Small Vessels

Nearly 2.4 billion people (about 40% of the world’s population) live within 100 km (60 miles) of the coast (United Nations, 2017). In conjunction with growth in coastal populations and infrastructure, the presence of small vessels has also risen. Small vessels include traditional, motor-powered recreational boats (“leisure boats” or “pleasure craft”). Rigid-hulled inflatable boats (RHIBs or “zodiacs”), marine tour boats, personal water craft (PWC or “jet skis”), sailboats, and canoes/kayaks are also considered small vessels. This section concerns sound production by small, motorized recreational vessels of lengths less than 7.9 m (26 ft), which tend to fall within Class A and Class I boats as defined by the National Oceanic and Atmospheric Administration (NOAA)’s Office of Marine and Aviation Operations (OMAO). Some sizes of fishing and whale watching boats, less than 19.5 m (64 ft) in total length, are also classified as “small vessels,” but are subject to different marine regulations and are not considered here.

Recreational boating is a major maritime activity and is growing steadily worldwide. Between 1980 and 2017, the number of registered recreational vessels in the United States increased by 1% per year, with the U.S. recreational boating industry valued at over $40 billion in 2019 (National Marine Manufacturers Association). Parts of Australia also saw an increase of 3% per year in the number of registered recreational vessels between 1999 and 2009 (New South Wales [NSW] Government, 2010). Small boat traffic on the Great Barrier Reef has risen steadily since the late 1980’s, and it is predicted that there will be half a million recreational motorboats using the inshore waters of the Reef by 2040, compared to 90,000 recreational boat registrations as of 2018 (Great Barrier Reef Marine Park Authority, 2014). Leisure boating is also significant in the European Union (EU), with over 48 million European citizens regularly participating in boating activities (EBI 2021). Much of this activity occurs in the Mediterranean, where more than 90% of vessels are considered recreational boats (Plan Blue, 2011). Managers calculate that the Mediterranean maritime tourism industry may be at overcapacity in many popular destinations.

The expansion of recreational boating impacts coastal environments in multiple ways. Effects include pollution, habitat disturbance, increased water turbidity, collision with marine animals, and increased underwater sound^[1]Carreño, A., & Lloret, J. (2021). Environmental impacts of increasing leisure boating activity in Mediterranean coastal waters. Ocean & Coastal Management, 209, 105693. https://doi.org/10.1016/j.ocecoaman.2021.105693.^[2]Lloret, J., Zaragoza, N., Caballero, D., & Riera, V. (2008). Impacts of recreational boating on the marine environment of Cap de Creus (Mediterranean Sea). Ocean & Coastal Management, 51(11), 749–754. https://doi.org/10.1016/j.ocecoaman.2008.07.001.. In coastal areas with extensive marine tourism, small vessels are a significant source of anthropogenic underwater sound. Most small vessels travel at speeds resulting in broadband, high frequency sounds between 1-10 kHz. Source levels are between 130 and 195 dB re 1 µPa @ 1 m^[3]Parsons, M. J. G., Erbe, C., Meekan, M. G., & Parsons, S. K. (2021). A review and meta-analysis of underwater noise radiated by small (<25 m length) vessels. Journal of Marine Science and Engineering, 9(8), 827. https://doi.org/10.3390/jmse9080827.

Vessel type, hull type, speed, engine type (inboard vs outboard), and the number of engines affect source levels and frequencies of sounds produced. Vessel speed has the greatest impact on both source level and frequency and is most likely related to increased propeller cavitation. Reducing vessel speed is an effective measure for reducing underwater sound levels from small boats. However, if vessels travel more slowly and therefore remain in an area for a longer period, this can result in higher cumulative sound exposure levels from animals being exposed to lower sound levels but for longer periods of time^[4]Parsons, M. J. G., Erbe, C., Meekan, M. G., & Parsons, S. K. (2021). A review and meta-analysis of underwater noise radiated by small (<25 m length) vessels. Journal of Marine Science and Engineering, 9(8), 827. https://doi.org/10.3390/jmse9080827.

