Historically, red drum were thought to move to ocean waters to spawn. Research in partnership with NASA explores whether estuarine spawning could be the prevailing strategy for certain populations.
The red drum (Sciaenops ocellatus) is a prized coastal sport fish in the southeastern United States and supports a valuable recreational fishery in Florida. Studies in many areas have concluded that while red drum are largely confined to estuaries as juveniles, most move to nearshore ocean waters as they mature. Spawning occurs each fall, often near tidal passes so that larvae can enter the estuarine seagrass and marsh nurseries where they will grow.
Evidence is accumulating that some adult red drum also spawn within the confines of certain estuaries. This behavior is particularly well documented in the Indian River Lagoon system (including Mosquito Lagoon and the Banana River) via the collection of spawning adults, eggs, and small larvae up to 56 miles (90 kilometers) inside ocean inlets. Traditional mark-recapture studies further suggest that some mature fish found here are year-round estuarine residents. It remains unclear whether estuarine spawning is the prevailing reproductive strategy locally or one adopted by a small percentage of mature fish. The answer to this question is important because estuarine spawning fish face a different set of management issues than their offshore counterparts.
To better understand the life history of Atlantic coast red drum, the Florida Fish and Wildlife Conservation Commission (FWC) has partnered with biologists at NASA's Kennedy Space Center to investigate the seasonal movement patterns of adult fish in the northern Indian River Lagoon system. Kennedy Space Center, which encompasses portions of Merritt Island National Wildlife Refuge and Canaveral National Seashore, serves as an ideal setting for this effort, as it harbors some of the healthiest seagrass and salt marsh in east Florida and supports a robust red drum recreational and guide fishery (Figure 1).
From May 2006 to June 2007, 44 adult red drum were collected from southern Mosquito Lagoon with trammel nets, longline, or hook-line (Figure 2). After capture, fish were transferred to an on-board tagging cooler and quickly sedated, then an acoustic transmitter (474-day minimum battery life) was inserted into the body cavity through a small incision. Fish were allowed to recover in an on-board flow-through tank, then were measured, weighed, marked with an external FWC dart tag, and released on site (Figures 3 and 4). The tags provided contact information for anglers to report recapture.
Red drum movements were tracked with an array of submerged acoustic receivers that can detect the presence of a tagged fish from 600 to 800 meters, or nearly half a mile away. Thirty-one of these listening stations were widely spaced throughout southern Mosquito Lagoon and the northern Indian River Lagoon proper. Three receivers were moored in Ponce Inlet at the northern end of Mosquito Lagoon to detect any red drum moving into the open Atlantic Ocean. Tag detections stored in each receiver were downloaded to a laptop computer every one to two months (Figure 5).
The receivers monitored red drum movements from May 2006 until September 2008, recording signals from individuals for an average of 271 days. Twelve red drum were regularly detected for a year or more; the longest tracking period was 654 days. From winter through early summer, most fish typically visited a few receivers nearest their release location, a strong indication of a trait known as site fidelity (Figure 6). Behavior changed markedly during fall spawning months as fish began to range widely throughout the estuary. The greatest movement, measured as the number of receivers that individual fish visited per month, occurred in September 2006 and again in September 2007 (Figure 7).
Despite the change in red drum behavior each fall, no singular pattern or direction of travel was apparent. Of the 34 fish tracked through a spawning season, nine were never detected leaving Mosquito Lagoon. Sixteen others made one or multiple round-trip excursions from Mosquito Lagoon into the northern Indian River Lagoon via Haulover Canal. Only nine red drum visited Ponce Inlet; seven of those were never detected again and most likely entered the Atlantic Ocean. Angler recaptures independently supported the notion that many red drum were long-term estuarine residents. When measured in July 2010, local anglers reported recapture of 18 of 44 tagged red drum (41%), occurring at intervals as early as eight days after tagging and release to as much as four years later. Except for one fish recaptured at Ponce Inlet, all tag returns came from southern Mosquito Lagoon.
Red drum spawning in Florida waters is thought to peak each September and October. The increased movement observed at this time is in part attributable to reproductive activity as fish locate mates and seek suitable spawning locations. That most tagged red drum remained within the Indian River Lagoon system through the 28-month study provides the strongest evidence to date that estuarine reproduction is an important life history strategy for the species in east-central Florida. The repeated short-duration movements of many fish into the northern Indian River Lagoon during the spawning season each fall suggest that this area serves a critical role in the life history of the species.
A combination of factors may explain why many adult red drum spawn deep within the estuary and do not move offshore with maturity, as they do elsewhere. For example, the brackish waters of the Indian River Lagoon system may aid in development of red drum eggs. The narrow widths and wide spacing of coastal inlets may constrain larval influx from nearshore waters, making estuarine spawning more advantageous. Finally, locally mild winter water temperatures may not compel adult red drum to move to deeper nearshore waters in winter, as they do in other portions of their range.
Questions for Further Study
Whereas this study provides insights into adult red drum behavior in the northern Indian River Lagoon, fish in other portions of the estuary may behave somewhat differently. To complicate matters, adult red drum inhabiting open Atlantic Ocean waters have proven difficult to locate and capture, so little is known regarding their abundance and distribution along Florida's Atlantic coast. As a result, many important life history questions remain unanswered: For example, do other important estuarine spawning sites exist, and do they remain stationary from year to year? Are maturing red drum found near ocean inlets more likely to migrate offshore? What fraction of juvenile red drum populating seagrass and marsh nurseries derive from estuarine versus nearshore spawning groups? Does this ratio vary substantially from year to year or between locations?
Given the tremendous ecological and economic value of red drum in Florida waters, efforts to further study the ecology and behavior via acoustic telemetry would be valuable.
To learn more about our telemetry studies, visit the Acoustic Telemetry Research section.
FWRI scientists collaborated on this study with the Kennedy Space Center Ecological Program-Innovative Health Applications. The following is a more comprehensive research report:
Reyier, E.A.; Lowers, R.H.; Scheidt, D.M.; Adams, D.H. 2011. Movement Patterns of Adult Red Drum, Sciaenops ocellatus, in shallow Florida lagoons as inferred through autonomous acoustic telemetry. Environmental Biology of Fishes, v. 90 no. 4, p. 343-360. Access Publication
Figure 1. Anglers sight-fish for red drum near Whale Tail Shoal in southern Mosquito Lagoon.
|Figure 2. A red drum rests in a tagging cradle after surgery, its incision visible below the pectoral fins.
||Figure 3. Researchers record measurements before releasing a tagged fish.
|Figure 4. A red drum is typically is ready for release 30 to 60 minutes after surgery.
||Figure 5. Tools of telemetry: an acoustic receiver, three battery-operated transmitters, and a field computer.
Figure 6. Mosquito Lagoon and northern Indian River Lagoon study area, indicating fish collection-and-release locations and sites of acoustic receivers, or listening stations. (map from Environmental Biology of Fishes (2011) 90:343-360)
Figure 7. Mean number (±1 SE) of receiver stations visited by tagged red drum during the study. Dashed lines indicate the July-October spawning season in east-central Florida.