Movement Ecology and Reproductive Resilience of Gag in the Eastern Gulf of Mexico

  • Gag are a prized catch in Florida. However, there have long been concerns about decreased size and male abundance. Pictured on the left is a client of Captain Eric Schmidt holding a 72 lb male gag caught in 1985 off SW Florida. On the right is Captain Ed Walker, who collaborated on this research, also holding a male displaying dark black pigmentation known by fishermen as a "rusty belly" or "copper belly." In six years of sampling, the two largest males came from marine protected areas (MPAs) and weighed 38 lbs (Steamboat Lumps) and 40 lbs (Madison Swanson).

  • Since 2009, annual commercial landings in most years have been well below commercial quota. MERR lab scientists collaborate with fishermen in the recreational, charter, and commercial sectors along Florida's west coast to identify potential drivers of declining stock productivity, including where and when sex change occurs and how fishing pressure affects male recruitment and survivorship.

  • Current understanding of gag life cycle space use: mature females migrate offshore to depths greater than 50 meters during spawning season (February-mid-April) where males are thought to remain all year. Eggs and larvae are pelagic and take 30 to 60 days to arrive in estuarine nursery habitat, where they settle in seagrass beds and remain as juveniles for 5 to 7 months. They then move to nearshore hard bottom as immature females (typically between 1-4 years old). As female gag mature, they begin to move offshore and form pre-spawning aggregations from November to February. Seasonal information is in parentheses, y=years, mths=months.

  • During the first study, samples were collected with video and rod and reel from three areas with varying spatial management: an area with no protection (star), the Madison Swanson marine protected area, and the Edges which is closed to fishing during the spawning season. Data collected included: gag abundance and habitat (left), blood for sex hormone analysis (right), length, weight, age, sex, pigmentation, mercury, gonadal samples for histological analysis, and egg samples to estimate fecundity. Fish samples were provided to the FWRI fish health group which took data to assess condition and health.

  • Four spawning grounds have been sampled in the two studies, the Madison Swanson (MS) marine protected area (MPA) and the Edges in the first study and the Steamboat Lumps MPS (SBL) and the Sticky Grounds (SG) in the second. In addition, the second study collected preliminary samples from a nearshore pre-spawning aggregation site (PSA). Sampling methods were the same for all studies.

  • The third study will use video surveys, soundscapes, and acoustic telemetry (top panel) to monitor gag throughout estuarine, nearshore, mid-water, and offshore habitats. Fish will be acoustically tagged (n=140) in estuarine and nearshore habitats and tracked for three to five years. Fish will be monitored across all four habitats using MERR arrays (red) and the arrays in the iTAG network (black).

Gag grouper (Mycteroperca microlepis) are an iconic Florida fish whose complex life history and spatial ecology makes it difficult to measure the stock’s productivity. Gag are sequential hermaphrodites with all fish beginning life as females in estuarine nursery grounds. It is not until age 12 that 50% turn into males, changing not just their sex but also their movement ecology, becoming resident in deep-water spawning habitat. Anglers have voiced concern about decreasing size of male gag and lower catches. Since 2009, annual commercial landings in most years have been well below commercial quota. Because gag do not turn into males until they are relatively old, there is potential for fishing to result in too few males left to fertilize females’ eggs. Recent stock assessment results show male sex ratios decreasing from ~9% in the 1990s to ~2% in 2012 and 1.4% in 2021.

Spawner-recruit systems are fundamental to predicting productivity in stock assessments but typically evaluate only mature female biomass or egg production. Emerging ecological understanding indicates that spawner-recruit systems are multi-faceted and that reproductive resilience, i.e., the capacity of a population to maintain the reproductive success needed for long-term population stability, will be affected by a number of factors, including whether a species is hermaphroditic or not, and their spatial ecology. The Movement Ecology and Reproductive Resilience (MERR) Lab at FWRI studies the complexity of marine fish spawner-recruit systems and spatial ecology to help improve our ability to manage for sustainability. They began a series of gag studies in 2015 to improve our understanding of long-term biological productivity of the stock and potential bottlenecks, specifically to confirm or refute the low male sex ratios suggested by the 2014 stock assessment, the processes that could lead to this and the role male abundance plays in gag productivity.