Understanding the types of underwater sound produced by small vessels and the distribution of these vessels in time and space is important to coastal marine management. However, there is a lack in standardized measurements for small vessel sounds, and many coastal countries do not monitor small vessel traffic. For example, in the Hauraki Gulf, New Zealand, recreational boats were detected nearly every day of the year at five shallow, coastal habitats, including two Marine Protected Areas (MPAs) designated to protect marine life from anthropogenic stress^[5]Wilson, L., Pine, M. K., & Radford, C. A. (2022). Small recreational boats: A ubiquitous source of sound pollution in shallow coastal habitats. Marine Pollution Bulletin, 174, 113295. https://doi.org/10.1016/j.marpolbul.2021.113295. While commercial shipping traffic can be tracked via Automatic Identification System (AIS) software, this technology is not typically installed on smaller boats, which may not meet size requirements for AIS data transmission. Motorized, non-AIS vessels (mainly recreational vessels) were found to elevate ambient noise levels up to 55, 47, and 51 underwater dB in the third-octave bands at 0.125, 2, and 16 kHz respectively (Hermannsen et al. 2019). Noise levels and impact assessments may therefore be underestimated when small vessel traffic is neglected^[6]Hermannsen, L., Mikkelsen, L., Tougaard, J., Beedholm, K., Johnson, M., & Madsen, P. T. (2019). Recreational vessels without Automatic Identification System (AIS) dominate anthropogenic noise contributions to a shallow water soundscape. Scientific Reports, 9(1), 15477. https://doi.org/10.1038/s41598-019-51222-9.^[7]Cope, S., Hines, E., Bland, R., Davis, J. D., Tougher, B., & Zetterlind, V. (2021). Multi-sensor integration for an assessment of underwater radiated noise from common vessels in San Francisco Bay. The Journal of the Acoustical Society of America, 149(4), 2451–2464. https://doi.org/10.1121/10.0003963..

Potential Effects

Unlike the large amount of research studying responses to commercial vessels, data are limited on the potential effects to small vessels when considering different species, geographical location, and vessel type. Underwater sounds associated with small vessels have been shown to produce behavioral and physiological effects in some marine fishes and mammals, and effects may be cumulative, as some animals encounter numerous vessels each day. Determining the responses of marine animals to small vessel sounds is also challenging because of variability in species and populations across regions as well as a lack of baseline control physiological and behavioral data in the absence of boats. It is also often difficult to determine whether the physical presence of a vessel, the sound associated with the vessel, or both, caused an observed response.

Fishes

Small vessel activity in coastal habitats overlaps in time and space with regions important to many marine fishes such as spawning grounds and nursery areas. Moreover, small boats produce sounds that overlap in frequency with the hearing range of many fishes (typically < 1 kHz). Thus, underwater sound from recreational boats may affect production and detection of biologically important acoustic signals for fishes, such as those that relate to spawning, feeding, and predation.

In the May River estuary (South Carolina, USA), which provides important spawning habitat for a variety of fishes, the frequency range of underwater sounds produced by small vessels was found to overlap with the courtship sounds and chorusing of silver perch, black drum, oyster toadfish, spotted seatrout, and red drum^[8]Smott, S., Monczak, A., Miller, M. E., & Montie, E. W. (2018). Boat noise in an estuarine soundscape – A potential risk on the acoustic communication and reproduction of soniferous fish in the May River, South Carolina. Marine Pollution Bulletin, 133, 246–260. https://doi.org/10.1016/j.marpolbul.2018.05.016.. The peak frequency bands of small vessel sounds overlapped with the frequency band of mating calls of all fish species investigated. The risk was determined to be greatest for red drum, whose choruses occur in the late afternoon, overlapping with high levels of boat noise. Further, the only known spawning area for red drum in the May River estuary is at the river mouth, an area close to the intracoastal waterway. This area has the highest and most continuous level of boating activity in the estuary, and thus the highest amount of detected boat noise.

