In this post Erin Ashe discusses a new article from Esther Jones and colleagues ‘Seals and shipping: quantifying population risk and individual exposure to vessel noise

An exciting new paper (Jones et al. 2017) outlines a rigorous and widely applicable framework to predict ship noise levels in coastal waters, assess noise exposure for two seal species, and explicitly incorporate these results into risk assessments and marine spatial planning. The contribution of shipping to ocean ambient noise levels has increased by as much as 12 dB over the past few decades (Hildebrand 2009) and UK shipping lanes are among the busiest in the world. Marine mammals use sound to communicate, find prey, and attract mates. Ship noise can interfere with an individual’s ability to feed, avoid predators, and communicate with conspecifics, and repeated disturbance can carry energetic costs. As top predators, any noise-induced changes in distribution or feeding rates of marine mammals could carry significant implications for the broader dynamics of the marine ecosystem, but quantifying such effects is extremely difficult. This study was motivated, in part, by the paucity of information to assess risks of ship noise on UK’s seal populations. “There has been little research around the impacts that shipping may have on pinnipeds [seals and sea lions], and in particular how underwater noise may affect them. We realised that providing a framework to explicitly link potentially acoustically sensitive areas with received levels of underwater noise from vessels would be useful for spatial planning,” Esther Jones noted by email.

Synthesizing telemetry data from hundreds of seals, the authors illustrate how the method could identify important seal habitats that may require mitigation or that warrant prioritization for protection given their relative pristineness (e.g., Williams et al. 2015). Although this study has wide application as a methods paper, the results are equally compelling. The team found that grey and harbour seals have high spatial overlap with shipping traffic in many of the UK’s Special Areas of Conservation (SACs). This may raise conservation concerns for some populations, because harbour seal counts in east Scotland have decreased by over 90% over the last decade or so, whereas other population units appear stable.

First, grey and harbour seal usage maps (Jones et al. 2015), were overlaid with ship usage maps (MMO 2014) to generate co-occurrence maps describing the spatial overlap of seals and ships. Second, cumulative sound exposure levels with corresponding estimates of uncertainty for 28 individuals were modelled from using 2 years of telemetry data and Automatic Identification System (AIS) from all ships within a study area. Third, acoustic predictions were validated using sound field measurements.

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Jones notes that the study area was based on “varying rates of spatial overlap between seals and vessels, including high co-occurrence. We predicted acoustic exposure in this area and it was interesting to be able to quantify the levels of noise that seals are experiencing due to vessel noise. Importantly, we were also able to quantify the elevation of vessel noise over the ambient underwater noise.”

Jones et al. (2017) found that more than half (11 from 25) of Special Areas of Conservation for seals spanned areas of high co-occurrence between ships and seals. Risk of co-occurrence appeared higher for harbour seals than grey seals, due to the inshore distribution of harbour seals. Exposure risk to seals was highest within 50 km of the coast, near seal haul-outs, and these sites were dramatically noisier (28.3 dB re 1µPa2 s higher, on average) than ambient noise levels throughout the study area. There was no evidence from this study that any seals were exposed to ship noise levels high enough to cause permanent hearing damage, but some sites were sufficiently noisy that seals living there could experience Temporary Threshold Shifts in hearing ability.

The paper maps a clear direction for future research to understand the ecological impact of chronic noise exposure. As Dr Jones notes, “[a] key issue is that research has not yet been carried out to determine whether exposure to chronic underwater noise affects behaviour in pinnipeds in SACs. Underwater vocalisations play a role in reproduction through male-male completion or advertisement to females. Although our study did not find levels of noise associated with acute impacts, for populations of animals already under stress, such as harbour seals in some regions around the UK, elevated underwater noise could contribute to detrimental cumulative impacts.”

Dr Jones is interested in how anthropogenic noise may affect large-scale patterns of seal distribution and habitat use. “Research has not been undertaken to determine whether habitat use has changed over time, primarily due to a lack of seal movement data in comparable locations. A useful experiment would be to identify an area that is not currently acoustically sensitive but where the seal population is known to be increasing, and therefore which may become acoustically sensitive. Assessing how behaviour changes over time (whilst controlling for other effects such as density dependence), would give insight into the impacts of shipping on seal behaviour.”

Inshore SACs are designed to protect important breeding habitat for seals, but if offshore SACs are designated in future, additional habitat that provides other important functions to seals could be identified, says Jones. “Offshore SACs have not been designated for either species around the UK, and the importance of foraging areas may be considered within the context of offshore SAC designation. The framework we have set out in the paper would be a useful tool to inform offshore SAC designation.”

The authors stop short of making policy recommendations, but their methods offer an objective way for managers to standardize the way they interpret “Good Environmental Status” language under the Marine Strategy Framework Directive. Jones highlighted this application of the study’s methods. “We created a framework so that anthropogenic stressors such as shipping traffic could be incorporated into spatial planning, which has not been done before. Metrics for Good Environmental Status (GES) in the Marine Strategy Framework Directive (MSFD) are currently being determined so this framework has utility in informing those decisions.”

The paper shows nicely that, because shipping lanes are so well entrenched, chronic ocean noise can be incorporated explicitly into marine spatial planning and management plans for existing marine protected areas. The authors note that policy-makers must articulate how much risk (e.g., temporary threshold shifts) they are willing to tolerate, but once those allowable noise levels are set, frameworks such as this one will become invaluable ways of assessing whether those objectives are being met. The analytical framework also lends itself nicely to a transparent process to explore alternative noise mitigation measures that maximize benefits to endangered marine mammals while minimizing costs to industry. As Dr Jones points out, “Urbanisation of the marine environment is inevitably going to continue. To build on the conclusions of this study we need to begin assessing any behavioural changes of seals as a result of chronic exposure to underwater anthropogenic stressors, and understand the implications of those changes on individuals and ultimately on population dynamics.”

As for next steps, researchers at the University of St Andrews have begun deploying high resolution sound and movement tags (DTAGs) to investigate the total noise exposure of individual seals. These tags record the GPS position and behaviour of seals, along with the received sound level, for up to a month. The tags are being deployed in Danish and German waters to assess the relative contributions of shipping and construction noise versus natural ambient noise (e.g. from rain) to the sound fields experienced by seals.

References

Jones, E.L., McConnell, B.J., Smout, S., Hammond, P.S., Duck, C.D., Morris, C.D., Thompson, D., Russel, D.J.F., Vincent, C., Cronin, M., Sharples, R.J. & Matthiopoulos, J. (2015) Patterns of space use in sympatric marine colonial predators reveal scales of spatial partitioning. Marine Ecology Progress Series, 534, 235–249.

Jones, E.L., Hastie, G.D., Smout, S., Onoufriou, J., Merchant, N.D., Brookes, K.L., Thompson, D. (2017) Seals and shipping: quantifying population risk and individual exposure to vessel noise. Journal of Applied Ecology, DOI: 10.1111/1365-2664.12911

Hildebrand, J. (2009). Anthropogenic and natural sources of ambient noise in the ocean. Marine Ecology Progress Series, 395, 5–20. https://doi.org/10.3354/meps08353

MMO. (2014) Mapping UK shipping density and routes technical annex. Report to the Marine Mammal Organisation. Project number 1066.

Williams, R., Erbe, C., Ashe, E., Clark, C.W. (2015) Quiet (er) marine protected areas. Marine Pollution Bulletin, 100, 154-161.

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