In a recently released scientific paper, the James Cook University (JCU) researchers said that human activities have been discharging increasingly large amounts of sediments into tropical coastlines for the past few decades. The solid particles end up suspended in the water, blocking the influx of light and making the crystal clear waters cloudier and darker.
Study author Jodie Rummer became curious about the effect of this diminished visibility on the performance of fish. In particular, she wanted to find out if their ability to evade predators decreased or increased.
Her team of researchers acquired cinnamon clownfish (Amphiprion melanopus) that were one month old. They kept the fish in a tank filled with suspended sediment for a whole week. During this time, they observed the response of the animals to sham predator attacks and compared it to earlier reactions in clearer waters. (Related: Sunscreens may be harming fish embryos.)
They found that clownfish living in turbid waters displayed faster reaction times compared to those that live in cleaner, clearer environments. The animals were also more successful at fleeing the simulated predator. It seemed that the loss of visibility caused the fish to act more nervously than usual.
Furthermore, the animals moved with much less energy during the times that they were searching for food. The fish also refused to go to open areas where they would be easily spotted by predators.
"But while the faster responses and more cautious foraging may increase survival rates in low-visibility environments where predators are present, there is a price to be paid," warned Rummer.
She said that the clownfish expended extra energy in order to evade potential predators in turbid waters. But doing so reduced the amount of remaining energy that the fish could use to grow, heal itself, and reproduce. So while an individual clownfish is more likely to survive in the short run, the population as a whole will be much less likely to survive due to reduced birthrates and slower, less successful recoveries from disease and injury.
"It's particularly bad for juvenile reef fishes, as survivorship is already quite low during this critical life-history stage," Rummer remarked.
Clownfish enjoy a symbiotic relationship with sea anemones. The two species protect each other from their respective predators. Clownfish even lay their eggs inside the stinging tentacles of anemones.
Young clownfish often move between different anemones for various reasons, such as their current home getting too crowded or becoming too small for them. However, murky water could make these journeys too dangerous for the juvenile fish.
Rummer said that high concentrations of suspended sediments caused clownfish to act more cautiously. Such wary behavior decreased their activity within the area where they lived, which could prevent them from feeding themselves properly. It could also prevent young fish from finding the right anemone for their needs, leaving them vulnerable to predators.
The results of the JCU study suggested that fish could be greatly affected by the seemingly simple issue of lowered visibility. Animals that believed they were always in danger would expend energy that was normally reserved for long-term survival processes. These animals would grow more slowly, remain smaller, and become more prone to diseases.
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