Bighead carp (Hypophthalmichthys nobilis) is native to Asia and was introduced to the United States as a species of interest for aquaculture in the 1970s but escaped from captivity soon after. This species is currently widely spread throughout the Mississippi River Basin and may already be present in the Great Lakes, which could impact this important sport fishery. Current control efforts have focused on preventing further spread through constructing barriers and physical removal of adults from rivers. Alternative methods for biocontrol of invasive fish species are becoming increasingly attractive and show promise as another tool that can be used to control or eliminate invasive populations. These methods rely on genetic technology designed and implemented in aquaculture, such as sex reversal, polyploidy and mass offspring production for stocking. A series of experiments were conducted to investigate novel aspects of sperm storage to perform in vitro fertilization and intensive larval/juvenile rearing of bighead for application in biocontrol stocking programs.
The first experiment addressed the short term storage of sperm on ice and cryopreservation. There is a lack of available information on the short term storage of silver carp (Hypophthalmichthys molitrix) and bighead carp sperm. The quality of fresh silver carp sperm stored without dilution in a sperm motility inhibiting (extender) solution was found to decrease rapidly after 24 hours becoming agglutinated or forming clumps by the third day. When sperm was stored in an extender solution, 45 % motility was maintained after 3 days. The trials involving cryopreservation investigated the applicability of utilizing original or modified protocols previously described for common carp and silver carp. The trials conducted with modified protocols were not successful, as post-thaw motility was sporadic or absent. However, the trials with original protocols using koi carp sperm identified post-thaw motility of at least 15 %. The protocol described by Bernath et al. (2016) had significantly higher post-thaw motility (75 %) of koi carp sperm than the other three protocols examined. A third trial with original protocols using silver carp sperm was also conducted but data is not yet available. Fertility trials were also conducted during the storage of sperm which utilized zebrafish Danio rerio ova as a surrogate. For the first time, silver carp sperm was found to successfully initiate the development of zebrafish ova with surviving hybrid embryos/larvae identified up through 48 hours post fertilization.
The second experiment attempted to optimize larval rearing of bighead carp by investigating a novel static method. I explored several environmental conditions, such as utilizing 24h light, salinity (feeding), algal turbidity, and continuous food availability as compared to a traditional water recirculating system. Various levels of salinity (2, 3, and 5 ppt) were examined in the novel rearing environment. A 2x2x3 full factorial ANOVA design was implemented to compare the main effects of larval rearing system, feeding regime, and stocking density (12.5, 25, and 50 fish L-1) and the interaction effects between the main effects on survival, biomass, length, weight, proportion of large individuals, and specific growth rate. A desirability function was then constructed that would maximize survival (%), biomass (g L-1) and proportion of large individuals to optimize larval rearing conditions. These data can be used in further experiments addressing hormonal sex reversal. Elevated salinity, 3 and 5 ppt, was found to increase survival (51.3 ± 3.5 % and 64.3 ± 10.1 %, respectively) and proportion of large individuals (0.76 ± 0.08 and 0.82 ± 0.02, respectively) as compared to 2ppt (33 ± 3 % and 0.63 ± 0.03). The desirability function identified the static rearing system, with high stocking density (50 fish L-1), initially fed Artemia nauplii as having the maximum desirability (0.733) for the selected variables. In addition, this methodology could be applied to any larval rearing program to select for conditions favorable to specific desired outcomes.
The third set of experiments consisted of two weaning trials of bighead carp, i.e. transitioning from live food to artificial diets. The first trial examined if larval rearing conditions (stocking density and live feeding regime) influenced the success of weaning bighead carp to a commercial diet (OTO). The second trial investigated how two formulated weaning diets, which included spirulina (SPN) or soybean meal (SBM), compared to the commercial diet. Aspects of growth, survival, feed conversion ratio, mineral concentrations of fish, and the rate of deformities recorded, were assessed. For trial 1, no significant effects of initial larval rearing conditions were identified between any of the parameters measured. However, the rate of deformities observed in fish at its conclusion was high (37 %). In the second trial, significant differences were observed in the growth, survival, mineral concentrations of fish, and the rate of deformities. Results for the OTO diet were similar between trials 1 and 2 and it had the best overall performance measured in the trials. Growth rates were similar between the two formulated diets but survival of SBM fed fish was significantly lower than OTO and SPN. There was an increased rate of deformities recorded for the formulated diets compared to control. This was most likely due to mineral imbalances which were recorded. The presumed cause of these severity of deformation was from an excess of zinc, which has previously been linked to vertebral deformities in another cyprinid species, the common minnow (Phoxinus phoxinus).