Essentially similar to brown trout, rainbows differ chiefly in having a reddish band along the side of the body, although this may be lacking in immature fish. The dark spots on the back are smaller and more numerous than in the brown trout, but red spots are absent. In New Zealand, rainbow trout occur principally in lakes, migrating into the adjacent rivers to spawn, but they are also present in river systems in the Bay of Plenty and Waimarino areas. The original stocks of rainbow were “steelhead”, i.e., sea-run rainbow, but no runs of “steelhead” occur in New Zealand waters. The eggs are laid in a depression (redd) dug into the gravel, then covered over again. They hatch a few weeks later and the larvae (fry) make their way up through the gravel to the free water, living for a brief time on the contents of their yolk sacs. The young fish (fingerlings) stay in the river till a few inches in length, then descend to the feeding ground, where most of their growth occurs. Well conditioned mature rainbows from lakes average about 4 lb, but much larger fish are not uncommon.
by Lawrence James Paul, B.SC., Fisheries Division, Marine Department, Wellington.