Depth distribution and abundance of the common bully, Gobiomorphus cotidianus (Eleotridae), in three oligotrophic New Zealand lakes, one of which is turbid

The depth distribution of the common bully, Gobiomorphus cotidianus, a smal l benthic forage fish, was measured by trapping at set depths from 0-70 m i n three large oligotrophic lakes, including one where inorganic sediment fr om a glacially-fed river produces turbid conditions. Bullies occurred at al l depths from 0.5-70 m in the clear lakes, but none were present below 25 m in the turbid lake. Two groups of bullies were present in the clear lakes; a high-density, littoral stock at depths of 0.5-25 m, and a low-density, p rofundal stock at depths of 30-70 m. These groups were further distinguishe d by differing buoyancy requirements and feeding habits. The swimbladders o f littoral bullies contained gas, but those of the profundal bullies, which fed more than littoral bullies by both day and night, did not. The variati on in mean CPUE with depth within the littoral zone was not related to wate r temperature, oxygen concentration, or conductivity. Nor was it related to a reduction in light levels or to reduced water transparency caused by inc reased turbidity. It may therefore be controlled by biotic factors. The abs ence of a profundal stock below the littoral zone in the turbid lake indica tes that the settlement of fines from turbid inputs may affect bully abunda nce in deeper waters. As conventional measures of the abundance of benthic fish in lakes are often restricted to littoral habitats, and do not reflect changes in abundance with depth, an index of overall abundance based on de pth distribution was developed to allow comparisons between lakes.