Exotic fish biomass exceeded native fish biomass in a central Florida spring.

In Florida, the problem of exotic species introduction into freshwaters is particularly acute. The USGS Nonindigenous Aquatic Species database includes 245 entries for exotic or hybrid fish that have been found in Florida waters, most from tropical or subtropical Central or South America, Africa, or southeast Asia. Volusia Blue Spring, a first magnitude spring in central Florida, supports varying densities of at least six species: blue tilapia (Oreochromis aurea), two armored catfish (Pterygoplichthys disjunctivus and Hoplosternum littorale), grass carp (Ctenopharyngodon idella), pirapitinga (piaractus brachypomus), and chanchita (Cichlasoma dimerus). To evaluate the contribution of exotic species to the fish biomass of the spring, we counted and measured native and exotic fish in fall 2020 at two locations, one near the headspring and one near the confluence of the St. Johns River. For small species, we seined three locations at each site and measured all of the individuals captured. For larger species, we snorkeled the entire site, counted the fish that we observed, and photographed representative fish with a GoPro outfitted with two underwater lasers mounted with parallel beams 7 cm apart. We measured the fish on the photographs and calculated their length using the 7 cm reference points on the side of each fish photographed. We converted these lengths into biomasses using published length-weight relationships and summed the biomasses of all native and exotic species separately. Then we calculated the percent difference between native and exotic biomass. In eight of the ten samples, the exotic species biomass exceeded that of native species (exotic species biomass = 17% lower to 2,154% higher than native biomass). Although the native species outnumbered the exotic species, these data suggest that the exotic species may have a greater effect on spring ecosystem function than their densities would suggest. [ABSTRACT FROM AUTHOR]