Gulf Gag: where have all the cowboys gone?
Six years, two studies, one species: untangling the processes driving low male abundance of Gag in the Gulf of Mexico.
Cause for concern:
Gag (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.
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 over-fished or undergoing overfishing, but the same assessment predicted only approximately 2-3% of the population was male. Since then commercial fishermen have not been meeting quota, leaving fishermen and scientists concerned that this stock may not be as healthy as assumed.
Note, this figure originally appears in: 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)
How we are addressing the concern:
Beginning in 2015, the Movement Ecology and Reproductive Resilience (MERR) Lab at FWRI began a series of studies working with both recreational and commercial fishermen to evaluate factors affecting gag reproductive potential (their capacity to produce viable offspring). Specifically, we wanted to know:
- estimates of fecundity-at-age (how many eggs are produced and how that changes with age)
- spawning frequency (how often during a spawning season does an individual spawn) and sex ratios (how many are males and how many are females)
- spatio—temporal patterns of sex change and sex ratio (how transition from female to male and sex ratios shift with space and time)
Data derived from these studies will help to refine estimates of long-term biological productivity of the stock and in turn better manage this important fishery. We describe our two major gag program initiatives below.
Is low male abundance limiting stock productivity? Assessing factors affecting reproductive potential of gag, Mycteroperca microlepis, in the Gulf of Mexico.
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, a marine protected area (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 Fisheries Independent Monitoring (FIM) surveys, Fisheries Dependent Monitoring (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.
Spawning aggregations and sex-specific reproductive potential in gag grouper in the Gulf of Mexico: Improving stock assessment inputs.
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) a deepwater 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 deepwater spawning site 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 18 hour days required to successfully sample it. 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 are documenting new gag spawning grounds, and assessing sex ratios, and reproductive potential at these sites, which will then be compared with the better-known spawning sites off the panhandle.
Preliminary sampling of nearshore Gag in the Gulf of Mexico.
Preliminary sampling in shallower water, collaborating with Captain Ed Walker, provided data indicating that gag can undergo sex change in pre-spawning aggregations. Previously, it was believed that this occurred only on the spawning grounds and that spawning ground MPAs would protect males. This finding indicates a need to better understand pre-spawning aggregations and we continue to work with Ed, who is dart tagging and releasing gag at his nearshore sites.
To build the foundation for future research we have begun a preliminary acoustic tagging study to better understand site fidelity and when these fish leave to spawn offshore.
Each recapture of a dart tagged fish helps us better understand the site fidelity and movements of this iconic species. Because of our extensive telemetry array in the Tampa Bay area and the iTAG (Integrated Tracking of Aquatic Animals in the Gulf of Mexico: https://www.itagscience.com/) network, these fish can be tracked as they leave the estuary and move into Gulf waters. iTAG is an FWRI/University of Florida initiative, with the data exchange developed and maintained by the FWRI Information Science and Management section. Gag have complex spatial ecology, but with years of studying them, help from fishermen, and new technology to track them we are beginning to better understand the spatial distribution of fishing effort, and how this may impact recruitment to the male population and the spawning grounds. This information is being shared with the Gulf Council and NMFS stock assessment scientists to help keep this valuable fish on dinner plates across the state.