Several studies show fish species modify their acoustic behavior in the presence of motorboat sounds. Some fishes, such as the Lusitanian toadfish and the black drum, reduce their calling rates^[9]Ceraulo, M., Sal Moyano, M. P., Hidalgo, F. J., Bazterrica, M. C., Mazzola, S., Gavio, M. A., & Buscaino, G. (2021). Boat noise and black drum vocalizations in Mar Chiquita Coastal Lagoon (Argentina). Journal of Marine Science and Engineering, 9(1), 44. https://doi.org/10.3390/jmse9010044.^[10]González Correa, J. M., Bayle Sempere, J.-T., Juanes, F., Rountree, R., Ruíz, J. F., & Ramis, J. (2019). Recreational boat traffic effects on fish assemblages: First evidence of detrimental consequences at regulated mooring zones in sensitive marine areas detected by passive acoustics. Ocean & Coastal Management, 168, 22–34. https://doi.org/10.1016/j.ocecoaman.2018.10.027.. Others such as the brown meagre, increase their calling rates^[11]Picciulin, M., Sebastianutto, L., Codarin, A., Calcagno, G., & Ferrero, E. A. (2012). Brown meagre vocalization rate increases during repetitive boat noise exposures: A possible case of vocal compensation. The Journal of the Acoustical Society of America, 132(5), 3118–3124. https://doi.org/10.1121/1.4756928..

Much research on small vessel sounds and fishes has used playback studies of underwater motorboat sounds to investigate behavioral and physiological responses of reef fishes. In response to small vessel sounds, field-based studies show effects such as reduced startle and anti-predator responses; decreased parental-care; decreased feeding; and reduced “boldness”, the propensity of an organism to behave with confidence in the presence of a perceived risk^[12]Holmes, L. J., McWilliam, J., Ferrari, M. C. O., & McCormick, M. I. (2017). Juvenile damselfish are affected but desensitize to small motor boat noise. Journal of Experimental Marine Biology and Ecology, 494, 63–68. https://doi.org/10.1016/j.jembe.2017.05.009.^[13]McCloskey, K. P., Chapman, K. E., Chapuis, L., McCormick, M. I., Radford, A. N., & Simpson, S. D. (2020). Assessing and mitigating impacts of motorboat noise on nesting damselfish. Environmental Pollution, 266, 115376. https://doi.org/10.1016/j.envpol.2020.115376.^[14]McCormick, M. I., Allan, B. J. M., Harding, H., & Simpson, S. D. (2018). Boat noise impacts risk assessment in a coral reef fish but effects depend on engine type. Scientific Reports, 8(1), 3847. https://doi.org/10.1038/s41598-018-22104-3.^[15]Nedelec, S. L., Radford, A. N., Pearl, L., Nedelec, B., McCormick, M. I., Meekan, M. G., & Simpson, S. D. (2017). Motorboat noise impacts parental behaviour and offspring survival in a reef fish. Proceedings of the Royal Society B: Biological Sciences, 284(1856), 20170143. https://doi.org/10.1098/rspb.2017.0143.^[16]Simpson, S. D., Radford, A. N., Nedelec, S. L., Ferrari, M. C. O., Chivers, D. P., McCormick, M. I., & Meekan, M. G. (2016). Anthropogenic noise increases fish mortality by predation. Nature Communications, 7(1), 10544. https://doi.org/10.1038/ncomms10544.. Physiological changes include increased ventilation rates and higher stress hormone levels in the presence of motorboat noise^[17]Mills, S. C., Beldade, R., Henry, L., Laverty, D., Nedelec, S. L., Simpson, S. D., & Radford, A. N. (2020). Hormonal and behavioural effects of motorboat noise on wild coral reef fish. Environmental Pollution, 262, 114250. https://doi.org/10.1016/j.envpol.2020.114250.^[18]Nedelec, S. L., Mills, S. C., Lecchini, D., Nedelec, B., Simpson, S. D., & Radford, A. N. (2016). Repeated exposure to noise increases tolerance in a coral reef fish. Environmental Pollution, 216, 428–436. https://doi.org/10.1016/j.envpol.2016.05.058.^[19]Palma, J., Magalhães, M., Correia, M., & Andrade, J. P. (2019). Effects of anthropogenic noise as a source of acoustic stress in wild populations of Hippocampus guttulatus in the Ria Formosa, south Portugal. Aquatic Conservation: Marine and Freshwater Ecosystems, 29(5), 751–759. https://doi.org/10.1002/aqc.3056..

