EVALUATION OF THE IMPACT OF PROLONGED HIGH TEMPERATURE EXPOSURE ON GONADAL DIFFERENTIATION IN JUVENILE YELLOW PERCH Perca flavescens

Yellow perch (Perca flavescens) are a popular food fish in the Great Lakes region. In this species, females grow faster and to a larger size than males, making them more valuable in aquaculture. In fact, a population of 1,000 female yellow perch can gross approximately $555 more than the same number of fish exhibiting a 1:1 male:female ratio at the current fillet value of $14 per pound. Attempts have been made to produce monosex populations of fish through sex reversal. This has been achieved in several species through environmental manipulation during sexual differentiation, with the most prevalent environmental factor being temperature. However, sex reversal in yellow perch has only been carried out by use of hormone treatments, and published information suggests attempts have only been successful in producing reproductively non-functional sex reversed males. In the current study, we assess if the exposure to steroid hormones could have been impacted by high temperatures during the period of gonadal differentiation and influenced sex ratio of juvenile yellow perch.

A first experiment was conducted in 2015 to determine the efficacy of hormonal sex reversal in juvenile yellow perch less than 16 mm total length (TL). The tanks were maintained at a temperature of approximately 21.8 ± 0.7^o C in order to maximize growth. The sex ratio of the experimental groups (control, MT-Diet, MT-Immersion, and E₂-Immersion) was evaluated in January 2016. The control group was found to have a significantly greater percentage of spermiating males than the expected 50% (Fig. 1). Therefore, it was hypothesized that the high rearing temperature induced masculinization.

Eggs from our domesticated females were fertilized with 2015 "control" males (n = 12), and embryos were incubated in 10.3 ± 0.6^o C until hatching. Consequently, offspring from five males were raised in high (23.1 ± 0.2^o C) or moderate (16.4 ± 1.0^o C) temperatures from 33 to 55 days post-fertilization. The fish are now in the growout phase, with all groups at the optimum temperature for growth (23.5 ± 0.6^o C). Analysis of the sex ratio of the groups, and the subsequent conclusions that can be drawn, are to be completed in December of 2016.