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Biology

Keep it Down!: Eels Having Problems Avoiding Predators in Noisy Waters

Citation: Simpson, S. D., Purser, J., & Radford, A. N. (2014). Anthropogenic noise compromises antipredator behaviour in European eels. Global Change Biology, 1–8. doi:10.1111/gcb.12685 

Background

Lots of different human activities create noise in the ocean – shipping, boating, oil drilling, and commercial fishing, to name a few – and until recently, no one really thought that would be a problem for animals.  However, now scientists have recognized that these noises are causing new and different challenges to animals that rely on senses other than sight to survive.   Most dramatically, animals could die from a major sound – for example, from an oil drill – but usually, other aspects of an animal’s life are impacted.

Anthropogenic noise could distract the animal from major tasks, like breeding; could act as a stressor, increasing the animal’s metabolic rate; or could hide crucial sound cues from the environment, like the sound of an approaching predator. That last aspect was explored in this study: the researchers were looking to see how this noisy ocean was affecting juvenile European eels (Anguilla anguilla).   They hypothesized that the eels would have a higher metabolic rate (using more energy and indicating a high level of stress), as well as an inability to hear an oncoming predator.

European eel – Anguilla anguilla

Methods

The researchers held the eels in natural looking aquariums, containing artificial plants and PVC tubes for the eels to take shelter.  They made underwater recordings from three major harbors in Britain, places where eels are known to live.  Harbor noise was recorded in each harbor at two different levels to play during the lab experiments – one ambient, which is the way the harbor sounds if no ships are going by, and the other with noise generated by large, slow moving ships. There were two major things tested: predator avoidance reaction times with and without noise, as well as stress levels with and without noise.

In each predation experiment, they simulated an ambush type predator (the most common type of predator for juvenile eels) with a mechanical arm, swinging overhead of the eel’s burrow.  The eels often identify predators using the shadow created by the predator’s movement, so this was a reasonable proxy.  They then played either the ambient or the noisy harbor and monitored how the eel reacted to the “predator,” as well as how long that reaction time was.

Then, in each stress experiment, the researchers measured the breathing rate of each eel with and without noise.  They did that by looking at the operculum of the fish (the bony structure protecting the gills) and counting how many times it went up and down in a minute. This is analogous to measuring the chest rises of a person.

They also measured the metabolic rate of the eels by placing them in an airtight container for two minutes while the noises were playing, taking water samples and measuring the dissolved oxygen content before and after.

Findings

In the predation experiment, the researchers found that the eels were 50% less likely to exhibit startled behavior when moving ship noise was played in their tank.  Additionally, the eels that were startled reacted more slowly (25% slower) in the noisy trials than in the control trials.

The first graph shows the eels' reaction to a predator - would they exhibit the "startle" response, or would they not? In the control trails, they did, meaning they would more likely escape the predator. In the additional noise trial, they did not always show that startle response, meaning they would be less likely to escape the predator. The second graph shows their reaction time to exhibit that startle response, if they did at all. The eels in the noisy harbor simulation took longer to startle than the control treatment.

The first graph shows the eels’ reaction to a predator – would they exhibit the “startle” response, or would they not? In the control trails, they did, meaning they would more likely escape the predator. In the additional noise trial, they did not always show that startle response, meaning they would be less likely to escape the predator. The second graph shows their reaction time to exhibit that startle response, if they did at all. The eels in the noisy harbor simulation took longer to startle than the control treatment.

For the stress experiment, the researchers did not see a difference in opercular beat rate (respiration rate) between the noisy and control sounds. However, if the sound was switched mid-trial from control to noisy, the eels increased their breathing rates significantly.  Also, the oxygen consumption rate measured was significantly higher in the noisy trials than in the control trials.

The first graph represents the opercular beat rate during the control (first column) and the noisy harbor (second column). It went up, indicating more stress, in the noisy harbor. The second graph represents the change in oxygen of the eel's tank. In the noisy harbor treatment (second column), the eel consumed more of the oxygen, causing the total oxygen in the tank to decrease at a faster rate.

The first graph represents the opercular beat rate during the control (first column) and the noisy harbor (second column). It went up, indicating more stress, in the noisy harbor. The second graph represents the change in oxygen of the eel’s tank. In the noisy harbor treatment (second column), the eel consumed more of the oxygen, causing the total oxygen in the tank to decrease at a faster rate.

Significance

The results of the predation experiment suggest that these juvenile eels will be more likely to be eaten by predators in a noisy harbor than in a quieter harbor.  If they don’t exhibit the startle behavior or do so slower, then they won’t escape the predator in time.  Adding that to the results of the stress experiment, it seems that these juvenile eels are more stressed in that noisy harbor environment.  The combined effects of metabolic stress and reduced predator avoidance don’t bode well for these juvenile eels, which in turn doesn’t bode well for the population of eels as a whole.  These eels are already critically endangered, so keeping it down in the harbors of Europe may help their population creep back up to where it used to be.

 

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