The Potential of Finfish Aquaculture
As wild fish stocks face stresses from overfishing and the changing environment, aquaculture is getting more attention as a sustainable addition to existing fisheries. Over the past few years, researchers at the University of New Hampshire (UNH) developed a finfish aquaculture system that is both sustainable and designed for small growers. Their system, which is called the AquaFort, is an integrated multi-tropic aquaculture (IMTA) system, meaning that it contains species from different levels of the food web in one system. The AquaFort that researchers built at UNH grows steelhead trout, blue mussels, and sugar kelp.
Nutrient-Neutral Finfish Farming
Kelp and mussels are both excellent at absorbing nutrients. Kelp is great at absorbing inorganic compounds, and mussels filter feed on organic materials. On the AquaFort, these nutrient-absorbing species offset the nutrients excreted by the steelhead trout and from excess feed. From laboratory experiments, the UNH research team determined that three pounds of mussels and kelp absorbs the nutrients emitted by one pound of steelhead trout. By growing these species together in the right proportions, the AquaFort design has the potential to mitigate eutrophication, or the build-up of harmful levels of nutrients, especially nitrogen and phosphate, into the water. The research team at UNH is constantly monitoring and improving their methods to continue working towards a nutrient-neutral farm system.
While recent years have seen some finfish aquaculture farms adopt nutrient control systems, such as the use of underwater cameras to deliver more precise amounts of feed, the IMTA AquaFort is designed to neutralize nutrient pollution on a scale that doesn’t require much complicated machinery. The fish are fed by hand and their feeding behavior is closely monitored to ensure they do not overfeed. This helps minimize excess feed ending up in the environment.
How the AquaFort Works
The AquaFort is a raft about 52 feet long and 28 feet wide. The system holds two fish nets in the middle and mussel and kelp lines along the perimeter. The fish nets are 20 feet square and extend 15 feet down into the water. A net runs over the top of the fish pens to keep birds from flying in and the fish from jumping out, and a deck extends around each of the pens where workers can stand to feed the fish or perform maintenance tasks. The outside of the deck is lined with weighted dropper lines extending downwards with kelp and mussels attached.
The trout are sourced from a nursery in New Hampshire and transferred to the AquaFort pens at around 8 months old. Instead of starting the fish growing process, which is referred to as “grow-out,” in the spring like most farms do, the UNH team found that a late fall grow-out helped avoid the stresses caused by summer water temperature fluctuations. The mussels that are grown off the raft start as wild baby mussels called spat. Fuzzy ropes are placed in the ocean when the temperature is right for mussel spawning to collect spat, then the ropes are hung off the raft to culture the mussels. Kelp usually starts its life in a hatchery, where wild kelp is spawned and placed on twine that can be wrapped around the kelp line when it’s deployed.
The AquaFort Produces High Quality Products
In addition to being a sustainable aquaculture option, multi-trophic aquaculture also produces high quality products. Both the mussels and steelhead trout grown in the system benefit from cultivation in close proximity to other species.
The trout benefit from the excellent water quality produced by the kelp and mussels, as well as the high water flow rates through the AquaFort nets. Because the AquaFort is moored on both sides, it stays stationary as the tide moves water through it. When the system is placed in an area with lots of moving water, the moving water allows the fish to swim constantly, which keeps them healthy. These factors create a great tasting trout that has high demand at the market.
Mussels have been found to grow stronger and healthier in the presence of kelp. A recent study at Bigelow Laboratory for Ocean Sciences found that kelp reduces the acidity of the surrounding water, creating a “halo” of water that’s less affected by ocean acidification. Because of this, the mussels on the AquaFort experience excellent growing conditions.
Locally Owned and Operated Seafood
The team at UNH developed the AquaFort with small communities in mind. The system is designed to be operated as an owner-operator businesses, which is what local fishermen are usually familiar with. Many small Maine coastal towns rely on fishing for their income. By diversifying to aquaculture (one AquaFort set-up offers the chance to grow three different crops!) then these communities could ensure the stability of their income in the years to come. The small scale of the system is ideal for island communities, and their existing connections to local seafood markets is just what the AquaFort was intended for.
The daily maintenance on the AquaFort is mostly straightforward. The fish have to be fed twice a day (less in the winter) and the nets need to be cleaned to get rid of biofouling about once every two weeks. Cleaning has to be done by a diver, and switching the nets is an involved process that requires 4-6 people. Considering that the entire raft can be left in the water year-round and produces thousands of pounds of trout, blue mussels, and kelp per year, the maintenance requirements are relatively manageable for a small crew or as a diversification tactic for fishermen – such as maintaining the AquaFort on their way out to their fishing grounds.
Steelhead trout is viewed as a tasty alternative to salmon, which is rarely a local product and is often imported into New England. Not only is eating local a proven way to make your diet more climate-sensitive, but it’s also better for the local economy. The AquaFort gives communities the chance to grow, process, and eat locally source sustainable finfish (as well as kelp and mussels) instead of importing fish from out of state.
Integrated multi-trophic aquaculture like the AquaFort offers a creative way to sustainably produce finfish, and do it at a scale that is designed for small businesses. If the future of sustainable seafood is going to include a combination of wild-harvested and aquaculture products, then this is a great way for coastal Maine communities to meet both their economic and ecological needs.
Innovations articles highlight creative devices or methods on aquaculture farms. They are informed by farm visits and interviews with experienced growers. See our About page for more information.