Author: Mackenzie E. Miller

Publisher:

Published: 2020

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Aquaculture is the fastest growing food production sector globally, with freshwater fish making up the majority of aquaculture production today. An important commercial species throughout the Great Lakes with current demand as both a game and food fish, is Yellow perch Perca flavescens. Yellow perch is a species well-suited for commercial aquaculture; however, there are several challenges to perch culture operations that have limited further growth of this species commercially. One challenge is dimorphism for body size in this species; males grow at a much slower rate (30%) and have a shorter life span than females. Another challenge is that the onset of sexual maturity begins before male Yellow perch reach marketable size (100-150g). The combination of dimorphic growth and the early onset of male sexual maturity results in a more rapid growth rate in female Yellow perch than in males. Therefore, traditional mixed-sex stocks of Yellow perch exhibit large size variation, with half of the stock (females) reaching market size significantly earlier than the other half (males). A possible solution to the challenge that Yellow perch sexual dimorphism presents to culture operations is the production of monosex female populations, as all-female stocks would reach market size faster than mixed sex stocks, with lower overall costs. There are several potential methods to produce all-female Yellow perch. This work focused on the development of gynogenesis methods, as well as the evaluation of the efficacy for temperature exposure. Production of Yellow perch gyngoens will directly result in all-female progeny. However, induction of gynogenesis requires application of physical shocks to newly fertilized embryos, which can negatively impact survival and cause deformities. There is the potential for gyngoens to be sex-reversed to phenotypic males (genetic (XX) females capable of producing sperm), which could then be used to produce an all-female generations that has not been exposed to physical shocks. Application of sex hormones has traditionally been used; however, temperature exposure has also shown to have the potential to produce phenotypic males, which can then be used to produce all-female Yellow perch in the next generation. Access to cryopreserved Yellow perch sperm will be crucial to the application of these methods to industry; therefore, we first improved the effectiveness of simple cryopreservation techniques of Yellow perch sperm by examining different cryoprotectants (dimethylsulfoxide [DMSO] versus methanol [MetOH]) utilizing two simple cryopreservation methods, the effect of absence versus presence of seminal plasma, and the effects of UV irradiation on sperm physiology following cryopreservation. Cryopreservation and cryoprotectants were evaluated based on post-thaw motility as well as on fertilization rate, embryonic survival, and hatching rate. We determined that both simple methods with either cryoprotectant in the presence of seminal plasma are efficient to cryopreserve Yellow perch sperm. We also determined based on survival and growth results from the larval rearing experiment that progeny derived from cryopreserved sperm do not experience growth suppression or higher mortality than progeny obtained from fresh control sperm. Results from the cryopreservation of UV irradiated sperm suggests that the combination of UV-irradiation and cryopreservation does not result in lower sperm motility than is expected after UV-irradiation alone, a promising result for future work with cryobanks for female monosex production. We then successfully produced triploid Yellow perch utilizing heat shocks and raised triploid individuals until sexual maturation in order to evaluate their reproductive ability, as well as the viability and performance of resulting progeny. Successful induction of triploidy was an important first step for determining conditions to achieve gynogenesis in Yellow perch. Further, results demonstrate that triploid Yellow perch females possess well-developed ovaries with no reduction, that are highly capable of being fertilized. However, triploid females are sterile, as majority of progeny produced were aneuploids and mass to total mortality was observed before progeny reached juvenile stage. We then utilized this experience with triploidy to begin developing methods of producing Yellow perch gyngoens. Methods of UV-irradiating sperm of Yellow perch and heterologous sperm were also evaluated. Various heat and pressure shock conditions were determined to be effective in restoring diploidy, including a pressure shock of 9,000 psi applied at 5 minutes post fertilization (mpf) for a 10 minute duration and a heat shock of 28-30°C applied at 5 mpf for a 12 minute duration. Unreliable availability of heterologous sperm from wild sources resulted in majority of gynogenesis trials to be conducted utilizing UV-irradiated sperm of Yellow perch. We found high variability in the required doses for full irradiation of sperm from different Yellow perch males, likely due to high and variable sperm concentration and inadequate dilution. As a result, majority of gynogen groups produced by irradiated perch sperm contained normal, diploid individuals produced by non-irradiated sperm fertilizing an egg. However, when heterologous Walleye sperm was used, the resulting gynogen group contained 100% gynogenetic individuals, as hybrids of Walleye and Yellow perch are nonviable. Based on our results, we suggest use of Walleye sperm (diluted 1:10 with extender and UV-irradiated at 7,000-9,000 J/m2) for production of Yellow perch gynogens. We also produced a series of progenies that included all-female, female-biased, male-biased, and mixed-sex cohorts in order to compare the growth and survival between these groups and further define the benefits of mono-sex culture of Yellow perch. We determined that all-female groups grew significantly better than all other groups, and there were no impacts on survival. Therefore, we can conclude that all-female Yellow perch stocks will increase the profitability and efficiency of aquaculture operations. Finally, we conducted an investigation into the influence of temperature on gonad differentiation in Yellow perch. Research has shown that exposure to high temperature during the labile sex determination phase can lead to the masculinization and production of phenotypic males in fishes. Such a mechanism in Yellow perch would provide an alternative, cost-effective method for farmers and industry personnel to produce their own stocks of neomales, which could then be used to produce all-female Yellow perch. We evaluated the effect of high temperature exposure from the time of first exogenous feeding, to the end of sex determination, in two generations of Yellow perch and observed masculinization effect in 6 of the 11 progenies produced in the first generation. The second generation has not yet reached sexual maturation, therefore, sex ratio results for the F2 will be reported in future studies. This collection of studies provides an integrated, comprehensive approach to the production of all-female Yellow perch stocks for increasing Yellow perch aquaculture production and provides highly valuable and novel information to the industry, which will open new avenues for increasing the production of Yellow perch through intensive culture.