Anthropogenic sound exposure of marine mammals from seaways: Estimates for Lower St. Lawrence Seaway, eastern Canada

Applied Acoustics 71 (2010) 1093–1098

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Anthropogenic sound exposure of marine mammals from seaways: Estimates for Lower St. Lawrence Seaway, eastern Canada Y. Simard a,b,*, R. Lepage a, C. Gervaise c a

Marine Science Institute, University of Québec at Rimouski, 310 Allée des Ursulines, Rimouski, Québec, Canada G5L 3A1 Maurice Lamontagne Institute, Fisheries and Oceans Canada, 850 Route de la Mer, Mont-Joli, Québec, Canada G5H 3Z4 c DTN, ENSIETA, 2 Rue François Verny, 29200 Brest, France b

a r t i c l e

i n f o

Article history: Available online 16 June 2010 Keywords: Shipping noise Marine mammals Seaway Sound level M-weighting Whales

a b s t r a c t The impact of shipping noise on marine life and quality of marine mammal habitats in oceans and coastal environments has become a major concern worldwide. Background noise can also limits detection of marine mammal sounds in passive acoustic monitoring (PAM) systems. Characterisation of this noise over long time periods and estimates of the exposure of the different marine mammal groups are still very fragmentary and limited to only a few locations. This paper presents such observations for a part of a busy seaway of North America, the St. Lawrence Seaway, which cuts through the Gulf of St. Lawrence and crosses several cetaceans and pinnipeds feeding areas. Noise was continuously recorded for a 5month period in summer 2005 by an AURAL autonomous hydrophone deployed close to the bottom in the 300-m deep seaway. The maximum received noise level in the 20 Hz–0.9 kHz band reached 136 dB re 1 lParms. The median level of 112 dB re 1 lParms was exceeded 50% of the time due to transiting merchant ships. Median spectral level tracks the reference curve for heavy traffic in oceans and 50% of the noise is within a ±6 dB envelope around it. Strong spectral lines were common at low frequencies and in the 400–800 Hz band. M-weighting functions applied for the three groups of cetaceans and pinnipeds indicate wideband median levels varying from 106 to 112 dB-M re 1 lParms surrounded by a ±5 dB twoquartile interval. Higher values are expected for animals frequenting the sound channel at intermediate depths. As expected, the highest M-weighting levels correspond to low-frequency specialists and pinnipeds. Criteria for assessing the behavioural and physiological impacts of long term exposure of marine mammals to such shipping noise levels need to be worked out. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.

1. Introduction Marine mammals make extensive use of sounds to accomplish their vital functions of communicating, remote sensing their environment, navigating and finding preys in an opaque medium by echolocation [1–5]. Long time exposure to noise may adversely affect these vital functions [6–10]. Noise also affects the performance of PAM systems and determines their detection functions vs. range [9]. Various levels of noise are however always present in the environment, either arising from natural sources, which the animals should have adapted to cope with through evolution, or from anthropogenic sources, which are relatively new from an evolutionary perspective, and have considerably changed in the

* Corresponding author at: Marine Science Institute, University of Québec at Rimouski, 310 Allée des Ursulines, Rimouski, Québec, Canada G5L 3A1. E-mail addresses: [email protected], [email protected] (Y. Simard), [email protected] (R. Lepage), [email protected] (C. Gervaise).

last century [3,8,11–13]. A major anthropogenic contributor to ocean noise is shipping [8] a critical link for maintaining world global commerce, characterised by a steady increasing trend of 8–14% in transported cargo in the last decade [14]. Shipping is considered to have doubled ambient noise levels in the low frequency band off California every decade since the 1960s [8,11–13]. Shipping is ubiquitous around the mid-latitude belt of the planet and often concentrated along busy seaways and choke points [15]. New routes are expected to develop in the high northern latitudes in the coming decades in response to the melting of the Arctic ice sheet as a consequence of global warming. Despite these facts and the importance of noise for marine mammals and other marine organisms, very few data are available on the noise levels animals are exposed to on seaways. Documenting this noise is therefore among the research priorities identified by recent papers and reviews on impacts of anthropogenic sounds [8,12,13]. The present paper addresses this priority by characterising shipping noise on a segment of a medium-traffic (20 ships per day) seaway that crosses marine mammal feeding habitats in the

