Fish may form spectacular seasonal aggregations, where 100’s of individuals get together to spawn under certain environmental conditions. In addition to being a fascinating example of reproductive ecology, these spawning aggregations are of conservation concern. Large numbers of individuals aggregate in a small area and can be heavily targeted by fishing activity. Furthermore, there may only be a few spawning events per year so anything that disrupts spawning may have negative consequences for the population. Storm events are increasing in frequency and may impact the behavior of animals such as spawning fish in coastal areas if they cause significant surge activity. If the aggregating fish are also large marine predators, then seasonal formation of spawning aggregations may have a significant effect on local marine communities by consuming prey or changing prey behavior via risk effects.

The Goliath grouper (Epinephelus itijara) can reach over 2 m in length and forms large spawning aggregations seasonally at wreck sites off southern Florida. Goliath grouper were overfished and listed as endangered but there are now plans to open the population up to limited fishing, although there remain conservation concerns. While the locations of goliath spawning aggregations are known, dynamics of spawning are less clear. For example, under what conditions does spawning occur and do all fish in the aggregation spawn at the same time? What effect do goliath grouper aggregations have on prey communities on the wrecks? We used acoustic telemetry to remotely measure spawning behavior in goliath grouper, by tagging fish with transmitters that measure swimming depth and acceleration. Data from these tags were detected by listening stations we placed on and around a wreck off southern Florida. We simultaneously measured current strength and water temperature using oceanographic equipment. Finally, we wanted to know the effects of goliath grouper on potential prey fishes, so we a) simultaneously tracked the movements of reef fish on the wreck and b) measured the feeding rates of surgeonfish on the wreck. In both cases, we compared our results within and outside of the known goliath grouper spawning period.

We were able to identify spawning like behavior from goliath grouper, which occurred close to the new moon, when spawning is thought to occur. Fish tended to use the same areas suggesting a tight aggregation, but not all individuals in the group displayed spawning like behavior. Most fish would perform one spawning like behavior and then leave the region and swim to other wrecks or further north. We found that current had a strong effect on goliath grouper behavior, as fish had to stay close to the seafloor when currents were strong. Our results therefore suggest that spawning times may be based around lunar phase and current strength, and that fish may only spawn once within an aggregation before moving on. We have also sent our results on the likely depths of spawning, to colleagues at University of Miami, who are constructing models to predict where Goliath grouper larvae end up. By constructing this model, they can predict how storms, which may change water current patterns, may alter the dispersal of goliath grouper larvae.

We found no change in the space use of reef fishes on the wrecks, during and outside of the goliath grouper spawning season. In other words, reef fish did not appear to avoid the space used by grouper even when they are in large numbers. We also found that reef fish foraging rates actually increased during the goliath grouper spawning season. If grouper were inducing risk effects, then we would expect reef fish to reduce foraging rates in their presence, so they can spend more time being vigilant. The increase in reef fish foraging rates when grouper are present may be due to warmer water temperatures (grouper aggregations occur in the summer). However, it may also suggest that groupers facilitate communities either by providing nutrients (e.g. through excretion or egestion) or by attracting planktivorous fishes which are foraging on the grouper eggs. Future research should investigate the potential for large spawning aggregations of grouper to facilitate marine communities.

Last Modified: 12/02/2022
Modified by: Yannis P Papastamatiou