At MinAqua, piling fish losses on the compost heap adds up to nutrient riches

By E. M. Morrison

Renville, Minn. — An old grade school story has it that Indians taught the Pilgrims to plant dead fish alongside their corn and squash seeds. In just a few weeks, natural bacteria and heat transformed the fish into rich fertilizer for the young plants.

In present-day America, MinAqua Fisheries of Renville, one of the largest indoor fish farms in the country, is using the same natural process to dispose of its livestock losses. This fall, with help from AURI, MinAqua will begin composting dead fish.

Composting, long a tool of the poultry industry, is an environmentally safe, economical alternative to burial or incineration, says Jack Johnson, AURI waste utilization scientist. Around the state, AURI projects are demonstrating the benefits of composting livestock carcasses.

Composting makes sense for MinAqua, says Mel Stocks, president and CEO of the 350-member cooperative. The co-op had been burying dead fish on site, but new pollution control regulations forced the company to find another disposal method.

“We had three choices,” Stocks says, “a rendering company, incineration or composting.” Now into its third year of tilapia production, MinAqua’s livestock mortality rate — about 100 pounds a day — is too low to interest commercial rendering companies. And incineration is expensive, he says.

“Composting is a good alternative,” Stocks says. “Fish compost very well, and management is not as difficult as one might think.”

In August, MinAqua raised a 500-square-foot hoop building, enclosing four wooden composting bins. Each day, fish carcasses are deposited in the bins and covered with poultry litter, which furnishes carbon and nitrogen to aid decomposition.

When a bin is full, “we cover it and let it cook,” Stocks says. As composting progresses, temperatures reach 130 to 160 degrees, killing harmful bacteria. “It all happens naturally,” he says. “Byproducts are water and carbon dioxide.”

Within six months, Mother Nature will have transformed MinAqua’s dead fish into a new product — clean, quality fertilizer rich in organic matter and nutrients. Managed properly, Johnson says, the whole process is odorless and free of vermin or pollutants.

MinAqua’s compost facility, which cost about $6,000, is patterned on a plan AURI helped develop several years ago. “We designed low-cost structures that could be adapted from existing farm structures or built for $2,000 to $10,000,” Johnson says. “We put up a number of them as demonstrations.”