Assessing potential perception of shipping noise by marine mammals in an arctic inlet

Abstract

Shipping is increasing in Arctic regions, exposing marine mammals to increased underwater noise. Noise analyses often use unweighted broadband sound pressure levels (SPL) to assess noise impacts, but this does not account for the animals' hearing abilities at different frequencies. In 2018 and 2019, noise levels were recorded at five and three sites, respectively, along a shipping route in an inlet of Northern Baffin Island, Canada. Broadband SPLs (10 Hz–25 kHz), unweighted and with auditory weighing functions from three marine mammal groups, were compared between times ore carriers (travelling < 9 knots) were present or absent. Clearly audible distances of shipping noise and exposure durations were estimated for each weighting function relative to vessel direction, orientation, and year. Auditory weighting functions had significant effects on the potential perception of shipping noise. Bowhead whales ( Balaena mysticetus) experienced similar SPLs to unweighted levels. Narwhals ( Monodon monoceros) and ringed seals ( Pusa hispida) experienced lower SPLs. Narwhals were unlikely to clearly perceive shipping noise unless ships were in close proximity (<3 km) and ambient noise levels were low. Detectability propagation models of presumed noise exposure from shipping must be based on the hearing sensitivities of each species group when assessing noise impacts on marine mammals.

Vocal count responses of narwhals to bulk carrier noise in Milne Inlet, Nunavut, Canada

“…They were placed either on the nominal ship transit route (location A), 1 km shoreward from the ship transit route (location D), or 6 km from the ship transit route within Koluktoo Bay (location C; Figure 1). The location names (A, C, and D) are the same as those in a related study of the bulk carrier underwater noise levels conducted using the same set of recordings analyzed here (Sweeney et al, 2022a).…”

“…The HFC adjusted SPLs can be considerably lower than the unweighted values (Heide‐Jørgensen et al, 2021; Sweeney et al, 2022a) and as they better reflect the variation in hearing threshold levels over the audible frequency range, they will provide better estimates of the perception of the sound levels. The mean SPLs of the 14‐min recordings were used in the analyses.…”

“…These categories were augmented for AMAR C since it was located in Koluktoo Bay and perpendicular to the shipping lane at the position of AMAR A (see Figure 1). Thus, bulk carrier distance could not be closer than 6 km from the AMAR C. The During category at AMAR C was therefore restricted to the period when a ship was crossing the mouth of the bay, which was roughly equivalent to the period when the ship was within 5 km of AMAR A. Additionally, the transit directions derived for AMAR A were used for AMAR C. Based on distance from the nominal shipping route, bulk carrier noise levels were expected to be highest at AMAR A, lower at AMAR D, and lowest at AMAR C. The During category at AMAR locations A and D generally reflected periods when ship noise would likely be clearly detectable by narwhals near the recorder (Sweeney et al, 2022a,b).…”

“…These levels were then adjusted for the assumed high frequency cetacean (HFC) weighted hearing function of narwhals (Southall et al, 2019) and converted to broadband levels (using 1/3 octave bands, 10 Hz to 25 kHz, dB re 1μPa [HFC]). The HFC adjusted SPLs can be considerably lower than the unweighted values (Heide-Jørgensen et al, 2021;Sweeney et al, 2022a) and as they better reflect the variation in hearing threshold levels over the audible frequency range, they will provide better estimates of the perception of the sound levels. The mean SPLs of the 14-min recordings were used in the analyses.…”

“…The standardized audiogram of the high frequency cetacean (HFC) group, which includes narwhals and belugas, shows a decrease below their maximum sensitivity of ~30 dB at 1 kHz (Southall et al, 2019). Large ships typically have the highest amplitudes below 1 kHz (Arveson & Vendettis, 2000) meaning that HFC species are less sensitive to the lower frequencies of ship noise (Southall et al, 2019; Sweeney et al, 2022a). However, ship noise can also occur at higher frequencies (1–40 kHz; Veirs et al, 2016) and those higher frequency components could be audible to narwhals.…”