Mapping Spawning Habitat of Spotted Seatrout in Tampa Bay

Biologists map data from passive acoustic surveys throughout Tampa Bay to better understand spawning site preferences of spotted seatrout.

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.

spotted seatrout
Captured spotted seatrout in the back of a research mullet skiff.


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.

netting fish
Traditional capture techniques involve catching and sacrificing fish.
Here, scientists remove a spotted seatrout from an entanglement net.
Using passive acoustics eliminates the need of sacrificing 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.

sonic muscle
A mature spotted seatrout produces spawning sounds by vibrating
sonic muscles lining the abdomen—a feature exclusive to males of the
species—against the swim bladder, an organ that helps regulate a fish’s
depth in the water column.


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. Select the following links to listen to the calls of spotted seatrout, sand seatrout, silver perch, red drum, and black drum:

 


Listen to the sound of
one spotted seatrout.

Listen to to the sound of
an aggregation of spotted seatrout.
 
Listen to the sound of
one sand seatrout.
 
Listen to the sound of
an aggregation of sand seatrout.
 
Listen to the sound of
one silver perch.
 
Listen to the sound of
an aggregation of silver perch.
 
Listen to the sound of
one red drum.
 
Listen to the sound of
an aggregation of red drum.
 
Listen to the sound of
one black drum.
 
Listen to the sound of
an aggregation of black drum.

Fish images © Diane Rome Peebles. Illustration provided for viewing purposes only.

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.

biologist dropping a hydrophone
A biologist lowers the hydrophone into the water, preparing to listen with
headphones or the mini amplifier on the console. A circular float keeps the
 hydrophone away from the boat to avoid noise from waves slapping the hull.


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

 


Listen to bottlenose dolphin, Tursiops truncatus.

Listen to snapping shrimp,
family Alpheidae. 
 
Listen to gulf toadfish, Opsanus beta.

Listen to cusk eels,
family Ophidiidae.

Listen to a passing freighter
about one mile away.


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.


Listen to Overlapping Aggregations of Spotted Seatrout,
Sand Seatrout, and Silver Perch.

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.

hydrophone survey map
By the size and color of the circles, the map indicates the number of
spotted seatrout detected and their estimated distance from the
 hydrophone, as described in the legend. Small black dots indicate
 sampling sites where the hydrophone detected no spotted seatrout sounds.


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

spotted seatrout spawning habitat
An aerial photograph of typical spotted seatrout spawning habitat.
Areas characterized by seagrass bordering a sharp depth change
are frequently used by Tampa Bay spotted seatrout for spawning.


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