Eucalyptus leachate inhibits reproduction in a freshwater fish

1. Dissolved organic carbon (DOC) can induce lethal and sub-lethal effects in exposed biota via hypoxic blackwater events and the toxicity of leached compounds. Little is known of how DOC exposure affects fish reproduction despite the fact that its release can coincide with spawning-associated flow pulses. 2. River red gum (Eucalyptus camaldulensis) leaf leachate is a major source of DOC in Australian freshwaters and includes the toxic plant secondary metabolites polyphenols and tannins. High concentrations of leachate are released when leaves on floodplains or dry stream channels are inundated by water. 3. Southern pygmy perch (Nannoperca australis) from naturally high and naturally low Eucalyptus leachate environments in south-east Australia were exposed to elevated leachate levels to investigate the effects of DOC on reproduction and to explore whether response patterns were consistent with populations becoming locally adapted to historical leachate levels. 4. Fish exposed to leachate were half as likely to reach sexual maturity as control fish. Fish from a naturally high-exposure population tended to reach sexual maturity earlier than those from a naturally low-exposure population. Leachate exposure had no effect on either egg size or fecundity. 5. Our results suggest that leachate-exposed mothers did not reproduce because they were physiologically stressed or perceive the environment to be unsuitable, which raises the potential of plastic or adaptive responses to this stressor. The negative sub-lethal effects observed have important fitness implications for individuals, the viability of populations and the management of environmental flows and riparian zones.