UNCOUPLING OF VISUAL AND SOMATIC GROWTH IN THE RAINBOW-TROUT ONCORHYNCHUS-MYKISS

Hatchery reared juvenile rainbow trout of common parentage were reared on measured low or high rations, or fed ad libitum for 4.3 months to produce fish with different growth rates. Fish from the different grou ps differed in size by up to 40 and 290% for length and weight, respec tively, by the end of the trial. Slow somatic growth was not accompani ed by correspondingly slow growth of either the eyes, optic tectum or the cerebellum, suggesting that above a certain maintenance ration, gr owth of these parts of the CNS is strongly age-dependent. Retinal char acteristics were also found to be age-dependent and not affected by th e somatic growth rate of the fish. The corollary that age-dependent oc ular growth should also be reflected in different relative eye sizes i n fish with differential growth rates in the wild was tested by compar ing fish of known age from lakes in which fish have markedly different growth profiles. Fish stocked from a single hatchery were recaptured from Lakes Okataina and Tarawera (rapid fish growth) and Lake Taharoa (slow fish growth) at times up to 5 years after release. As predicted, eye sizes of fish from Lakes Okataina and Tarawera were smaller for a ny given body size, than those of fish from Lake Taharoa. Absolute eye size was correlated with fish age, although there was some variation in eye size for any given age. Taken together, these results show that up to a certain level of food deprivation, growth of the eye is maint ained at the expense, or in spite, of low somatic growth. However, the re is a level of somatic growth below which ocular growth is affected.