USE OF SPECTRAL-ANALYSIS TO ESTIMATE SHORT-TERM PERIODICITIES IN GROWTH-RATES OF BROOK TROUT SALVELINUS-FONTINALIS

We used spectral analysis to investigate the periodic components of re lative growth rates (on a mass and length basis) in two groups of six young-of-the-year brook trout (Salvelinus fontinalis). Fish were measu red every 2 days for 25 days according to a repeated measures design i n time. The predominant frequency components were estimated at the end of a stepwise procedure involving the analysis of the multifrequentia l periodogram. We found mainly two superimposed periodic components th at resulted in short rhythms; these had a period of about 5 and 10 day s for the growth rate in mass and of about 4 and 6 days for the growth rate in length (R-2 = 91% for mass and 90% for length, both including an autocorrelation term). Repeatability of rhythms was assessed by AN OVA of the finite Fourier transform, which showed that the two groups of fish reared under the same experimental conditions exhibited growth rhythms that were not significantly different from each other. On tha t basis, we present a harmonic model which predicts short-term variati on in growth rate of brook trout in mass (R-pred(2) = 0.73, with an au tocorrelation term) and in length (R-pred(2) = 0.72).