0003-682X/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apacoust.2010.05.012

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Estuary and Gulf of St. Lawrence during the feeding and busiest traffic season. The exposures of the different marine mammal categories are assessed using their respective M-weighing functions recently proposed by Southall et al. [1]. 2. Materials and methods The acoustic data were recorded from an AURAL autonomous hydrophone [16] moored on the floor of the 30-km wide Laurentian channel, in the Lower St. Lawrence Estuary, 20 km off Matane (48.9802°N, 67.7417°W), from 19 May to 17 October 2005 (Fig. 1). The hydrophone was deployed at a depth of 286 m, 10 m above the bottom, on a 20-m long ‘‘I” mooring line, with an acoustic release, a current meter, and a 0.76-m diameter steel float (Fig. 1). The instrument was set for continuous recording 16-bit wav files at 2000 samples s 1 with a gain of 17 dB. The hydrophone is a HTI 96-min (High Tech Inc., Gulport, MS, USA), with a flat low-frequency (61 kHz) receiving sensitivity (RS) of 163.7 ± 1 (median ± range) dB re 1 V/lPa at low frequencies, from a calibration performed for the 50 Hz–20 kHz band at the Defense Research and Development Canada calibration facility (DRDC, Dartmouth, NS, Canada). Then, for every 6-h period, a 5-min segment was randomly chosen for analyzing the ambient noise. This random selection is used to avoid possible aliasing that can occur when choosing a sampling frequency, notably the tidal component frequencies. The segment was first checked for possible interferences from strumming and flow noise that could bias the noise estimate by tracking an energy detector in the 0–30 Hz low-passed signal. When present, corre-

sponding time intervals were excluded from the analysis and another random segment was chosen. A total of 597 5-min segments were extracted from the 5-month time-series. Spectral rms levels were computed for (1) ANSI third octave bands covering the frequency range of [8.9–897 Hz] and (2) per 1 Hz band with a frequency resolution of 1.58 Hz. Broadband rms sound pressure levels (SPLrms) were obtained by summing the third octave levels over the [17.8–897 Hz] bandwidth. These metrics were also computed using the M-weighting functions proposed for three cetacean groups and pinnipeds [1]. Their respective probability distribution (pdf) was assessed for the 5-month period and summarized by box-plots or a series of cumulative percentiles. More details on material and methods are available in [17]. To check the effect of the hydrophone depth on the measured noise levels, total loss at different frequencies as function of range and depth was computed for a 5-m deep source representing a ship, using a normal mode propagation model. The ORCA model [18] was parameterized with the mean local sound speed profile, from Fisheries and Oceans Canada temperature and salinity profiles realized in the study area during the recording period, and the same bottom sound speed profile used in [9] for the Laurentian channel. Total loss difference relative to a receiver located at a depth of 286 m was computed for ranges from 0 to 20 km, representing the distances from the sources. This ratio was averaged over the 0–20 km range interval for each depth in linear domain and the result converted to dB to get the correction to add to our measured noise levels at a depth of 286 m for other measurement depths in the water column. This correction assumes that transiting ships are recruited over a 20 km radius from the hydrophone,

Fig. 1. Location of the AURAL mooring (+) in the navigation corridor (bold dashed lines) in Lower St. Lawrence Estuary, and sketch of the mooring configuration (drawing not to scale).

