Advanced ovarian development of Murray cod Maccullochella peelii peelii via phase-shifted photoperiod and two temperature regimes

The timing and characteristics of reproductive development in adult female Murray cod exposed to a simulated seasonal photothermal cycle (14:45 to 09:45 daylight h; 12–26 °C) (CONTROL) were compared to the development of females exposed to a three month phase-shifted (advanced) seasonal photothermal cycle (PHOTOTHERMAL) and to females exposed to a three month phase-shifted (advanced) photoperiod cycle in combination with constant temperature (19.5 °C) (PHOTOPERIOD). Females in PHOTOTHERMAL and PHOTOPERIOD treatments reached maturity up to three (June) and four (May) months in advance of CONTROL fish (October), respectively. Biannual maturation was also observed in four PHOTOPERIOD females (13%). Mean ovary diameter and relative fecundity of mature females were similar between treatments (p > 0.05), and viable eggs were produced in all groups (100% ovulated; 14.02%–39.12% mean survival to hatching). Ovary diameters and plasma levels of E2 and T in phase-shifted females remained at basal levels and/or were significantly reduced (p < 0.05) relative to CONTROL fish throughout the early to mid phases of the maturation period. However, rapid increases in plasma T (0.54–4.39 ng ml− 1) and ovary diameter (20.0–42.4 mm) in the 60 to 90 days preceding the onset of maturity in phase-shifted females revealed a capacity of Murray cod to accelerate development processes to compensate for earlier delays in photo-responsiveness. Low levels of E2 that persisted throughout the maturation period of PHOTOTHERMAL and PHOTOPERIOD females did not appear to greatly affect ovarian growth. The successful maturation of photoperiodically-manipulated females under constant temperature demonstrates an alternative approach for influencing maturation patterns in Murray cod that may improve the versatility and cost-effectiveness of broodstock conditioning procedures.