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Seagrass Meadows Shelter Fish in Silence
25 January 2013 3:25 pm
A tiny fish darts into a clump of seagrass, hiding from the beady eyes of seagulls and the jaws of bigger fish. But these underwater pastures may provide an additional advantage. By dissipating the chirps that enable hungry dolphins to capture food, the meadows may protect the fish with a cone of silence, new research suggests.
Seagrass provides fodder for grazers like sea turtles and manatees, and breeding grounds for many marine organisms at the bottom of the food chain. Because of their ecological importance, scientists have been trying to find ways of replacing seagrass meadows that are disappearing because of coastal development, pollution, and other threats. However, imitation meadows, which often contain plastic plants that resemble the real thing in color, leaf shape, and density, don't seem to support the same diversity and abundance of life, says Christopher Wilson, a marine ecologist at the University of Texas, Austin. One reason, he suspects, may be that plastic meadows don't have the same acoustic properties as real seagrass, making them less effective hiding places.
Unlike fake seagrass, living plants contain bubbles of gas and produce bubbles through photosynthesis that help scatter sound waves. To test the idea that these bubbles provide acoustic protection from dolphins, in 2011, Wilson and a small team of ecologists and engineers struck out in a boat for the shallow waters off the coast of Port Aransas, Texas. Using sensitive hydrophones, they measured how sound waves at the same high frequency as dolphin echolocation—about 500 Hz—dissipate across the beds. Compared with areas without seagrass, the meadows diminished the 500-Hz tones by up to 88%, the team reports this month in the journal Marine Ecology Progress Series, suggesting that it would indeed be harder for dolphins to detect the fish inside the meadows.
To determine whether certain kinds of seagrass provide better protection than others, the researchers compared beds of two different species: shoal grass, which thrives after disturbance to the seabed, and turtle grass, which prefers a more stable environment. Shoal grass didn't perform as well as turtle grass, suggesting that established beds are better at muffling sound. In the future, Wilson says, it might be possible to use acoustic sensors to monitor seagrass abundance and health.
In an area of research that's largely confined to the lab, the team's efforts to learn about an actual acoustic environment in which dolphins hunt is "commendable," says biosonar researcher Peter Madsen of Aarhus University in Denmark. However, he says that the researchers overestimated the dolphins' detection ranges in a cluttered environment like the seabed. By overlooking variables such as water depth and type of sediment, both of which dramatically affect the physics of sound transmission, the authors likely miscalculated how quickly acoustic signals degrade in seawater, he says.
Damon Gannon, a marine ecologist at Bowdoin College in Brunswick, Maine, agrees. "Since neither of these variables was included in the analysis, it is difficult to interpret the results," he says. Gannon also points out that dolphins rarely venture into water as shallow as that in which the experiment was conducted.
Despite these misgivings, both researchers see value in the study. Gannon says the study "helps our understanding of the complex nature of sound propagation in shallow estuaries." Madsen concedes that the idea that seagrass provides acoustic refuge for fish makes sense. "It's an interesting question," he says, "and I think their overall conclusion is correct."