Ovarian development of captive F1 wreckfish (hāpuku) Polyprion oxygeneios under constant and varying temperature regimes - Implications for broodstock management

In order to better understand how photo-thermal conditions affect oogenesis in captive-bred F1 hāpuku, a wreckfish considered for aquaculture in New Zealand, juvenile (pre-pubertal) fish were assigned to one of two regimes: exposed to a constant temperature of 17 °C (CT group) or to seasonally varying temperatures (VT group range: 10-17 °C), both under simulated ambient photoperiod, for nearly 2 years. Development in females was monitored through repeated gonadal biopsies (histology; target gene mRNA levels) and blood sampling (plasma levels of estradiol-17β; E2). Very little evidence of advancing oogenesis was found in the first year of study, when fish were in their 4th year. In the subsequent year, a proportion of fish reached the pre-spawning stage (fully-grown ovarian follicles); the proportion of females reaching this stage was notably higher in the VT (62%) than the CT (28%) group. Of the few females that did reach maturity in the CT group, significantly lower levels of plasma E2 were observed relative to those in fish from the VT group possibly indicating a temperature-induced endocrine impairment during oogenesis. Interestingly, females that did not reach the pre-spawning stage presented with a small transient, but significant increase in oocyte diameters and plasma E2, suggestive of a dummy run. Clear seasonality was observed for fish under both photo-thermal regimes, and this was reflected in plasma E2 levels and transcript abundances of aromatase, fshr and luteinizing hormone receptor in the ovary; these end points all peaked in maturing females during the late or post-vitellogenic stage. We conclude that captive female F1 hāpuku first mature as five-year-olds and that exposure to a decreased temperature is important for appropriate progression of oogenesis.