Recording spotted sea trout's mating calls in Indian River Lagoon focus of spawning study

Drop a microphone — waterproof, of course — into the Indian River Lagoon after sunset from about April to September and the snap, crackle, pop can be deafening.

It’s the sound of dozens of fish species calling their mates to spawn, under the din made by other lagoon denizens such as silver perch and snapping shrimp.

Spotted sea trout, however, is the one that most interests fish researcher Grant Gilmore, who has been studying them for about 40 years.

His latest study, a three-year endeavor that began in April, involves nighttime runs along the lagoon with several stops to record the trout's mating calls and log information about the time, location and water conditions.

The Coastal Conservation Association Florida’s Treasure Coast chapter funded the study, and about a dozen of its members assist Gilmore in finding spawning fish and recording their nightly grunts, groans and frog-like croaks. Gilmore has developed his own dictionary of descriptions of the sounds, like “bullfrog” and “buffalo herd.”

There are about 100 spawning hotspots from Jupiter to Ponce inlets, but the liveliest is in the Queens Cove area, and Gilmore speculates that’s thanks to the Fort Pierce Inlet flushing the lagoon with salty ocean water. Below 15 parts per thousand, the trout won’t call their mates, Gilmore said.

The one place you won’t find a single spawning sea trout — since about 2013 — is Martin County, which Gilmore attributes to Lake Okeechobee discharges causing increasingly worse toxic algae blooms.

So why didn’t the trout disappear after severe discharge events in 1998 and 2005?

“We didn’t have microcystis,” Gilmore said. “Now we have the green slime.”

When the Army Corps of Engineers discharges excess Lake Okeechobee water into the St. Lucie River, the water sometimes contains blue-green algae that sometimes contains the toxin microcystin. Blooms are fed by nutrients — mostly nitrogen and phosphorus —from farms, lawns, leaky septic tanks and others polluters.

Spawning sites are sacred sites

Elsewhere in the lagoon, the eggs are threatened by poor water quality from similar pollution carried by stormwater runoff, as the eggs can’t evade dirty water like adults.

When they spawn, the male and female rise to the surface in unison and lay/fertilize about a million eggs a week, which hatch in about a day. Only a very small amount survive to adulthood, but Gilmore couldn’t put a percentage on it.

“Spawning sites are sacred sites,” Gilmore said, adding one of his biggest concerns is the overuse of lawn fertilizer and chemicals such as RoundUp. When asked about his biggest takeaway after 40 years of research, he said, “Just advocate to everybody that you don’t have to do this. The grass is going to grow. It’s going to be green.”

Other tidbits from Gilmore’s research into trout spawning include:

• Each fish species has its own frequency
• Trout have three distinct sounds
• Individual trout always spawn in their same favored spot
• They don’t spawn in sea grass, but on the clear bottom
• They don’t take bait when they spawn
• Activity peaks one hour after sunset
• 23.5 degrees is their optimal water temperature
• They prefer deeper water because their calls travel better
• Vibrations, such as trains, make them stop calling their mates

Gilmore's last recording trip was on Sept. 25, but he'll be collecting data again once the spawning season resumes in April.