The effects of constant photoperiods on the timing of spawning in the rainbow trout

Abstract

Groups of up to 700 broodstock rainbow trout were exposed to a variety of light regimes. Long days early in the year followed by short days advanced spawning, and constant short days, or short days followed by constant long days delayed spawning by up to 4 months. Provided that fish were exposed to some long days before the summer solstice, and 6 weeks of long days or possibly less is sufficient, spawning occurred at the same time as in control fish under a seasonally-changing annual light cycle. This indicates that there is a period of photosensitivity in the first half of the annual cycle during which the trout can respond to long days. Constant long days or continuous light both advanced spawning. In those fish previously exposed to ambient light before exposure to the test regimes, spawning was advanced 2–3 months. Fish which were stripped under these photoperiods subsequently entered repeat 4–6 month spawning cycles. Thus light regimes of constant length are alternatives to altered seasonal cycles for the modification of spawning time of rainbow trout. Such light cycles produce no adverse effects on the fecundity of broodstock, or the size, quality and fertility of eggs, provided that spawning is not advanced more than 2–3 months. Cycles producing even earlier spawnings are unlikely to produce eggs of acceptable size for sale, although the eggs may be retained by the farmer for the production of fry.