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Mapping Spawning Habitat of Spotted Seatrout in Tampa Bay

Spotted seatrout (Cynoscion nebulosus) are the top catch for anglers in the Gulf of Mexico and one of the marine species most targeted in Florida waters, according to federal fisheries data. Given such popularity, it is important that researchers understand the fish’s reproductive biology and protect its spawning stocks. The FWC’s Fish and Wildlife Research Institute (FWRI) has been conducting several studies addressing these matters.

One such study, completed in 2005, focused on the spotted seatrout’s spawning habitat in the Tampa Bay area. Scientists examined where and when spawning occurred, as well as associated environmental parameters such as temperature, salinity, dissolved oxygen, and bottom type.

To conduct this research, scientists used passive acoustics, a noninvasive method based on fish sounds. This technique uses an underwater microphone called a hydrophone to record the spawning sounds of male spotted seatrout. The advantage of this technique over more traditional capture methods is that scientists can learn about where and when fish spawn without having to disrupt spawning or sacrifice fish.

Male spotted seatrout produce sounds by vibrating specialized sonic muscles against the swim bladder. Scientists believe that these males collectively call to aggregate (gather together) a large group of fish at specific spawning locations. In the typically murky bay waters, sound is an effective method for these fish to communicate their location. “Calling” or “drumming” begins around sunset and continues for 3 to 12 hours. Once the males and females are together, they release sperm and eggs into the water, where fertilization occurs externally.

Other species in the drum family (Sciaenidae) also spawn in or near Tampa Bay. Each has a specific sound that scientists can recognize, as one might identify a bird by its song. 

Fish in the drum family usually spawn in the evening, but various species do so at different times of the year. During the spotted seatrout spawning season, which typically starts in late March and ends in late September, scientists sampled two nights a week. They lowered the hydrophone into the waters in passes, off nearby beaches, and all over Tampa Bay. At each sampling site, scientists noted temperature, salinity, dissolved oxygen, depth, and bottom type while recording fish sounds on a digital audiotape recorder.

Fish are not the only animals making sounds underwater. Scientists using the hydrophone frequently detected bottlenose dolphins, snapping shrimp, toadfish, eels, and passing boats.

During the spotted seatrout spawning season, researchers also detected the calls of sand seatrout and silver perch. On rare occasions, spotted seatrout, sand seatrout, and silver perch were present together in large aggregations, causing incredibly loud sounds underwater.

When positioned directly over an aggregation of fish, scientists could hear drumming through the bottom of the boat without the aid of the hydrophone. On summer evenings, boaters on Tampa Bay area waters may be able to hear these remarkable sounds by putting an ear against the hull.

At the conclusion of the spotted seatrout study, scientists mapped locations where they detected drumming, estimating the number of fish and their distance from the hydrophone.

Collecting data over multiple spawning seasons, 2003-05, allowed the researchers to cover a range of spawning sites and document changes in site selection. Scientists examined temperature, salinity, dissolved oxygen, current, and bottom type to determine which factors may have influenced spawning site selection.

Spawning, as determined by the sounds produced by large aggregations, was detected throughout Tampa Bay except for Hillsborough Bay. Spawning was most common in the lower bay and the eastern part of the middle bay. Spawning aggregation sounds occurred in a wide range of habitats, including channels, seagrass, sand bottom, and beach areas; however, the majority of aggregation sounds occurred at sites with seagrass habitat. Proximity to shoreline, shallow depth, and high dissolved oxygen values were all characteristics of these spawning locations.

The spawning season of spotted seatrout overlaps those of sand seatrout and silver perch, whose aggregations occurred with even greater frequency. Aggregations of all three species rarely were detected simultaneously, though, as sand seatrout and silver perch use different habitats within Tampa Bay to spawn.

Download complete results of this research project: Using a Passive Acoustic Survey to Identify Spotted Seatrout Spawning Sites and Associated Habitat in Tampa Bay, Florida