Red drum and the role of birthplace on maturity movements and spawning location

This article is a summary of the following publication:

Walters Burnsed S, Lowerre-Barbieri S, Bickford J, Hoerl Leone E. 2020. Recruitment and movement ecology of red drum Sciaenops ocellatus differs by natal estuary. Mar Ecol Prog Ser 633:181-196. https://doi.org/10.3354/meps13183

Does the site of where redfish are born affect when and where they spawn as adults?

Do individual redfish return to spawn in the coastal waters they were born - resulting in their offspring using the same estuary they used when young?

To begin answering these questions, FWRI research biologists with the Movement Ecology and Reproductive Resilience (MERR) laboratory, implanted acoustic transmitting tags into subadult (teenage) redfish in the neighboring estuaries of Tampa Bay and Charlotte Harbor, then monitored their movements with acoustic receivers for three years.

Their research was published in the January 2020 issue of scientific journal Marine Ecology Progress Series and is summarized below with the full content available through the FWRI library:

Recruitment and movement ecology of red drum Sciaenops ocellatus differs by natal estuary.

Who and what were we studying?

Red drum (Sciaenops ocellatus) live in different habitats depending on their age. Juvenile and subadult fish reside in estuaries while adult fish mainly live in coastal and offshore waters. In eastern Gulf of Mexico waters, red drum typically mature and become adults after they have grown out of the Florida recreational fishing slot (18-27” total length), generally corresponding to ages 3-5. When red drum recruit or leave the estuary to join the mature adult fish offshore, they typically remain offshore for the duration of their lifespan, up to age 30. Mature red drum make annual spawning migrations to coastal nearshore waters each fall, arriving in late August and staying through November. These large, mature red drum aggregate together in the hundreds to thousands as they move up and down the coast to feed and spawn within a few miles of the beaches and inlets (to read more on the adults, please see the article Red Drum Spawning Movements Off Tampa Bay). Each year, a new group of subadult red drum recruit from their estuarine nursery habitat to join the nearshore spawning aggregations. Two major estuaries along Florida’s Gulf coast, Tampa Bay and Charlotte Harbor, serve as nursery grounds for red drum and are ecologically and climatically similar systems. Because nearshore areas off both estuaries also serve as red drum spawning habitat, we had an opportunity to compare when red drum recruit out of each estuary to spawn. By catching and acoustically tagging 20 subadult red drum within each estuary and monitoring their movements for 3 years after tagging, we determined if a red drum’s estuary of origin affected 1. when fish joined the spawning population 2. where they spawned and 3. how they moved within the spawning grounds.

How did we do it?

Fish tagging

As red drum predictably aggregate within certain areas of each estuary during early fall, we wanted to collect fish to tag from these schools which were longer than Florida’s recreational maximum length limit (27” total length) to prevent these tagged fish from being harvested. All fish over the length limit were implanted with an acoustic tag during a small surgical procedure securing the tag inside the abdominal cavity. An external dart tag was affixed across the back of the fish to inform anglers to release the fish if captured and report their catch. We clipped off the second dorsal spine in order to determine the age of the fish and also assessed each fish for gender (male or female) before release. In both estuaries, recreational fishing guides were instrumental in helping us collect and tag fish. We thank Captains Rhett Morris and Tyler Gulau in Charlotte Harbor and Captain Jason Stock in Tampa Bay for their assistance. Fish in Tampa Bay were tagged over a series of four dates in fall 2012 between August 30 and October 10, while in Charlotte Harbor all were tagged on one day in September 2013.     

Acoustic monitoring

Each implanted tag emits a unique ultrasonic code approximately every 40 seconds for 3.5 years.  When a tagged fish swims within detection range of a compatible underwater or handheld hydrophone (~100-400 yards depending on bottom depth) that tag number is decoded and recorded along with the date and time. We monitored the Tampa Bay estuary for tagged fish using a weekly mobile survey technique with a handheld hydrophone. In Charlotte Harbor we deployed 2 underwater receivers, 1 at the tagging site and 1 near the mouth of an anticipated high-traffic area for fish within the estuary (Figure 1). We monitored the nearshore spawning areas off both Tampa Bay and Charlotte Harbor with an array of 50 underwater receivers, 33 off Tampa Bay and 17 off Charlotte Harbor (Figure 1) and thank recreational fishing guide Captain Jimmy Burnsed for sharing aggregation locations off Charlotte Harbor.

What did we find out?

