By Sarah Burnsed, Hayden Menendez, and Sue Lowerre-Barbieri
Gag grouper (Mycteroperca microlepis) are an iconic Florida fish that may be in trouble. All fish begin as females in estuarine nursery grounds, but as they age they move further offshore, with the oldest, largest fish turning into males (Figure 1). This life history and spatial ecology pattern makes it difficult to decide on the best measure of reproductive potential. Based on females-only, the last stock assessment found them to not be overfished or undergoing overfishing. But the same assessment predicted only approximately 2-3% of the population was male and since then commercial fishermen have not been meeting quota, leaving fishermen and scientists concerned that this stock may not be as healthy as assumed.
Beginning in 2015, the Movement Ecology and Reproductive Resilience (MERR) Lab at FWRI began a series of gag studies to evaluate factors affecting gag reproductive potential. Empirical data derived from these studies, including estimates of fecundity-at-age, spawning frequency and sex ratios, and spatio—temporal patterns of sex change and sex ratio, will help to refine estimates of long-term biological productivity of the stock and in turn better manage this important fishery. We describe our three major gag program initiatives below.
Our initial study (December 2015- May 2018) off the Florida Panhandle targeted the best-known gag spawning habitat ~50-100 miles off Panama City Beach. Three areas were sampled, with varying protection from fishing: (1) Madison Swanson, an MPA (2) The Edges, open half the year to fishing (3) an open area. Twice monthly during gag spawning season (December-May), we departed lab headquarters in St. Petersburg to drive to Panama City and boarded chartered fishing boats for multiple day cruises. We captured gag using hook-and-line and recorded parameters of time landed, location, depth and ventral pigmentation. We collected video data using an unbaited camera array with a ~360° field of view to assess habitat, spawning behavior and abundance. We evaluated all gag for lengths, weight, genetics, mercury, age, sex, hormones and maturity. Data from our collections were integrated with data from FIM surveys, FDM, and a collaborating commercial fisherman to test assumptions about sex change and spatial management in gag. Results indicate overall gag abundance is low, MPAs do not protect all recruiting males (as previously assumed) and current regulations are not sufficient for males to recover to historic levels. To read more on this study, please see: Lowerre-Barbieri S, Menendez H, Bickford J, Switzer TS, Barbieri L, Koenig C (2020) Testing assumptions about sex change and spatial management in the protogynous gag grouper, Mycteroperca microlepis. Mar Ecol Prog Ser 639:199-214. https://doi.org/10.3354/meps13273.
To assess how these results might differ with location, a second study was begun in December 2018 (extending through May 2021). This study uses similar methods but is focused ~100 miles offshore of Tampa Bay at (1) Steamboat Lumps, an MPA and (2) the Sticky Grounds, an open area south of Steamboat Lumps, originally brought to our attention by fishermen and confirmed as a spawning site by preliminary sampling. Both habitats are quite different from those in the Panhandle with Steamboat Lumps having considerably less relief than Madison Swanson and the Sticky Grounds characterized by patchy high relief in depths greater than sites sampled before. Because gag spawn at these offshore sites in the windy winter months, it is not surprising previous sampling in this area has been limited due to uncooperative seas and day trips requiring 18 hour runs from shore. We are fortunate to again work with an impressive group of captains willing to safely execute these trips and share their knowledge of the gag fishery to increase our success. By expanding the area of collection, we’ll be able to identify and quantify gag spawning aggregations, sex ratios, and reproductive potential off the West Central Florida Shelf and compare these parameters to those collected in the initial study to determine spatial and temporal differences within and between study areas.
These studies, and the integration of their results with the larger sampling efforts of the FIM reef fish survey and FDM, changed our understanding of where and when gag change sex. Previously, it was believed that this occurred only on the spawning grounds and that spawning ground MPAs would protect males. However, we found fish transitioning from females to males not only on the spawning grounds but also in pre-spawning female-only aggregations. Thus, there is a need to better understand pre-spawning aggregations, their seasonal cues, and spatial consistency, as well as sex-specific movement ecology. Research on these questions was started in December 2017 when the same fisherman who provided samples from a gag pre-spawning aggregation site for our first study began working with us to dart tag and release gag at his nearshore site.
In 2019 and 2020 we increased this effort to include scientific sampling of pre-spawning gag aggregations, dart tagging a larger number of fish, and beginning a program to acoustically tag females (Figure 3 and cover image). Our recaptures so far suggest very high site fidelity of females to pre-spawning aggregation sites, as well as much higher catch per unit effort at these sites than on the spawning grounds. We expect recapture rates at these sites to decrease as fish begin moving offshore. In addition, acoustic tag detections will help us understand where these females move to once they leave these sites. Our ability to detect them throughout the Gulf is made possible because of the iTAG (Integrated Tracking of Aquatic Animals in the Gulf of Mexico) network, which enables researchers to share detections on their receivers that are not their study species. This network and the data exchange are FWRI initiatives, with the digital exchange developed and maintained by the FWRI Information Science and Management section. These telemetry detections along with dart tag recaptures and reproductive data will collectively enable us to better understand how this species’ spatial ecology affects vulnerability to fishing and the measures needed to allow more males to recruit to the population, hopefully in turn keeping this valuable fish on dinner plates across the state.