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one by one, and they are uniformly or randomly distributed over this radius. This correction profile allows using our measurements to assess the mean noise level at different depths in the water column. 3. Results The daily merchant ship traffic in the Lower St. Lawrence Estuary from the Coast Guard and pilot station at Les Escoumins during the 5-month period averaged 17.5 ± 3.7 (SD) merchant ships per day. Additional traffic from the car and truck ferry crossing the St. Lawrence at Matane adds six daily transits from mid-June to the first week of September and four daily transits for the rest of the time. Fishing vessel traffic from the shrimp trawler fleet (the main local fishing activity) and other fishing boats is less intense off Matane than further downstream in the Gulf, from the analysis of fishing effort west of 66°W meridian during the study period. Shrimp fishing effort peaks in May and shows a steady decreasing trend from May to October. We estimate to a minimum of zero and a maximum of five the number of possible daily transits in a 25-km radius from the station, from the trawlers landing in Matane. The median received noise level in the 20 Hz–0.9 kHz band was 112.0 dB re 1 lParms (Fig. 2). Therefore, half of the time the ambient noise goes beyond this level in response to local merchant ship

traffic. The highest levels above 125 dB re 1 lParms are encountered when a ship is at close ranges from the hydrophones (0.3–1 km), based on the tracking of an individual merchant ship of know GPS position (details in [17]). The envelope of the spectral levels covers a wide range, varying between 60 dB at 10 Hz and 40 dB at 1000 Hz (Fig. 3). The distribution of spectral levels is slightly skewed towards lower values although the median is not far from the center of the envelope. Median levels are around 90 dB re 1 l Pa2rms =Hz below 80 Hz and decrease to 71 dB re 1 l Pa2rms =Hz at 1000 Hz. Minimum levels oscillate between 65 and 70 dB re 1 l Pa2rms =Hz below 100 Hz, and decline to 60 dB re 1 l Pa2rms =Hz at 1000 Hz. The typical shipping signature, centered at 50–100 Hz, is evident on all percentiles of the distribution. From 20 Hz to 200 Hz, the median spectral level tracks the Wenz reference curve for heavy shipping traffic in oceans [3,19]. The maximum spectral values exceed the median by 25–30 dB on average. Strong spectral lines exceeding the mean maximum levels by 10–15 dB were common at low frequencies and in the 400–800 Hz band (Fig. 3, max curve). The shipping imprint is still visible in the M-weighted third octave spectra computed for the three groups of cetaceans and the pinnipeds (Fig. 4). It is 5–10 dB higher for the low-frequency specialists and the pinnipeds, as expected from their respective Mweighting functions. This translates in a 5–7 dB higher broadband SPL-M for these two groups compared to the mid- and high-frequency specialists (Fig. 5). Broadband SPL-M values for all groups range from 90 to 135 dB-M re 1 lParms with 50% of the distribution in a ±5 dB envelope around the median, which varies from 106 to 112 dB-M re 1 lParms depending on the group (Fig. 5).

4. Discussion With 20 ships transiting per day, the St. Lawrence Seaway can be considered as a medium traffic seaways compared to the English Channel or Gibraltar strait, where the traffic can be between 5 and 10 times higher. Results clearly show that such medium traffic seaways of the world are areas where mean ambient noise can be high. Half of the time noise level was higher than the 1960s reference levels for heavy shipping in oceans [3,19]. For instance, the median spectral noise levels over the 1 kHz band were 1–3 orders of magnitudes (10–30 dB) higher than what was observed over similar long-term series at two sites on Southern California continental shelf [12,13] (Fig. 3). Below 50 Hz, the median spectral levels were comparable to median estimates of Andrew et al. [11] off Point Sur, California. For higher frequencies, Point Sur observations track the first quartile line of the spectral level pdf presented in this

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SPL-M [20-900 Hz] (dB re 1µParms) Fig. 5. Box-plots of M-weighted SPL[20–900 Hz] over 5 months for the three cetacean categories and the pinnipeds.

