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? And it’s also what leads to conversations like the one Chris Hartleb, professor of fisheries biology at the University of Wisconsin-Stevens Point, 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 and desirable 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 the University of Wisconsin Sea Grant Institute, Hartleb set out to determine whether organic or inorganic fertilizer would promote the particular type of algae and zooplankton fledgling perch need to survive–and, by extension, a larger fingerling survival rate for fish farmers.
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 research has shed important new light on a fish-farming process that’s still developing best practice models. Some farmers, for instance, make the mistake of assuming that the key to success is as simple as achieving green pond water.
“The “green water” phase is actually a false assumption,” Hartleb said. “The perch want the plankton that eats the algae, and just because you have green water doesn’t mean you have the right type of algae.”
In Wisconsin, fish farming is big business. The state is home to 2,300 fish farms, around 50 of which are dedicated to growing feed-trained yellow perch fingerlings that are then sold to larger aquaculture outfits and raised to market size. Wisconsin is the largest producer of farmed fish in the Midwest, to the tune of $2.1 million a year in revenue.
Obviously, the financial stakes for successful fish farming are high. Yet Hartleb said farmers have been initially resistant to alter their organic-based approaches, even in light of his research findings.
One issue is cost: Inorganic fertilizer is nearly ten times more expensive to purchase than organic, especially when some fish farmers have a cheap and ready supply of cow manure nearby.
The other is simple intransigence: “Famers tend to be resistant to change,” said Hartleb.
Hartleb is hoping that time, experience and information will alter that. His original research proposal aimed at calculating the proper ratio of nitrogen, phosphorous and carbon to create a successful inorganic fertilizer formula. Hartleb 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.”
Now that Hartleb has solved the fertilization question for yellow perch, he’s turning his attention to how best to create favorable farming conditions for walleye, a fish that poses a quite different challenge.
“Walleye are night feeders, so what we’ve had to do is find a way to make the ponds darker,” explained Hartleb. “We’ve focused on increasing the turbidity of the water. But what does it shade? The algae, which is what is needed to fuel the food chain to feed the walleye.”
SIDEBAR: Tanks for the Advice
In addition to the question of organic versus inorganic fertilizer, Hartleb partnered with Jeff Malison, retired director of the University of Wisconsin-Madison’s aquaculture program, to tackle a different either-or question in the fish-farming universe: Which kind of water was better for feed-training yellow perch grown in indoor tanks—pond water or heated municipal water?
The latter option won out, largely because in several studies, the pond water developed an army of deadly fish disease organisms. Interestingly, it’s not an inherent issue with the pond water, but rather what happens after it’s transferred to the tank—high concentrations of substrate and organic particles cause the disease organisms to multiply. Sand filtration may help reduce the risks.
