Sea Grant Institute Research
Producing Perfect Plankton for Picky Perch
By Aaron Conklin
Among fish farmers, it’s known as “the bottleneck,” the six-week period between the point when a fish fry is released into a pond and the point when it’s grown large enough to become feed-trained. For many fish farmers, it’s the make-or-break phase of the operation—will enough fish survive for me to turn a profit? Chris Hartleb, professor of fisheries biology at the UW-Stevens Point, recalls a conversation he had with a yellow perch farmer a few years ago.
“I asked him, ‘How many fish do you put in your pond?’” recalled Hartleb. “He told me, ‘I put half a million in, and I expect that after six weeks I’ll only get 10,000.’ These farmers are massively overstocking their ponds just so they’ll meet their quota. That’s such an inefficient way to operate.”
The problem stems from the fact that yellow perch, the delicious staple of the Midwest’s Friday night fish fry, aren’t as domesticated a fish species as catfish or rainbow trout. In other words, perch fry first have to learn to feed on tiny zooplankton produced by specific types of algae. If the zooplankton aren’t present or develop too late, the perch will die. Often in disturbingly large numbers.
“Getting the first feed is a very tricky proposition,” said Hartleb. “Yellow perch are extremely finicky and are gape limited; that is, they have a very small mouth for a predator.”
Fueled by funding from Wisconsin Sea Grant, Hartleb set out to determine whether organic or inorganic fertilizer would promote the particular type of algae and zooplankton fledgling perch need to survive.
Over a two-year period, Hartleb and his research team used 34 ponds, located at the Lake Mills State Hatchery and Coolwater Farms in Deerfield, Wisconsin, to compare the results of organic fertilization, a process that involves allowing manure to decay in the pond, and inorganic fertilization, a process that involves mixing nitrogen, phosphorous and carbon and spraying the pond water.
The results may surprise those who’ve come to believe that “organic“ always equals “better.”
“What we found is that inorganic fertilization resulted in the proper type of algal blooms, and, as a result, a far greater survival rate for the perch larvae,” said Hartleb.
The difference boils down to timing and type of algae. It takes far longer—around 8-10 weeks—for the organic fertilizer to break down, enrich the pond water and produce algae blooms. By that time, large chunks of the perch fry population have starved and died. Inorganic fertilizer moves more quickly to produce a particular type of cladoceran plankton called Bosmina. Bosmina is the right-sized zooplankton perch fry need to thrive. Organic fertilizer favored a different type of plankton called Copepod that’s far less desirable to the perch and resulted in lower survival.
“Using inorganic fertilizer yields a twofold benefit: it produces quickly and it produces the appropriate type of algae,” said Hartleb.
Hartleb’s original research proposal aimed at calculating the proper ratio of nitrogen, phosphorous and carbon to create a successful inorganic fertilizer formula. He hoped that a process used by fish farmers in the South would also work in Wisconsin, but it wasn’t to be. Wisconsin’s colder water disrupts the absorption process.
“There are rough guidelines for the ratio,” said Hartleb. “But a lot of it is still trial and error. Due to a variety of factors, each farmer’s pond behaves differently.”