paper. Observations for the Atlantic continental shelf in eastern Canada, summarized by Desharnais and Collisson (Fig. 14 of [25]), are included within 80% of the spectral envelope of the observations reported in this paper, and centered around the median below 80 Hz and on the first quartile curve at higher frequencies. Contributions of other sources to ambient noise are likely but thought to be minor compared to shipping. Fin and blue whales may have contributed with their strong infrasounds (190 dB re 1 lPa source level (SL)) in the low frequency band below 25 Hz, but, in contrast to [11–13,25], the spectral pdf pattern did not evidence strong effects in the area where the instrument was located. Wind contributions at frequencies above 200 Hz are likely and may have slowed the decreasing spectral slope with frequencies. Although the separation of the wind and ship contributions may not be possible, shipping noise has a high probability of being the dominant contributor in this band, given the relatively sheltered conditions of the study area compared to open ocean, and reverberations in the 30-km wide U-shaped Laurentian channel. The observed high noise levels testify significant traffic. The relatively centered spectral pdf, contrasting with the long-tailed spec-

tral pdf off California, may relate to this medium-traffic as well as its regularity and the time interval between the ships along the seaway. The ship transit is detectable by above-median broadband levels for about 45 min at an average cruising speed of 14–15 knots. This corresponds to a strong noise footprint of 20 km diameter around the ship. With an average of 1 ship per hour (i.e. 27 km between ships), there is very little time/space (7.5 min/3.5 km) out of the ship footprint series when the noise can reach the lowest levels. This is in large part the reason why our lowest spectra are high, as well as all the percentiles of the spectral pdf, compared to the sites off California, where the noise source is more distant-shipping than seaway-shipping. The relatively deep and flat-bottom trench of the Laurentian channel allows significant propagation of the noise radiated by ships transiting on the seaway. The sound speed profile in this region is progressively changing from May to October, from an upward refracting mode to a channeling into the summer cold intermediate layer between 50 and 150 m, in response to winter surface cooling and summer warming ([9,22], Fig. 6g). The 100– 150 m thick upper water column affected by seasonal fluctuations should trap more noise in surface waters than deeper depths, where the hydrophone was located. These surface waters are also where the whales are expected to be feeding in summer and fall because of the location of the krill aggregations [23,24]. The modeled total loss from a 5-m deep source at the main shipping noise frequency (50 Hz) clearly shows this channeling effect in this krillrich summer sound channel at intermediate depths (Fig. 6a, b and h). The profiles of the mean differences with the 286 m depth (Fig. 6h) indicate the relative importance of low-frequency trapping in the upper 2/3 of the water column compared to the bottom where our measurements were taken. Differences can reach 35 dB at ranges where the acoustic rays converge (Fig. 6b). A whale feeding in the sound channel can be exposed to low-frequency shipping noise levels 15–22 dB higher than what we have measured at 286 m (Fig. 6h). For higher frequencies, large positive and negative differences are observed compared with our measurements depending on depth and range, because of the ray-concentrated propagation (Fig. 6d and f). The mean is an increase of 10 dB in

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plishing their vital functions is still largely unknown [20]. Modeling based on observations made in different environments have shown that it has the potential of limiting their communication space to only a fraction of what it would be under natural conditions [9,21], assuming that the animals cannot entirely compensate by modifying their calling pattern and SL. Such modeling of noise levels and exposure probability [26] can help assessing the risk that chronic high anthropogenic noise levels reduce the quality of marine mammal habitats, eventually subtracting vital spaces through habitat desertion or generating permanent damage to the acoustic apparatus (e.g. [10]). Criteria for assessing the risk of impacts of such chronic noise at individual and populations levels are still to be developed. In all cases however, the models must be fed with adequate high-resolution noise observations in frequented habitats over the relevant periods of occupation, including the frequency of occurrence of low noise conditions. Such information is presently uncommon for marine mammal habitats. This paper is a first contribution to fill this gap for an important habitat of Northwest Atlantic whales in eastern Canada. 5. Conclusion Results indicate that the ambient noise on a medium-traffic (20 merchant ships per day) continental seaway such as the St. Lawrence Seaway is high and exceeds 1960s reference levels for heavy traffic in oceans 50% of the time. The envelope from the lowest ambient noise level to the highest noise level spans over 50 dB. Given the noise footprints of the ships and their succession rate along the seaway, there is very little time for low noise levels without direct ship influence. Modeling indicates that noise is higher in the upper half of the 300-m deep water column, and channeled at intermediate depth during summer where the krill is known concentrate in daytime. Noise levels were also weighted for the different marine mammal groups using the M-weighting functions. Fig. 6. Total loss modeled for a 5-m deep source as function of range and depth (a, c and e), and difference relative to the hydrophone depth (b, d and f) for three frequencies: 50 Hz, 200 Hz and 900 Hz. Sound speed profile used for the normal mode propagation model (g). Mean correction to add to our noise level measurements to get estimates at different depths in Laurentian channel (h), assuming uniform distribution of ship sources over a 20 km recruitment radius (see text). Positive values indicate higher levels compared to the hydrophone depth.

