The common bully (Gobiomorphus
cotidianus), a small-bodied New Zealand native fish species, was used to
monitor population impacts of multiple effluents in the
Tarawera River, New Zealand. In an initial survey, the
absence of reproductive development at the expected
spawning time for common bully was observed in a population
downstream of effluent discharges. Subsequently, we
examined the hypotheses that the observed changes were
due to effluent exposure, migratory patterns, or genetic
differences between populations. Liver detoxification enzyme
activity and stable isotopes provided evidence against
upstream migration of sexually mature bully. The observed
presence of developed gonads in the downstream
population during winter season resulted in the rejection
of the hypothesis that reproductive failure was due to effluent
exposure, and it was concluded that there were substantial
differences in reproductive timing. Genetic analyses of
two upstream, one downstream, and one population from
a nearby coastal river indicated the upstream (reference)
and downstream (effluent exposed) bully in the river formed
genetically distinct populations. The identification of a
nearby river population with similar reproductive timing
and high genetic similarity to the effluent-exposed population
suggests that the observed differences in the genetics of
the downstream population were not caused by effluent
exposure. The genetic analysis did highlight the lack
of downstream dispersion and gene flow in the river which
could possibly be related to anthropogenic stress.