Members of Kevan Main’s aquaponics team demonstrate cooking techniques for nutritious sea purslane they are harvesting from a prototype system which grows fish and vegetables in a saltwater environment.

Researchers at Mote Marine Laboratory are testing a novel farming system that grows marine finfish and saltwater veggies — all at the same time.

With Florida Sea Grant funding, a team of investigators at Mote’s Aquaculture Research Park are developing a prototype of an aquaponics system,  a hybrid of aquaculture, or fish farming, and hydroponics, growing plants in water without soil. In aquaponic systems, fish and plants rely on each other to thrive, much like in the wild. Waste produced by fish in grow-out tanks helps fertilize the plants, and the plants help purify the water recirculated to the fish.

What sets Mote’s system apart is the harvest — saltwater fish filets and sea vegetables. Typically, aquaponics systems have been primarily developed for production of freshwater fish and vegetables.

“We’re trying to build off of the ‘farm-to-fork’ concept that is so popular right now, and produce locally grown fish and vegetables for distribution to local markets,”  said Kevan Main, director of Mote’s aquaculture research program.

Main and her team have completed preliminary studies that have tested smaller benchmark systems, and she says the outlook is positive for a new, bigger operation.

The prototype system will consist of three fish tanks that are about eight feet in diameter and four feet in depth. The goal is to produce 600 market-sized red drum the first time around. The system was constructed in a greenhouse that also contains two raceways, or rectangular plant troughs, growing seaweed and sea vegetables.

Production begins in the fish culture tanks containing red drum fingerlings. Water flows out of the fish tanks and runs through a swirl separator, which removes the solid waste. Then, an additional filtering step is used to convert the ammonia produced by the fish into nitrates, a form of nitrogen usable by plants.

Next, the water with the nutrients hits the plant raceways. The plants take up the dissolved nutrients in the water, and also filter additional forms of nitrogen produced as the water circulates.  After that, the filtered water goes back into the fish tanks. The separated solid waste is used as fertilizer for other marine plants at restoration sites around the state.

“Just by introducing the plants, you’re not only producing fish to go to market, you’re also producing plants to go to market,” Main said. “I’m able to use the nutrients from the feed to feed the fish and then the nutrients that come from the fish tank system to feed the plants. You’ve produced two things for the price of one.”

In this zero-discharge system, all of the water is reused. Small amounts of fresh water are added to make up for whatever water evaporates.

“This particular system is designed to be as simple as possible but to complete all the necessary steps,” Main said. “And it’s both simple and low-cost. We want to make sure we have as little pumping with the system as possible.”

This project comes after a renewed enthusiasm and interest in aquaponics, which has been growing around the world for the past several years. Small and large projects are underway in cities and agricultural areas in the United States, United Kingdom, Australia, Israel, Canada, Nigeria, Abu Dhabi, and others. Most of the projects, however, have focused on producing freshwater fish, such as tilapia and catfish, and vegetables, such as lettuce, tomatoes and herbs.

“What I’m trying to do is look at opportunities to expand aquaponics technology to incorporate saltwater fishes,” Main said.

The project team includes engineers from the University of South Florida in Tampa and aquaponics experts from Morningstar Fishermen in Dade City. Graduate, undergraduate and high school students are all participating in collecting data and studying this system.  Main said if the prototype system is successful, she hopes similar aquaponics operations will be implemented across the country.

“Our goal is to develop marine aquaponics technology that can be implemented on a small scale, not large megafarms,” she said. “This small-scale production unit could be located in cities across the country providing local fish and sea vegetables to local markets.”