Fish tagged in Tampa Bay and Charlotte Harbor were similar in size and age (Figure 2) but Charlotte Harbor had significantly more mature fish at the time of tagging (75%) compared to Tampa Bay (15%). Although Charlotte Harbor fish were more mature, they recruited out of the estuary to join the spawning adults much later than Tampa Bay fish. Fish tagged in Tampa Bay took, on average, 94 days to recruit while in Charlotte Harbor it took over triple that time, 309 days (Figure 2). Fourteen fish in Tampa Bay recruited out within the same fall as being tagged while in Charlotte Harbor only four fish shared this timing. Within both estuaries, the fish that did not recruit the fall they were tagged did so the following fall spawning season (Figure 3).

Once a fish recruited to the nearshore spawning habitat, its return to the estuary was rare as the majority of recruited fish (90%) appeared to remain offshore once that transition was made. Movements of tagged fish on the nearshore receivers during the first (when they were tagged) and second fall spawning seasons show distinct differences in how space is used between Tampa Bay and Charlotte Harbor fish (Figure 4).

We used a spatial analysis technique called network analysis to help us better understand how tagged fish movements compared between the two estuaries. Network analysis takes the actual movement data of the tagged fish on the nearshore receivers and produces visual linkages based on the frequencies and directionality of these movements compared to all the possible paths between receivers. It also gives us a numeric value for the amount of space used, called edge density, with a larger edge density value corresponding to greater space use. Figure 4 shows a sequence of maps with the results of the network analysis. During the first spawning season, Tampa Bay fish show more complicated movement compared to Charlotte Harbor, resulting in a higher value of edge density. Six of the 14 Tampa Bay fish that recruited out during the fall they were tagged also made long movements to reach receivers off Charlotte Harbor at the end of the spawning season (Figures 3 and 4). Conversely, the 4 Charlotte Harbor fish that recruited out the first spawning season only used a few receivers close to their exit point out of the Charlotte Harbor estuary. In Charlotte Harbor, the majority of fish stayed close to the tagging site until the second spawning season when the remaining 11 fish recruited to the nearshore. During the second spawning season in Tampa Bay, the final 2 fish recruited out and joined 6 tagged fish that returned from the previous year and together these 8 fish had similar movement patterns as the 14 recruits the year prior but with an increase in space use.

Movements of fish from both estuaries were most similar during the third spawning season. All 18 returning fish came back to spawn off the estuary they were born (9 in each estuary), exhibiting natal homing. Four of the Tampa Bay fish were also detected on the Charlotte Harbor receivers either as first or last detections within the spawning season, likely migratory transitional movements to or away from their natal Tampa Bay site. Only 1 fish from Charlotte Harbor visited Tampa Bay receivers and for only 2 days in the middle of the last spawning season. Although Tampa Bay fish initially showed greater space use during the first spawning season compared to Charlotte Harbor fish, as the number of recruited fish from Charlotte Harbor met that of Tampa Bay, fish from both estuaries displayed an increase in space use and movement over time and ended the last spawning season with similar movements.

Why is this study meaningful?

Red drum are an important recreational fishery in the state of Florida and are heavily targeted for sport and for dinner while they reside in the estuaries as subadults. By finding out when red drum transition to adult spawning habitat and the role of natal estuary on that movement as well as how it influences spawning site location can help improve conservation and management strategies.

Our study found that despite being the same age and size, fish tagged in Tampa Bay recruited out to spawn much earlier than fish in Charlotte Harbor. Given that the Charlotte Harbor fish had more advanced maturity than Tampa Bay fish, it was surprising that they remained within the estuary longer. The majority (85%) of Charlotte Harbor females were mature but it is unlikely they were spawning within the estuary because the low salinity (11ppt) at the tagging site would prevent successful hatching of eggs and larvae survival (high salinity of 30 ppt is optimal). This disconnect between maturity and movement to the nearshore spawning aggregations in Charlotte Harbor is puzzling and further study involving more fish than just 20 would help us understand if what we found was out of the ordinary or possibly a locational effect (typical of fish in this area). 

