Reproductive biology and maturation control of Brook Trout (Salvelinus fontinalis, Mitchill) in Tasmania

Pilot scale commercial production of brook trout (Salvelinus fontinalis) in Tasmania encountered a high incidence of early sexual maturation particularly in males. Attempts by commercial growers to produce all-female populations (by neomale‚Äö-female crosses) using standard industry techniques were unsuccessful. As maturation has a deleterious effect upon flesh quality, growth and immunocompetency, commercial production was aborted until this issue could be resolved. This study firstly aimed to describe early gonadal development, sex differentiation, developmental and endocrine changes during puberty and throughout the annual reproductive cycle to establish baseline information pertinent to the species reproductive strategies under Tasmanian environmental conditions. Once determined, the results from that study facilitated an experiment investigating the most appropriate timing and approach for the production of neomales. Photoperiod manipulation in mixed populations of brook trout was likewise investigated as a further means of reducing the incidence of early maturation. Sex differentiation and gonadal development in brook trout was studied from 8 degree days post-hatch (¬¨‚à´dph) until the age of 27 months. Gonadal development was histologically studied while plasma profiles of testosterone, estradiol-17˜í‚⧠and 11-KT were measured by RIA and ELISA. Gonadal development began during pre-hatch period and undifferentiated gonads directly developed into testes and ovaries at 3354 ¬¨‚à´dph. Males had attained puberty by the age of 14 months. However, females did not achieve maturation during their first year. During the second year, maturation was observed during March and May for males and females respectively. This study determined that pre-hatch period might be the appropriate time for sex inversion treatment as development of undifferentiated gonads commenced during this time period. Moreover, variations in gonadal development and profiles of sex steroids were controlled by seasonal changes in photoperiod during second year thus suggesting the possible inhibition of maturation under manipulated photoperiod conditions. Neomale production was undertaken by immersion and in-feed treatment (and a combination of both) with 17˜í¬±-methyltestosterone (MT). Sex inversion treatment targeted the pre-hatch, hatch and post-hatch period to determine the most sensitive window of time. Treatment given during 4 to 6 days pre-hatch (at 400 ˜í¬¿g/L MT for 4 hours) was found to be the most successful producing a population containing 75% of male fish without the occurrence of any sterile fish. Feeding fish a MT supplemented diet (3 mg/kg) for 60 days from first feeding did not affect the normal sex ratio and MT oral treatment combined with immersion treatments resulted in high percentages of sterile fish. In the present study, 4 to 6 days pre-hatch was found to be the most sensitive window of time to produce the highest male population with only two immersions thus significantly reducing the dose of MT and treatment duration. Three photoperiod regimes over a 10 month period during the second reproductive year were tested. The regimes were simulated natural photoperiod (NP), advanced photoperiod accelerated by 8 weeks (AP) and continuous illumination (CP). Fish exposed to advanced photoperiod corrected their maturation cycle by an advanced phase shift of their endogenous rhythm. Advanced photoperiod inhibited maturation by 6% and 8% in males and females, respectively, relative to natural photoperiod treated fish in which 100% maturation occurred. However, most of the fish successfully recruited for maturation presumably because the threshold of growth during the critical phase‚ÄövÑvp of photoperiod treatment was surpassed. Similarly, treatment of continuous photoperiod failed to inhibit the onset of maturation however continuous photoperiod did inhibit the final stage of maturation. Overall, the present study provided the baseline data about gonadal differentiation, puberty and annual reproductive cycle of brook trout under Tasmanian climate conditions, reported for first time for the southern hemisphere. Furthermore, this PhD project provided the most efficient and commercially applicable sex inversion protocol to masculinize genetic brook trout females, the most critical step of indirect feminization. Progeny testing by crossing these neomales and normal females can be conducted as a future study to produce all-female population. Furthermore, this protocol may produce 100% male population if applied to gynogenetic females instead of mixed sex culture which needs to be investigated. Moreover, this study suggested the possible success of photoperiod manipulation to control maturation in brook trout but further refinement of photoperiod regimes is required before its commercial trial.