Lunar periodicity of growth cycles in rainbow trout,Salmo gairdneriRichardson

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Abstract
Rainbow trout (Salmo gairdneri Richardson) maintained in the laboratory under a fixed artificial 12 hrs light: 12 hrs dark photoperiod exhibited alternating periods of rapid and slow growth. The cycles of growth were rhythmic, and evident in ‘longitudinal’ and ‘transverse’ experimental series. The length of the cycle was approximately two weeks in duration. Peaks of growth rate predicted by least square cosine models describing the relative growth rate patterns were associated with new and full moon suggesting that the lunar calendar is a Zeitgeber used in the regulation of the growth cycles. Changes in growth rate cycles and activity of the fish were associated with the spring equinox, indicating that the fish are sensitive to the event, and that they may re‐adjust their growth rate cycle at that time of the year.