Biologists implant acoustic tags in adult red drum to determine
habitat use and site fidelity in association with reproduction.
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
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
Figure 1. A spotter plane is essential in locating
aggregations of red drum
such as this one off Tampa Bay. Although
the aggregation is obvious from
700 feet in the air, these fish can be
difficult to spot from the water.
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
- 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
Figure 3c. An absorbable stitch or two closes the
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
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.
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.
To learn more about our telemetry studies, visit the Acoustic Telemetry