It is important to note that responses to small vessel sounds, and the duration of those responses, differ among fishes and will depend on factors such as vessel type, duration of exposure, time of day, fish species, and life stage. For example, underwater sound from small boats were found to impact the behavior of juvenile common damselfish but only in the short term. After one minute of exposure to underwater sound produced by a small motorboat operating 30-80 m from the reef site, scientists noted the fish moved less and hid more. However, after 20 minutes of continuous motorboat noise exposure, behavior returned to pre-exposure levels^[20]Holmes, L. J., McWilliam, J., Ferrari, M. C. O., & McCormick, M. I. (2017). Juvenile damselfish are affected but desensitize to small motor boat noise. Journal of Experimental Marine Biology and Ecology, 494, 63–68. https://doi.org/10.1016/j.jembe.2017.05.009.. By contrast, in a study on juvenile domino damselfish, changes in behavior and in stress hormone levels did not diminish after two days of repeated, playback exposure (one motorboat sound playback every 5 min during daylight hours), but did so after one and two weeks of exposure^[21]Nedelec, S. L., Mills, S. C., Lecchini, D., Nedelec, B., Simpson, S. D., & Radford, A. N. (2016). Repeated exposure to noise increases tolerance in a coral reef fish. Environmental Pollution, 216, 428–436. https://doi.org/10.1016/j.envpol.2016.05.058.. Some fishes, such as the slippery dick wrasse, showed no significant behavioral or physiological response to small vessel sounds^[22]Staaterman, E., Gallagher, A., Holder, P., Reid, C., Altieri, A., Ogburn, M., Rummer, J., & Cooke, S. (2020). Exposure to boat noise in the field yields minimal stress response in wild reef fish. Aquatic Biology, 29, 93–103. https://doi.org/10.3354/ab00728..

Marine Mammals

Much of the research on marine mammals and recreational boating has focused on small odontocetes, such as bottlenose dolphins. This species is often found in coastal habitats with extensive small vessel activity. Resident bottlenose dolphins in Sarasota, Florida, have been estimated to be exposed to a small vessel passing within 100 m approximately every six minutes during daylight hours^[23]Buckstaff, K. C. (2004). Effects of watercraft noise on the acoustic behavior of bottlenose dolphins, Tursiops truncatus, in Sarasota Bay, Florida. Marine Mammal Science, 20(4), 709–725. https://doi.org/10.1111/j.1748-7692.2004.tb01189.x.. During times of high boat traffic (e.g., weekends and summer months), exposure may be continuous. Behavioral responses to small vessels include changes in surface, swimming, and diving activities. Changes in acoustic behaviors include shifts in whistle frequency, amplitude, rate, and/or duration^[24]Guerra, M., Dawson, S., Brough, T., & Rayment, W. (2014). Effects of boats on the surface and acoustic behaviour of an endangered population of bottlenose dolphins. Endangered Species Research, 24(3), 221–236. https://doi.org/10.3354/esr00598.^[25]Heiler, J., Elwen, S. H., Kriesell, H. J., & Gridley, T. (2016). Changes in bottlenose dolphin whistle parameters related to vessel presence, surface behaviour and group composition. Animal Behaviour, 117, 167–177. https://doi.org/10.1016/j.anbehav.2016.04.014.^[26]May-Collado, L. J., & Quiñones-Lebrón, S. G. (2014). Dolphin changes in whistle structure with watercraft activity depends on their behavioral state. The Journal of the Acoustical Society of America, 135(4), EL193–EL198. https://doi.org/10.1121/1.4869255.. However, as noted above, it is difficult to discern whether the physical presence of boats, associated underwater sounds, or both, cause observed effects. Responses vary also based on vessel speed, type, and/or proximity, as well as animal behavioral state, group size, and calf presence.

