A University of Missouri researcher's strategy to favor male bluegill over females could be the key to establishing a new aquaculture species for Missouri fish farmers.
For the past several years, Rob Hayward, MU associate professor of fisheries and wildlife, has been developing an economically viable system for producing farm-raised bluegill, a popular native sportfish. The major hurdle he has faced is improving the fish's growth rate.
“For farm-raised bluegill to be profitable, you need to grow half-pounders within two years,” he said. “With conventional feeding, it takes about three years.”
To help solve the problem, Hayward is taking advantage of a natural phenomenon, called sexual dimorphism, to bring bluegill from the pond to the plate.
“Sexual dimorphism is a particular trait difference between males and females of the same species. In the case of the bluegill, that trait is size. Males get bigger than females, so it's the males we're targeting for aquaculture.”
Because young bluegill are sexually ambiguous until they are 40 to 70 days old, male-only populations can be produced by treating young fish with androgen, a male sex hormone, effectively reversing the sex of fish that would have become females. While this type of sex reversal is not harmful to the fish and ensures mostly males, it adds another cost to an aquaculture system for producers, Hayward said.
“Instead of using hormonal sex reversal, we've developed a practical grading system based on size,” he said. “Once young fish reach 3 inches in length, we can select males with 80 to 90 percent success using the system.”
Hayward also borrowed from nature to develop a feeding regimen using a biological response called compensatory growth.
“By restricting food for a period of time, then providing the bluegill with a full ration, we tapped into the fish's natural ability to make up for lost growth opportunity,” he said. “In laboratory experiments, fish fed this regimen grew twice as large as bluegill given all the food they wanted.”
With an optimal feeding regimen developed, Hayward is now putting his system to a real-world test. This summer, he is stocking two bluegill populations — one all-male and one male-female — to determine if half-pound bluegill can be pond-raised within two years in Missouri.
The project is funded through a U.S. Department of Agriculture grant in collaboration with researchers at Ohio State University.
“We know the regimen works with individual fish in the lab, but when you put them in groups, the dynamics can be different,” he said. “One issue we'll be monitoring closely is density. If it's too high, aggression among males increases and subordinate males won't grow as fast.”
Work continues in the lab, too, as Hayward attempts to solve another aquaculture problem: reducing the amount of wasted fish food.
“Feed is the largest expense for a fish farmer, and in some systems, up to 20 percent is wasted,” he explained. “Not only is this money right out of the producer's pocket, but the waste can promote algal growth that can lower the oxygen content in the water, which can hurt fish growth rates.”
One cause of wasted food is that fish, like people, eat different amounts of food on different days. Hayward believes this waste could be reduced by adjusting the compensatory feeding regimen.
“Rather than feeding the fish to satiation, we'll leave them a little hungry,” he said. “That way, they'll be less likely to waste food, and it should allow us to maintain good growth rates.”
Jason L. Jenkins is a senior information specialist, University of Missouri Extension and ag information (JenkinsJL@missouri.edu).