Study 1 (2015-2018, Offshore Florida Panhandle): Is low male abundance limiting stock productivity? Assessing factors affecting reproductive potential of gag, Mycteroperca microlepis, in the Gulf of Mexico

This study drew on the expertise of collaborative fishers as well as state, federal and academic scientists, with expertise in: fisheries management, reproductive dynamics, and gag grouper life history, health and fisheries independent (FIM) and fisheries dependent monitoring (FDM). Sampling targeted a historic gag spawning site, Madison Swanson MPA, as well as a seasonally closed area, The Edges, to the south and an area open to fishing just northeast of Madison Swanson. Twice monthly from December to May, FWRI researchers drove to Panama City and boarded chartered fishing vessels for multiple day cruises extending 50 to 100 miles offshore to collect biological and video data. Results from this directed research was integrated with the FIM and FDM data, and data from a nearshore collaborative fisher to increase sample size and the spatio-temporal scale of inference—especially important for understanding where and when gag change sex. It was previously believed that sex change occurred only on the spawning grounds after the spawning season ended.  Histological analyses from this study showed sex change occurred before, during and after the spawning season and both on the spawning grounds and in pre-spawning female-only nearshore aggregations. Low male sex ratios were observed during the spawning season: 5% in the MPA and 0% in the other areas.

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.

Study 2 (2018-2021, Offshore Tampa Bay): Spawning aggregations and sex-specific reproductive potential in gag grouper in the Gulf of Mexico: Improving stock assessment inputs

To increase biologists’ understanding of gag reproductive potential and how it may vary with region, a second study, using similar methods, was conducted in shelf habitat south of the first study area and approximately 100 miles offshore of Tampa Bay. It again included an MPA, Steamboat Lumps, and an unprotected area (The Sticky Grounds) 20 miles to the south which was identified by collaborative fishers as a likely spawning site. Habitats in this study differed from the first study, with Steamboat Lumps having considerably less relief than Madison Swanson, whereas the Sticky Grounds was characterized by patchy high relief, high current and deep water. In addition, a shallow-water, nearshore pre-spawning site was opportunistically sampled to compare catch rates with those seen in the MPAs and to collect preliminary data to improve our methods for assigning maturity. 

Spawning was documented in both Steamboat Lumps and the Sticky Grounds. The grueling 18-hour winter trips to these sites helped clarify why this had not been documented previously and it would not have been possible without the expertise of the collaborative captains. Analysis of this data is on-going, but preliminary results suggest male sex ratios in the Steamboat Lumps MPA are comparable to those seen in the Madison Swanson MPA, but slightly higher in the Sticky Grounds. A simulation study is being conducted to improve our understanding of how sex change and male abundance affects stock assessment results.

Study 3 (2021-2024, Nearshore Tampa Bay): Nearshore pre-spawning aggregations, productivity, and stock structure of Gag grouper in the Gulf of Mexico.

A third study is just beginning and focusses on an improved understanding of gag spatial ecology and how it intersects with fishing pressure. In this study biologists will assess gag density, fishing pressure, movement in different habitats (estuarine, nearshore, mid-water and offshore), and continue to enhance our understanding of what drives sex change in pre-spawning aggregations and at spawning sites. A remotely operated vehicle (ROV) video survey will be used to assess changes in aggregate behavior/density before and during the spawning season, coupled with soundscapes to track engine and anchoring sounds as a proxy for fishing pressure. One hundred and twenty females will be acoustically tagged throughout estuarine and nearshore pre-spawning habitats and monitored with a series of acoustic receiver arrays in the four habitats mentioned above, as well as throughout the Gulf of Mexico through the iTAG (Integrated Tracking of Aquatic Animals in the Gulf of Mexico) network. iTAG is a MERR initiative, in collaboration with the FWRI Information Science and Management section and the University of Florida, that allows researchers to receive detections of their acoustically tagged animals throughout the area where iTAG members have acoustic receiver arrays. These telemetry detections along with dart tag recaptures and reproductive data will improve biologists’ understanding of gag behavior. Population structure and connectivity will be assessed with genetics, dispersal models and otolith microchemistry.

Integrating results from these studies with simulation results will help us build a better model of factors affecting gag reproductive resilience and productivity, as well as what factors need more protection to increase productivity of this iconic species and improve catches.

These projects were funded through NOAA MARFIN grants and Sportfish Restoration. FWRI’s Fish Health and Genetics Groups also collaborated on these projects.