Effect of six photoperiod protocols on the spawning time of two rainbow trout, Oncorhynchus mykiss (Walbaum), populations in north-west Greece

The effect of four seasonally changing and two static light photoperiod regimes was investigated in two populations of rainbow trout, Oncorhynchus mykiss shasta and kamloops (Walbaum), kept under almost constant temperature, diet and management from 1981 to 1990 in Epirus, north-west Greece (39,40′N, 20,51′E). We studied how different populations (determined according to the different spawning time) responded to the same artificial photocycles and how age influences that process (underyearling, 1 +, 2+ years old). Early sexual maturation with a 2 to 11-week acceleration of the peak spawning time occurred in fish kept under protocol P₁ ('long'spring days followed by light reduction during autumn).‘Long'days exposure of fish (≥14h light/day) from June (P₃) or September (P₄) had little (4 weeks) or no effect respectively in advancing spawning time. Trout kept under a‘reverse'(P₂) photocycle attained the peak spawning time 4-9 weeks later than the controls. An 18-week delay in spawning time occurred in fish kept under P₂ from the fry stage. Static light regime P₅ (20 L:4 D to 15 May, then 4L:20D) advanced peak spawning by 6 weeks, while P₆ (4L:20D from March to August then 20L:4D) delayed spawning by 2-4 weeks. Photoperiod also influenced the duration of spawning: the Pi protocol increased by spawning spread three times, while P₂ decreased it (time between the spawning of the first and last fish was 9-12 days). No significant difference was observed in the percentage of mature females between tested and control groups, except of kamloops fish kept under P₁ (higher than control, P <0-01), P₂ (lower than control, P <0-05) for their second reproductive cycle. A higher percentage of early mature males occurred under protocol P₂. Fecundity expressed as number of ova/kg of fish weight was similar between tested and control groups. Absolute fecundity (total number of ova/female) was lower in trout kept under the‘reverse'P₂ regime (P <0-05). No statistical difference was found in egg diameter and weight. Hatching percentage of fertilized eggs was similar between tested (47-64%) and control (50-70%) fish.