Decline of Common Smelt (Retropinna retropinna) in Turbid, Eutrophic Lakes in the North Island of New Zealand

Abstract

Densities of larval smelt (Retropinna retropinna) and common bullies (Gobiomorphus cotidianus) were estimated by a closing Wisconsin drop-net on two occasions following the spring/summer spawning season in 11 geographically localised lakes varying in size and trophic status. Mean densities of smelt ranged from 0 to 65 fish m−2 and were directly related to water clarity as determined by secchi disc depth. In contrast, densities of larval bullies ranged from 0 to 117 fish m−2 and were inversely related to water clarity. Larval smelt dominated in all the clear water, oligotrophic lakes, but bullies dominated in lakes where water clarity was less than 6 m. The depth distribution and depth range for schools of juvenile smelt was also reduced in turbid lakes. As the ability of juvenile smelt to feed on zooplankton is not reduced by turbidity, and as predation by trout on smelt decreases in turbid lakes, these two factors are not responsible for the decline of smelt in the more turbid, productive lakes. We propose that reduced larval recruitment caused by a higher egg mortality is responsible. A higher egg mortality would be expected in such lakes because of increased siltation of littoral spawning substrates, and increased predation by common bullies.