Assessing the Impact of Underwater Clearance of Unexploded Ordnance on Harbour Porpoises (Phocoena phocoena) in the Southern North Sea

Abstract

Large amounts of legacy unexploded ordnance (UXO) are still present in the North Sea. UXO are frequently accidentally encountered by fishermen and dredging vessels. Out of concern for human safety and to avoid damage to equipment and infrastructure from uncontrolled explosions, most reported UXO found in the Dutch Continental Shelf (DCS) are detonated in a controlled way. These underwater detonations produce high amplitude shock waves that may adversely affect marine mammals. The most abundant marine mammal in the DCS is the harbour porpoise (Phocoena phocoena), a species demonstrated to be highly sensitive to sound. Therefore, an assessment of potential impacts of underwater explosions on harbour porpoises was undertaken. Information regarding UXO cleared in the DCS provided by the Netherlands Ministry of Defence was used in a propagation model to produce sound exposure maps. These were combined with estimates of exposure levels predicted to cause hearing loss in harbour porpoises and survey-based models of harbour porpoise seasonal distribution on the DCS. It was estimated that in a 1-y period, the 88 explosions that occurred in the DCS very likely caused 1,280, and possibly up to 5,450, permanent hearing loss events (i.e., instances of a harbour porpoise predicted to have received sufficient sound exposure to cause permanent hearing loss). This study is the first to address the impacts of underwater explosions on the population scale of a marine mammal species. The methodology is applicable to other studies on the effects of underwater explosions on the marine environment.

Harbour porpoise movement strategy affects cumulative number of animals acoustically exposed to underwater explosions

“…Instead, following Southall et al (2007), PTS-onset was defined as 164 dB re 1 µPa 2 s (= TTS onset SEL) + 15 dB = 179 dB re 1 µPa 2 s, and considered a lower limit below which PTS is unlikely to occur. Experi-mental exposure of fresh dead marine mammal carcasses to underwater explosives (Ketten 2004) showed that ear trauma can occur at peak over-pressures between 10 and 25 psi (or 69−172 kPa), which corresponds to approximately SEL 190 to 203 dB re 1 µPa 2 s for explosions in a shallow-water environment (von Benda-Beckmann et al 2015). It was therefore assumed that the explosion sound would very likely cause PTS at the lower limit at which ear trauma occurs (i.e.…”

“…controlled underwater detonations of recovered bombs, mines and torpedoes from WWII) are unique as they represent single sound events, with the peak sound pressures of the blast wave being one of the highest anthropogenic sound levels produced in the marine environment (Ainslie et al 2009, Koschinski 2011. Such detonations are conducted on the Dutch Continental Shelf (DCS) by the Royal Netherlands Navy in a controlled manner, to reduce the risk of uncontrolled explosions, which could be dangerous to human life and infra-structure (Koschinski 2011, von Benda-Beckmann et al 2015. Each year, approximately 100 detonations occur on the DCS.…”

“…To marine mammals, the sound produced by these explosions could cause acute stress (Reeder & Kramer 2005), trigger behavioural reactions (Nowacek et al 2007, Southall et al 2007, and may lead to temporary or permanent hearing impairment or other physical injury (Ketten 2004, Lucke et al 2009, Kastelein et al 2012. Since the detonations occur irregularly in time and space (von Benda-Beckmann et al 2015), animals are likely to be unable to anticipate and avoid them.…”

“…Von Benda-Beckmann et al (2015) investigated potential impacts of explosive clearance activities conducted by the Royal Netherlands Navy on harbour porpoises on the DCS. That study focused on a temporary or permanent shift of the auditory threshold, known as TTS (temporary threshold shifts) and PTS (permanent threshold shifts), respectively, and estimated that these underwater explosions likely resulted in hundreds to thousands of PTS events annually.…”

“…Here, we first conducted a pilot simulation to investigate how movement choices of individuals can influence numbers exposed to a potentially detrimental sound source. We then extended the study of von Benda-Beckmann et al (2015) by applying individual-based models to investigate the effects of the extremes of movement strategies that harbour porpoises could exhibit (i.e. random dispersal and site fidelity), and thereby deriving improved estimates on the number of individuals receiving PTS or temporary threshold shifts (TTS).…”