Competitive interactions and community-level impacts of an invasive fish

Background/Question/Methods

Invasive species are among the top three greatest threats to global biodiversity, and competition, and while not often cited as the ultimate cause of native population extirpation by invasive species, competition is a major contributor to native population declines.  Cuatro Ciénegas, in Coahuila, Mexico is an oasis in the Chihuahua desert and an endemic hotspot.  A common aquarium trade fish, the West African jewel cichlid (Hemichromis guttatus), was introduced in 1995 and isotope data suggest it is a potential competitor of two endemic, threatened fishes, the Cuatro Ciénegas cichlid (Herichthys minckleyi) and two-line pupfish (Cyprinodon bifasciatus).  Experiments are being performed in cattle tanks to quantify this competition and to determine any differential effects on the macroinvertebrate community by the two species.  Treatments include tanks with all H. minckleyi, all H. guttatus and mixed H. minckleyi and H. guttatus, each with and without vegetation.  The six treatments were replicated 5 times each and there were 2 no-fish controls.  Each tank was inoculated with macroinvertebrates taken from a nearby pond.  Prior to the introduction of fish, macroinvertebrate samples were taken and fish were weighed and measured, and this will be repeated at the end of the experiment.  Behavioral observations are being taken throughout.

Results/Conclusions

Mortality has been high in both species among all treatments, likely due in part to cold winter temperatures followed by high algal growth in the recent warmer temperatures.  No fish were found in the H. guttatus only/no cover treatment and only H. minckleyi were found in the mixed/no cover treatment supporting previous findings that vegetation is correlated to H. guttatus presence.  When comparing the growth of the two fishes in the one species/with cover treatments, growth was similar between the two groups (mean growth was 2.4 cm for  H. minckleyi and 2.7 cm for H. guttatus) suggesting similar growth patterns for the two species when in intraspecific competition.  Fry of both species have been noted, specifically of H. minckleyi in the H. minckleyi only/no cover treatment and of H. guttatus in the mixed/no cover treatment earlier in the trial period, which contradicts the discovery that vegetation is important for H. guttatus presence.  Analysis of the macroinvertebrate community data and interspecific competition effects between the two fish species is ongoing.  The full analysis will contain more replicates and more fish, shedding light on some of these apparently contradictory preliminary results.