Scientists and environmental managers are concerned that underwater sound associated with small vessels may interfere with dolphin communication signals. Small vessel sounds range up to 25 kHz, which overlap with biologically significant dolphin signals such as whistles used for individual recognition that are important to mother calf communication and group cohesion^[27]Haviland-Howell, G., Frankel, A. S., Powell, C. M., Bocconcelli, A., Herman, R. L., & Sayigh, L. S. (2007). Recreational boating traffic: A chronic source of anthropogenic noise in the Wilmington, North Carolina Intracoastal Waterway. The Journal of the Acoustical Society of America, 122(1), 151–160. https://doi.org/10.1121/1.2717766.. In addition to documented behavioral changes^[28]Nowacek, S. M., Wells, R. S., & Solow, A. R. (2001). Short-term effects of boat traffic on bottlenose dolphins,Tursiops truncatus, in Sarasota Bay, Florida. Marine Mammal Science, 17(4), 673–688. https://doi.org/10.1111/j.1748-7692.2001.tb01292.x.^[29]Pennino, M. G., Pérez Roda, M. A., Pierce, G. J., & Rotta, A. (2016). Effects of vessel traffic on relative abundance and behaviour of cetaceans: The case of the bottlenose dolphins in the Archipelago de La Maddalena, north-western Mediterranean sea. Hydrobiologia, 776(1), 237–248. https://doi.org/10.1007/s10750-016-2756-0.^[30]Pirotta, E., Merchant, N. D., Thompson, P. M., Barton, T. R., & Lusseau, D. (2015). Quantifying the effect of boat disturbance on bottlenose dolphin foraging activity. Biological Conservation, 181, 82–89. https://doi.org/10.1016/j.biocon.2014.11.003., dolphin groups in busy coastal bays and waterways temporarily change the production rate and/or frequency of their whistles in the presence of small vessels^[31]Buckstaff, K. C. (2004). Effects of watercraft noise on the acoustic behavior of bottlenose dolphins, Tursiops truncatus, in Sarasota Bay, Florida. Marine Mammal Science, 20(4), 709–725. https://doi.org/10.1111/j.1748-7692.2004.tb01189.x.^[32]Luís, A. R., Couchinho, M. N., & dos Santos, M. E. (2014). Changes in the acoustic behavior of resident bottlenose dolphins near operating vessels. Marine Mammal Science, 30(4), 1417–1426. https://doi.org/10.1111/mms.12125.^[33]Rako Gospić, N., & Picciulin, M. (2016). Changes in whistle structure of resident bottlenose dolphins in relation to underwater noise and boat traffic. Marine Pollution Bulletin, 105(1), 193–198. https://doi.org/10.1016/j.marpolbul.2016.02.030.. These temporary changes in sound production may be a means to compensate for communication in noisy waterways.

The presence of calves has also been observed to affect group response to small vessel sounds. For example, groups with mother-calf dolphin pairs in Doubtful Sound, New Zealand were significantly less cohesive and coordinated when tour boats were detectable underwater via hydrophones. Groups with calves also increased whistle rates during interactions and when boats were departing, while groups without calves became quieter^[35]Guerra, M., Dawson, S., Brough, T., & Rayment, W. (2014). Effects of boats on the surface and acoustic behaviour of an endangered population of bottlenose dolphins. Endangered Species Research, 24(3), 221–236. https://doi.org/10.3354/esr00598.. A similar response was noted for Indo-Pacific humpback dolphins that occur in a narrow channel at Amity Point, Australia^[36]Van Parijs, S. M., & Corkeron, P. J. (2001). Boat traffic affects the acoustic behaviour of Pacific humpback dolphins, Sousa chinensis. Journal of the Marine Biological Association of the United Kingdom, 81(3), 533–538. https://doi.org/10.1017/S0025315401004180.. Groups including mother-calf pairs showed an increase in whistles in response to transiting vessels within the channel and within 1.5 km from the group. Groups with no calves produced significantly fewer whistles. Whistles are often used by delphinids when socializing. When calves are present, there may be a greater need to maintain or reestablish acoustic contact via whistling; increased signaling may be necessary to compensate for higher noise levels associated with motor boat traffic.

