Invasion of the Atomic Sunfish
By Conservancy Research Manager Dr. Jeff Schmid
Atomic sunfish have invaded our coasts! Not in the sense of a nuclear disaster followed by a monstrous creature emerging from the ocean to wreak havoc on coastal communities. Rather, these vividly colored fish were introduced somewhere, somehow to south Florida and were first documented within Everglades National Park in 1983. The species’ tolerance to a wide range of salinity conditions, combined with seasonal flooding of aquatic habitats and the network of man-made canals throughout southern Florida, allowed for the rapid expansion of atomic sunfish from coast to coast. Their kryptonite, however, is the inability to tolerate low water temperatures which appears to have kept these invaders from establishing themselves beyond central Florida.
Atomic sunfish is one of many common names for the species Mayaheros urophthalmus and other names include orange tiger, false red terror, mojorra castaricca, and schwanzfleckbuntbarsch. Perhaps best known as the Mayan cichlid, this freshwater fish is native to the Gulf of Mexico and Caribbean coast drainages of Central America from southeast Mexico to Nicaragua. It is a highly valued food fish in its native range and is related to the African cichlids sold as “tilapia” in U.S. supermarkets. The edibility and aggressive behavior of Mayan cichlids has made them popular with anglers in Florida waters. There are no bag limits for this non-native fish and state wildlife agencies encourage anglers not to release their catch . It is commonly assumed that introduced fish species such as these have a negative impact in the aquatic environment but quantitative data are often lacking to determine their effects.
Knowledge of Mayan cichlid feeding habits is vital to understanding the impacts of their invasion of Florida waters. The species is considered omnivorous (feeding on plants and animals) with the most comprehensive descriptions of its diet from the brackish coastal lagoons of Mexico. Diet studies in Florida have focused on freshwater habitats and indicated small fish were common prey for Mayan cichlids. Detailed information was lacking on food items they consumed in estuarine tidal creeks and backwater bays which are also important nursery grounds for sportfish such as snook and tarpon. Analyses of mangrove fish communities have suggested an inverse relationship with the abundance of Mayan cichlids and that of native fishes but the mechanism for this relationship is not clear. One line of thought is that an introduced piscivorous (fish-eating) predator could have negative impacts on the small fishes inhabiting these developmental habitats.
It was during fishery surveys of mangrove tidal creeks that I was contacted by Copley Smoak, a long-time member with the Conservancy of Southwest Florida. He had seen a newspaper article telling of how we were sampling fish in our newly-constructed filter marsh and then feeding the cichlids to rehabilitating shorebirds in the von Arx Wildlife Hospital. Cope is an avid angler with an insatiable appetite for Mayan cichlids, so we developed a research project to investigate the diet of fish he was catching in the tributaries to Estero Bay and Wiggins Pass estuaries. During the study he caught a total of 747 Mayan cichlids, extracted the digestive tract of each fish, and then identified and measured the volume of the food items in each tract.
Mayan cichlids in southwest Florida tributaries primarily consumed organic debris (detritus), as has been documented in most other diet studies for the species, but some researchers have suggested these debris were accidentally consumed while feeding on animal prey. We observed Mayan cichlids picking at red mangrove prop roots and submerged tree snags and this behavior would lead to their ingesting bark, rootlets, wood fragments, and other organic debris when feeding on organisms stuck to the surface of these substrates. In fact, estuarine mussels were prevalent in the diet of fish collected from Estero Bay tributaries and earlier studies found these root-fouling organisms were the most abundant in the mangrove tidal creeks. As such, Mayan cichlids were exhibiting a generalist feeding strategy by consuming the most available prey and this strategy likely enhances their ability to invade and become established in coastal waterways.
In addition to mussels, Mayan cichlids in estuarine tributaries consumed other hard-shell invertebrates such as snails, crabs, barnacles, scarab beetles, and calcareous worm tubes. There was a pronounced seasonal pattern to their diet with higher percentages of detritus consumed during the dry season and higher percentages of invertebrates during the wet season. Mayan cichlids in Mexico, as well as other cichlid species in Central and South America, exhibit similar seasonal variation for these food items. Another pattern emerging from the available diet information is that Mayan cichlids exhibit different feeding habits depending on the salinity and habitat characteristics of the waters they inhabit. Durophagy (eating hard-shell prey) appears more common in estuarine habitats and piscivory more prevalent in freshwater habitats. Our study provides yet another piece in the puzzle of how the atomic sunfish was able to rapidly invade southern Florida.
For more detailed information, the results of our Mayan cichlid diet study were recently published in the scientific journal Gulf and Caribbean Research. Our project was funded with proceeds from the RedSnook Charity Fishing Tournament and this year’s tournament is scheduled for Saturday, October 9 through Sunday, October 10, 2021.
