Abundance and daytime vertical distribution of planktonic fish larvae in an oligotrophic South Island lake

Abstract

The vertical distribution of common bully (Gobiomorphus cotidianus) and koaro (Galaxias brevipinnis) larvae in the limnetic zone of Lake Coleridge were determined using a high-frequency (200 KHz) echosounder. Planktonic bully larvae first appeared in appreciable numbers in January. By February, they formed a scattering layer between depths of 12 to 24 m during the day, where they achieved a maximum density of 0.59 fish m-3. Larger (> 18mm) fish migrated to the littoral zone and densities declined to < 0.01 fish m-3 by July, when remaining larval fish occupied greater daytime depths. Their vertical distribution during the day appeared to be influenced mainly by light levels and water temperatures. Larvae grew more slowly (0.12 mm d-1) than in more productive North Island lakes, and were also present in lower densities for a more restricted period of time. Koaro larvae first appeared in November and December and were found in low numbers (< 0.01 fish m-3) in summer at depths of 10 to 26 m. Salmonid production in the limnetic zone is probably limited by the small size and relative scarcity of forage fish present.