefore, Southall et al. (2019) noted that, while considering the same source categories, the exposure criteria for impulsive and non-impulsive sound should be applied based on the signal features likely to be perceived by the animal rather than those emitted by the source. This is likely to change the way in which we assess the impact of impulsive noise on marine mammals in future. Southall and colleagues don’t go as far as suggesting where this transition might occur but indicate that specific methods to estimate the transition from impulsive to non-impulsive noise are being developed and will be published at a later date. We’re also aware of an upcoming publication from our colleagues here at the Sea Mammal Research Unit (SMRU), led by Dr Gordon Hastie, that will provide valuable data to inform this question – watch this space!
Like the 2007 paper, the 2019 update gives a comprehensive overview of the most up-to-date research on the auditory system of marine mammals. It includes a detailed review of all available data on direct measures of hearing, auditory anatomy, and emitted sound characteristics for all marine mammal species, explains how the weighting functions and TTS/PTS thresholds for the functional groups were derived, and provides research recommendations to improve quantitative methods for evaluating the auditory effects of noise on marine mammals. It is a great source of knowledge for the scientific community and guidance for regulators – so we recommend you have a look!
On a last note: while Southall et al. (2007) discuss the influence of noise on marine mammal behaviour, this was not included in either Finneran (2016), NMFS (2016/2018) or Southall et al. (2019), but an update is on the way.
National Marine Fisheries Service (2016). Technical Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing: Underwater Acoustic Thresholds for Onset of Permanent and Temporary Threshold Shifts. Silver Spring, U.S. Department of Commerce. NOAA Technical memorandum NMFS-OPR-55: 189.
National Marine Fisheries Service (2018). Revisions to: Technical Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing (Version 2.0): Underwater Thresholds for Onset of Permanent and Temporary Threshold Shifts. Silver Spring, U.S. Department of Commerce, NOAA. NOAA Technical memorandum NMFS-OPR-59: 167.