The effects of reversal of seasonal changes in photoperiod on the growth and food consumption of cod, Gadus morhua L.

Abstract

The effect of reversing the natural seasonal change in photoperiod on the feeding and growth of juvenile cod, Gadus morhua. was investigated in two experiments. In an experiment conducted in spring no photoperiod effects on overall growth and food consumption were observed; however, tissue analysis at the end of the experiment showed that treatment groups differed significantly in their muscle water content. A repeat experiment conducted in early winter similarly failed to reveal an effect of photoperiod reversal on overall growth and feeding, although a very marked decline in growth rate was observed for all fish. The decline in growth rate observed in the second experiment cannot be accounted for by any changes in experimental procedure and is interpreted as possible evidence for an endogenous circa‐annual rhythm in growth rate.