EFFECTS OF OXYGEN, TEMPERATURE AND LIGHT GRADIENTS ON THE VERTICAL-DISTRIBUTION OF RAINBOW-TROUT, ONCORHYNCHUS-MYKISS, IN 2 NORTH-ISLAND, NEW-ZEALAND, LAKES DIFFERING IN TROPHIC STATUS

Vertical distributions of adult rainbow trout (> 25 cm fork length, FL ) were determined with a SIMRAD ES470 split-beam echosounder in two 80 -90 m deep lakes differing in water quality. Between November 1993 and February 1994, most trout (> 80%) were between 10 and 40 m, within or close to the thermocline. However, a small group of fish occupied col der waters, deeper than 50 m. In February, surface water temperatures > 21.0 degrees C and hypolimnetic oxygen levels < 2.5 g m(-3) compress ed the habitable depth range for trout in Lake Rotoiti to 12-35 m comp ared with 12-80 m in Lake Rotoma. Deeper-dwelling trout inhabiting wat ers over 50 m in Lake Rotoiti would have been forced into shallower wa ters at this time. However, the vertical distribution of the remaining trout in Lake Rotoiti was not compressed. In March 1994, adult trout were still present in waters 10-40 m deep in both lakes, but many of t he smaller fish had moved into shallower waters (< 10 m deep), probabl y because of declining water temperatures in the epilimnion and increa sed densities of their preferred prey. At this time, the lower depth r ange for trout in Lake Rotoiti was limited to 28 m by the 2.5 g m(-3) oxygen level and trout occupied warmer waters than they did in Lake Ro toma. When oxygen was not limiting, water temperature was the main var iable determining the depth of the trout, and monthly changes in the m ean depth of trout between both lakes and months were explained by a t hermoregulatory model for trout movement.