Delayed recruitment by Charlotte Harbor fish potentially increases their mortality rate as exposure to the recreational fishery is higher within the confines of the estuary compared to the open nearshore environment. Although fish tagged in this study were outside of the legal limits of a slot fish, the longer residence within the estuary increases the opportunity for catch and release fishing and the associated post-release stress or post release mortality (5.6%, Flaherty et al. 2013) coupled with illegal harvest as experienced by the residential adults in the Indian River Lagoon (Reyier et al. 2011). Professional guides in both estuaries have voiced concerns about the escalating fishing pressure on these subadult schools as the angler population has swelled and there has been a perceived decline of the number of fish and reduced cohesiveness of the remaining schools (DMFM 2016). Current Florida management for red drum divides the state into four regions: northern, southern, the Gulf coast, and the Atlantic coast (Chagaris et al. 2015). Smaller scale management units, such as by estuary, may be more biologically appropriate for red drum given the difference in recruitment timing but enforcement challenges at this small scale may prove to be impractical. Increased estuarine-time raises risk to fishing pressure, plus other local-scale effects such as freshwater inflow and water quality as they could influence the population dynamics of a fishery shown to have local production (Rooker et al. 2010). Although the latest state-level stock assessment did not raise concerns over management benchmarks of escapement rates (Chagaris et al. 2015), continued monitoring of the subadult and juvenile fish will be necessary to ensure that increased fishing pressure or ecosystem stressors (most recently, a protracted red tide bloom of Karenia brevis affecting both Tampa Bay and Charlotte Harbor in from September 2017 to January 2019, Weisberg et al. 2019) are not detrimental to recruitment.

This extended time within the Charlotte Harbor estuary and consequent longer exposure to the fishery may eventually result in a localized effect on the population. Our data showed that after recruiting to the adult population, fish returned to their natal estuary to spawn. If less red drum in Charlotte Harbor are able to escape the estuary to join the spawning aggregations off Charlotte Harbor due to fishing mortality (or other environmental pressures), the population would ultimately be affected by having less adults to spawn. In Tampa Bay, although fish in this study recruited more quickly from this estuary, all tagged fish were caught and released at the mouth of the estuary and further work to target fish in other areas of the estuary would help gauge if the estuarine mouth site is a primary or secondary source for recruits to the spawning population. Additional work to further understand connectivity between red drum subadult and adult populations will only improve the management conservation of this iconic species, especially in light of the increasing ecosystem challenges associated with coastal development as experienced by the central west coast of Florida.

Figure 1

The study site, including (A) nearshore receiver arrays, and (B) mobile hydrophone survey sites within the Tampa Bay estuary. (A) VR2W receivers were deployed at sites where red drum aggregations had been previously located off of Tampa Bay (triangles), with additional receivers (black circles) to ensure relatively even coverage. The arrow indicates range test location for the Vemco VR2W receivers. In nearshore Charlotte Harbor, the initial array (black circles) was based on a grid pattern, as there was no preexisting data on aggregations, and two receivers added within the nearshore array (triangles) in 2013 based on aggregation locations in 2012. Two receivers were deployed within the Charlotte Harbor estuary following subadult tagging in fall 2013. PR denotes the Peace River and CR denotes the Caloosahatchee River. Tagging sites within Charlotte Harbor and within Tampa Bay are denoted by a rectangle (B) Within the Tampa Bay estuary, 19 fixed sites were monitored in a mobile hydrophone survey; black circles correspond to sites where subadults were captured, tagged, and released and open circles where subadult schools had been spotted and/or captured during previous research.

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Figure 2

Recruitment timing as measured by the number of days between the first date an individual fish is detected on any nearshore receiver and the mean arrival date (September 8) of repeat adult spawners to the Tampa Bay nearshore array (Lowerre-Barbieri et al. 2019) according to sex and age at the time of tagging.  Only tagged fish that were detected in the nearshore arrays are included in this figure. (A) fish tagged in Tampa Bay (n = 16) and (B) fish tagged in Charlotte Harbor (n = 15). Sex is designated by color with females in pink and males in blue. Several instances are indicated when a pair (by n = 2) or triad (n = 3) of fish of the same sex and age were detected on the same day. Note, although the x-axis (horizontal) scale for the number of recruitment days is the same for both estuaries, the actual years referenced are offset with Tampa Bay (A) fish tagged in 2012 and Charlotte Harbor (B) fish in 2013.

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Figure 4

Network analysis during three consecutive reproductive periods (August-December) for fish tagged in Tampa Bay (top row, first reproductive period is 2012) and in Charlotte Harbor (bottom row, first reproductive period is 2013). Visited receivers are indicated by gray dots (nodes) and unvisited receivers by green dots (nodes). Detection frequency and relative importance of each receiver is indicated by node size, the bigger the node, the more important it is. Edge weight (line thickness) is proportional to the interaction frequency between connected receivers (i.e. more frequent interactions are depicted by thicker edge weight).  The number of fish included in the analysis is reported (n) as well as the edge density metric, with a large edge density corresponding to greater space use.

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