Case Studies Resulting from the COVID-19 Pandemic

The COVID-19 pandemic provided a unique opportunity to investigate the potential effects of anthropogenic sound on underwater soundscapes. During the height of the pandemic, many countries entered into stages of “lockdown” in which movements within some areas were restricted, and commerce and international trade were substantially reduced. Changes in terrestrial and underwater soundscapes were observed in response to these periods of restricted human activity.

For example, New Zealand entered a very strict period of lockdown during March, 2020. All vessel activity in the Hauraki Gulf Marine Park, a 4,000 km2 area in the waters off and around Auckland, New Zealand, abruptly declined, with all recreational boating banned from the area for seven weeks. Commercial shipping continued but at a significantly reduced level. Acoustic recordings within the Gulf showed ambient sound levels in normally busy channels decreased rapidly^[39]Pine, M. K., Wilson, L., Jeffs, A. G., McWhinnie, L., Juanes, F., Scuderi, A., & Radford, C. A. (2021). A Gulf in lockdown: How an enforced ban on recreational vessels increased dolphin and fish communication ranges. Global Change Biology, 27(19), 4839–4848. https://doi.org/10.1111/gcb.15798.. Median sound pressure levels decreased by 8-10 dB on the first day of lockdown (March 26, 2020). After the first day of lockdown, in one channel in the Gulf, which is typically busy with small boats, underwater sounds associated with recreational boating were recorded initially 34% of the time and subsequently dropped to 8% after five days. Scientists calculate that these reductions in small vessel sounds significantly increased the communication range for bottlenose dolphins and bigeyes fish, the two species that were the focus of this study^[40]Pine, M. K., Wilson, L., Jeffs, A. G., McWhinnie, L., Juanes, F., Scuderi, A., & Radford, C. A. (2021). A Gulf in lockdown: How an enforced ban on recreational vessels increased dolphin and fish communication ranges. Global Change Biology, 27(19), 4839–4848. https://doi.org/10.1111/gcb.15798..

DOSITS Links

• Science of Sound > How does shipping affect ocean sound levels?
• Science of Sound > Sound Movement > Reflection
• Science of Sound > Sound Movement > Refraction
• Science of Sound > Sound Movement > Scattering
• Science of Sound > Sound Absorption
• Science of Sound > Advanced Topic > How does sound travel in shallow waters?
• Animals and Sound > How do marine fishes use underwater sound to communicate?
• Animals and Sound > Effects > Fishes > Behavioral Change
• Animals and Sound > Marine Mammal Communication
• Animals and Sound > Effects > Marine Mammals > Behavioral Change
• Animals and Sound > Anthropogenic Sound Sources > Commercial Vessel Traffic
• Audio Gallery > Ship
• Audio Gallery > Outboard motor
• Audio Gallery > Personal watercraft
• Audio Gallery > Red drum
• Audio Gallery > Black drum
• Audio Gallery > Lusitanian toadfish
• Audio Gallery > Bottlenose dolphin
• Audio Gallery > Indo-Pacific humpback dolphin

Additional Resources

• Plan Blue: Cruises and recreational boating in the Mediterranean.
• European Boating Industry (EBI), Facts and Figures
• Great Barrier Reef Marine Park Authority (GBRMPA). (2014). Great Barrier Reef Outlook Report.
• Maritime Queensland
• Matzner et al. 2010. Small vessel contribution to underwater noise. OCEANS 2010 MTS/IEEE SEATTLE. DOI: 10.1109/OCEANS.2010.5663818
• National Marine Manufacturers Association, Statistics
• NSW Government Maritime. (2010). NSW Boat Ownership and Storage: Growth Forecasts to 2026. Sydney, NSW: NSW Government Maritime
• US Department of Homeland Security. (2018). 2017 Recreational Boating Statistics (Report No. COMDTPUB P16754.31). Washington, DC, US Department of Homeland Security
• Vieira, M., Fonseca, P. J., Zuazu, A., Lopes, A. F., & Amorim, M. C. P. (2019). The effect of boat noise on calling activity in the Lusitanian Toadfish. 070007. https://doi.org/10.1121/2.0001213.