Scientists Track Florida Redfish

Dr. Sue Lowerre-Barbieri and team from Florida’s Fish & Wildlife Research Institute implant acoustic tags in adult red drum to determine habitat use and site fidelity in association with reproduction as part of a three-year study to evaluate red drum spawning stock size and structure in the eastern Gulf of Mexico.

The red drum (Sciaenops ocellatus) fishery is one of the largest and most popular in the state of Florida. Whether it consists of one stock or subpopulations more strongly affected by local fishing pressure is a persistent question for resource managers. To begin to address it, there is a need to better understand reproductive behavior, such as the number and spatial distribution of spawning aggregations and movement to and from these aggregations. Of particular interest is whether red drum exhibit spawning site fidelity by returning consistently to some specific location, such as where they were spawned (known as natal homing).

Large aggregations of red drum form every fall in nearshore Gulf waters to spawn (Figure 1). Spawning typically starts in mid-September and continues for about two months. Since 2009, biologists with the FWC’s Fish and Wildlife Research Institute have used aerial surveys to assess the distribution and number of spawning aggregations, as well as target aggregations for acoustic tagging.

Figure 2.
On an acoustic tagging trip, biologists in the air guide colleagues on the water to an aggregation where they catch fish with a rod and reel (Figure 2). Within three to four minutes of bringing the fish on board, biologists

make a small, shallow incision between the pectoral fins along the midline of the belly, taking care to avoid the egg-laden ovaries, which lie close to the body cavity wall (Figure 3a).
insert an acoustic tag into the abdomen (Figure 3b) and close the wound with one or two stitches made of absorbable material (Figure 3c).
take length measurements and insert a dart tag behind the dorsal fin before returning the fish to the water.

Figure 3a. A small incision allows insertion of an acoustic transmitter tag.

Figure 3b. The tag slides into the belly of the fish.

Figure 3c. An absorbable stitch or two closes the incision.

Figure 4. The study area in which biologists conduct aerial surveys and deploy underwater receivers (shown in red) extends from Pinellas County to Sarasota County, roughly 0-7 miles west of the coastline.

During the tagging process, biologists take ovarian samples from females to determine whether fish were about to spawn that night or spawned the day before. A catheter (a small tube with a syringe at the end) is used to extract a few eggs. Later in the laboratory, biologists process the samples to make histology slides, which they examine under a microscope.

One of two types of acoustic tags, continuous or coded, is implanted into a fish. Continuous tags are used to follow an aggregation when reflected sun on the water would make an aggregation difficult to see from the air. Continuous tags constantly emit a signal, allowing biologists to immediately follow and track a released fish with a handheld receiver. The goal–dependent on the tagged fish remaining with the aggregation–is to track the aggregation to its spawning location around dusk.

To evaluate longer-term movements, biologists use coded tags that are passively detected by underwater receivers. Depending on research goals, biologists place 20 to 40 receivers at historic and recent aggregation locations off the mouth of Tampa Bay (Figure 4) to determine whether aggregations return to the same sites within the same spawning season and from year to year. Data from these fixed stations indicates site fidelity, while tracking data may reveal spawning locations. Although both tag types provide movement information, they differ in that continuous tags show both movement and direction over the short term (as long as biologists are able to track it), while coded tags show movement over the long term (up to two years of battery life) but without directionality.

Since 2009, biologists have worked on this methodology to test and improve sampling design as well as collect preliminary data. Results indicate that red drum survive the tag implantation, return to previously identified aggregation sites, and can move up to 16 kilometers (10 miles) a day. Using data collected from these pilot studies, researchers are developing larger-scale studies to estimate red drum spawning stock abundance and assess spawning site fidelity and potential mixing along the Florida west coast.