Humic substances affect physiological condition and sex ratio of swordtail (Xiphophorus helleri Heckel)

Exposure of young swordtail (Xiphophorus helleri) to increasing concentrations of synthetic humic substances (HS1500) significantly effected the physiological condition and slightly effected the sex ratio of the fish. Any exposure enhanced the body mass development of the swordtail compared to the control. No dose-dependent effect of HS on growth was detectable. After a two-week period of stressful handling, the HS-exposed fish recovered quickly whereas growth stagnated in the control fish. The examination of gills, fins, and skins revealed no infestation with ectoparasites. We assume that the growth promoting effect of HS1500 was due to an overall stimulation in metabolism. Furthermore, the sex ratio of fish exposed to HS1500 for 21 weeks slightly shifted in favor of females in a dose-dependent manner. Although the mode of action is still obscure, there is a probability that alkylphenol structures in HS1500 may be responsible for this weak feminization.