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Scientist reports a rare piece of good news for lagoon

STORY BY STEVEN M. THOMAS, (Week of August 8, 2013)

Notwithstanding the problems of the Indian River Lagoon, spotted sea trout are continuing to spawn in some of their traditional breeding locations between the 17th Street Bridge and the Fort Pierce Inlet, according to Dr. Grant Gilmore, senior scientist with Estuarine, Coastal and Ocean Science, a Vero Beach ecology consulting firm.

"It is really good news for the lagoon that the trout are still there," Gilmore says.

On a nighttime excursion last week, Gilmore used an underwater microphone called a hydrophone to listen in at sea trout spawning spots he has charted in the past. Male trout make a range of resonant sounds to attract females during breeding season and Gilmore says that based on what he heard, there was plenty of action going on below the waterline.

“The section of the river near the Moorings and Harbor Branch is the most important trout spawning area south of Kennedy Space Center,” Gilmore says. “With all the recent problems in the lagoon, I was concerned we might not still have breeding activity.”

The problems Gilmore refers to include a series of unprecedented algae blooms in 2011 and 2012 that led to the loss of most seagrass and greatly reduced fish populations north of the 17th Street Bridge. The seagrass die-off and other as yet unexplained factors have contributed to mass dolphin, manatee and pelican deaths this year.

Seagrass continues to flourish south of the 17th Street Bridge because of the flushing action of sea water in and out of the Fort Pierce inlet, according to scientists.

With the underwater meadows in good condition, Gilmore was hopeful trout would still be present but he could not be sure until he lowered his hydrophone into the water shortly before sunset on the last Tuesday in July and heard the racket the fish were making.

When the speaker in his lap exploded with loud clicks, chirps and a sound like galloping horses, Gilmore got a big smile on his face, same as everyone else in the 18-foot Hell’s Bay skiff piloted by Captain Brendan Burke.

“Those are spotted sea trout,” Gilmore told the others, none of whom had ever heard the sounds before.

Gilmore, one of two founding scientists at Harbor Branch Oceanographic Institute, has been using hydrophones to study fish behavior in the lagoon since the 1970s. During the 1990s and early 2000s, he ran research programs that charted breeding locations for trout and other sound-producing fish along most of the lagoon’s 150-mile length.

Male spotted sea trout have a special sonic muscle that enables them to vibrate their hollow swim bladders to generate sound, sometimes making enough noise to be heard in an open boat without a hydrophone.

The sounds attract females that release eggs into the water at the same time males release sperm to fertilize the eggs in a group spawning process known as oviparity.

Gilmore says other scientists at Harbor Branch thought he was losing it when he first started listening to fish reproduce, but his research has been fruitful. “Until we used the hydrophones no one had any idea where or when sea trout spawned,” he says. “They did not know the time of year or the time of day or night.”

The information is important because it helps scientists understand the fish’s reproductive biology and enables them to correlate water conditions such as temperature, salinity and turbidity with reproductive success. Those correlations in turn help biologists and conservation officials protect breeding stocks and spawning locations.

One of the things Gilmore discovered is that the level of sound at breeding sites is directly related to the number of eggs in the water column. When the males get loud it means a lot of little fish embryos are being made. Which is why he was so pleased by the clatter that burst from the speakers on his hydrophone at each location he tested in the lagoon last week.

The spotted sea trout is a key species in the Indian River Lagoon, in part because it is a favorite food of bottlenose dolphins. The trout rely on the seagrass beds for shelter and food, and the dolphins rely on the fish for much of their diet. “Basically, no seagrass, no dolphins,” Gilmore says.

Lange Sykes, president of the Treasure Coast chapter of the Coastal Conservation Association, invited Indian River County Commissioner Bob Solari along on last week’s mini-expedition.

“I believe it is prudent for citizens to work with both county and city elected officials to help inform them and offer additional perspective on important local issues,” he says.