Underwater sound pollution leaves juvenile European eels vulnerable to predators
Many of us know about the familiar sources of water pollution: fertilisers running off agricultural fields, sewage leaking from underground pipes and oil and fuel leaking from boats, amongst many others. But what if the pollutants and stresses on aquatic environments weren’t chemical and visible, but sonic and audible? How might noise pollution affect underwater life, and how might we manage it? How, in fact, in a crowded, noisy world do we even define what noise pollution might be?
A recent study published by Stephen Simpson and colleagues at the Universities of Exeter and Bristol in England investigated how the noise made by ships affects the behaviour of juvenile European eels. They found that underwater sound pollution significantly affects the behaviour of juvenile eels in ‘life or death’ scenarios when ambushed or pursued by a predator. Their findings suggest that sound may need to be increasingly taken into account when assessing the multiple pollutants and stressors that aquatic life is exposed to, both in oceanic and freshwater ecosystems.
The European eel’s life-story is fascinating, and (perhaps surprisingly for such a well-studied species) still has an element of scientific uncertainty. Eels spawn in the Sargasso Sea in the Western Atlantic Ocean, and larvae are carried along ocean currents back to rivers in Western Europe. When the larvae approach river estuaries they metamorphosise into ‘glass eels’ with almost transparent bodies measuring around 10cm in length. The glass eels go through subsequent growth cycles into elvers and finally adult eels as they migrate upstream, sometimes living for over 20 years in rivers and lakes (they have been known to travel across wet ground like snakes in order to find suitable habitat) before migrating downstream and out to the Sargasso Sea to begin the cycle again.
Unfortunately, European eel populations are critically endangered. Studies by the Zoological Society of London suggest that eel numbers in the River Thames have dropped by around 95% in the last 30 years, due to a combination of overfishing (particularly at the glass eel stage – these are a delicacy), habitat loss, barriers to migration (such as weirs), chemical pollution (particularly by this chemical) and climate-related shifts in Atlantic ocean currents. In short, eel populations are struggling, and it is important to understand what factors are threatening the species, and how they might be managed. And in this context, the study by Simpson and colleagues reminds us to consider sound as an important, if not always considered, source of aquatic pollution that may affect their populations.
In their study conducted, glass eels were collected in the River Severn and transferred to laboratory aquariums. There, the eels were exposed to recordings of large ferries, tankers and container ships moving around three UK harbours, along with ambient recordings of the harbours with no ship noise, which acted as a control, both in laboratory and open-water conditions. The recordings were made with a special microphone known as a hydrophone, which is dropped under the water’s surface and records the otherwise inaudible (to humans, at least) underwater soundscape.
A hydrophone recording of a dredger boat at the confluence of the River Lea and River Thames in East London. Taken as part of the Surface Tension project. This recording is for reference to show how loud underwater sound pollution can be, and was not used in the study by Simpson and colleagues.
The glass eels in the laboratory study were then subjected to two simulated ‘predator attacks’. In the first ‘ambush’ simulation, a single eel was acclimatised to a new tank with a ‘predator window’ where a model fish on a pendulum arm was swung. When the eel passed the small glass window, the model predator was swung (in an admirable adherence to control conditions, the researcher doing the ‘swinging’ listened to loud music on headphones so as not to know which soundscape was being played to the eel). In the second ‘pursuit’ simulation, eels were chased with a handnet through an experimental tank arranged as a maze with Perspex blocks. In both simulations, the response of the eels to ‘predation’ was carefully noted, under both ship noise and ambient soundscape conditions.
The results were significant: when the ship noise was played, eels were 50% less likely to startle to an ‘ambush’ predator compared to the ambient, control soundscape, and when they did startle, this reaction was 25% slower. Similarly, eels in the ‘pursuit’ simulation were caught more than twice as quickly when exposed to the ship noise soundscape. Additionally, the eels exposed to ship noise altered their spatial behaviour and movement, and heightened their stress (as observed by the ventilation and metabolic rate).
Lead author Dr Steve Simpson, Senior Lecturer in Marine Biology & Global Change at the University of Exeter, said: “Our findings demonstrate that acute acoustic events, such as the noise of a passing ship, may have serious impacts on animals with direct consequences for life-or-death behavioural responses. If these impacts affect whole populations then the endangered eel, which has seen a 90 per cent crash in abundance over the past 20 years due to climate change, may have one more problem to deal with as they cross busy coastal areas.”
Co-author Dr Andy Radford, Reader in Behavioural Ecology at the University of Bristol, outlined that: “The fact that eels were affected physiologically and spatially suggests that other important functions may also be affected. We focused on anti-predator responses as, unlike impacts on movement or feeding, there is no way to compensate for being eaten after the disturbance goes away.”
The findings remind us that documenting the multiple stressors that affect freshwater life is not a simple process. Here, the pollutants affecting the eels behaviour and possible survival rates are not chemical but sonic. Sound pollution is a commonly observed problem in oceanic environments, but less so in freshwaters. So the question here is: how can aquatic sound pollution be monitored and managed, if at all?
The management of anthropogenic noise is already included in the US National Environment Policy Act and the European Commission Marine Strategy Framework Directive, and as a permanent item on the International Maritime Organisation Marine Environmental Protection Committee agenda, but given the thousands of busy, interconnected shipping lanes that criss-cross the world, how effective can these policies be? And given that the findings of this study may be replicated in other fish species, we might ask what about the management of sound pollution in freshwater environments?
This leads to another complication in understanding the full suite of multiple stressors impacting eel populations: spatial and temporal scale. In other words, the eels move through different environments (oceanic, estuarine, freshwater) at different points in their life cycles, covering thousands of miles from birth to death. Sound pollution stressors that affect freshwater populations of eels may occur hundreds, or even thousands, of miles away in the ocean or in river estuaries. These are dynamic populations with complex life cycles affected by different stressors at each stage of growth. So the second issue here is how to foster co-operative, interlinked management strategies for such migratory species, which help mitigate and manage the effects of stressors in oceanic, estuarine and freshwater environments?