the upper 2/3 of the water column relative to our measurements (Fig. 6h). Briefly, marine mammals frequenting the sound channel depths would experience 10–22 dB higher noise levels over the 20–900 Hz band compared to what we measured at depth, because of the trapping of the shipping noise band in the upper part of the water column. Bottom measurements do not represent the actual conditions in the sound channel, at the main baleen whale foraging depths, but they present the advantage of not being exposed to the strong tidal currents of the Estuary, generating mooring strumming and high flow noise affecting low-frequency measurements [9,22]. The U-shaped submarine valley of the Laurentian channel, with steep walls on both sides, likely favours basin reverberations, which adds to the direct contribution of the sources. The channel is narrowing by a factor of about two in the head region, 175 km upstream of the study area, where the baleen whale feeding ground of the Saguenay St. Lawrence Marine Park is found [24]. A more detailed study of shipping noise in this area is underway (Simard et al., in preparation) and will present a more complete picture of the shipping noise of the segment of the St. Lawrence Seaway cutting through the marine mammal habitats. How such high ambient noise affects marine mammals and their capacity to efficiently exploit their acoustic tools in accom-

Acknowledgements We thank the students and staff at ISMER-UQAR and Fisheries and Oceans Canada who have contributed to the success of the field work and data analysis. This research was sponsored by Fisheries and Oceans Canada, ISMER-UQAR research chair in applied underwater acoustics, NSERC Grant to Y.S., and benefited from the FQRNT Grant to Québec-Ocean. We tank the anonymous reviewers for their contribution to improve the manuscript. References [1] Southall BL, Bowles AE, Ellison WT, Finneran JJ, Gentry RL, Greene CRJ, et al. Marine mammal noise exposure criteria: initial scientific recommendations. Aquatic Mammal 2007;33(4):410–522. [2] Au WL, Hastings MC. Principles of marine bioacoustics. Springer; 2008. doi:10.1007/978-0-387-78365-9. [3] NRC. Ocean noise and marine mammals. Washington, DC: The National Academies Press; 2003. [4] NRC. Marine mammal populations and ocean noise: determining when noise causes biologically significant effects. Washington, DC: National Academies Press; 2005. [5] Richardson WJ, Greene Jr CR, Malme CI, Thomson DH. Marine mammals and noise. New York: Academic Press; 1995. [6] Andre´ M, Johansson T, Delory E, van der Schaar M. Cetacean biosonar and noise pollution. In: Proc. IEEE oceans 2005 – Europe 2, 2005. p. 1028–32. art. no. 1513199. [7] Weilgart LS. The impacts of anthropogenic ocean noise on cetaceans and implications for management. Can J Zool 2007;85:1091–116. [8] Hildebrand J. Impacts of anthropogenic sound. In: Reynolds JE et al., editors. Marine mammal research: conservation beyond crisis. Baltimore: Johns Hopkins University Press; 2005. p. 101–24. [9] Simard Y, Roy N, Gervaise C. Passive acoustic detection and localization of whales: effects of shipping noise in Saguenay–St. Lawrence Marine Park. J Acoust Soc Am 2008;123:4109–17.

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