Fathead minnow (Pimephales promelas) reproduction is impaired when exposed to a naphthenic acid extract.
Previous studies have demonstrated that oil sands process-affected water (OSPW) impairs the reproduction of fish and that naphthenic acids (NAs), a natural constituent of oil sands, are suspected of being responsible. This study evaluates the potential impact of NAs on the reproduction of adult fathead minnows (Pimephales promelas) under laboratory conditions. Fathead minnows exposed to a 10 mg/l naphthenic acid extract (NAE) for 21 days spawned fewer eggs and males had reduced expression of secondary sexual characteristics. Male fathead minnows exposed to a 5 mg/l NAE had lower plasma levels of 11-ketotestosterone whereas those exposed to a 10 mg/l NAE had lower concentrations of both testosterone and 11-ketotestosterone. Since OSPW also contains high concentrations of salts, this study also investigated whether they modify the toxicity of NAEs. Spawning was significantly reduced in fathead minnows exposed to a 10 mg/l NAE alone and in combination with NaHCO₃ (700 mg/l), typical of concentrations in OSPW(.) Interestingly, the addition of NaHCO₃ reduced the inhibitory effects of the NAE on the numbers of reproductive tubercles and plasma testosterone levels. Further studies showed that NaHCO₃ acted by reducing the uptake of the NAE to the fish. NaHCO₃ but not NaCl or Na₂SO₄ reduced the acute toxic effects of the NAE on fathead minnow embryo and larvae mortality. Collectively, these studies show that the NAs in OSPW have the potential to negatively affect reproduction in fathead minnows and that HCO₃⁻ reduces the acute and chronic toxicity of NAs.