Aquaculture for all

Secondary school: learning to utilise waste in RAS

Recirculating aquaculture systems (RAS) Aquaponics Oysters +9 more

The use of aquaculture-derived waste products to grow secondary crops – such as used in biofloc systems or aquaponics – can increase profits by more than 20 percent, as some of the leading proponents of this field explain.

by Aquaculture consultant and producer
Dr Bill McGraw thumbnail

Growing plants from waste produced by aquaculture has been practised for thousands of years. Seaweeds grown in nutrient-rich coastal waters have always been the biggest aquaculture product by weight, accounting for 90 percent of the total world aquaculture production. However, according to a recent article in the European Journal of Phycology, climate change is threatening the natural, uncontrolled production of seaweeds in coastal areas. The production of plants in closed aquaculture systems provides a more continual, sustainable increase for this major field of agriculture. Growing valuable biomass from excess wasted nutrients can provide some of the greatest increases in efficiency and profitability as well. As disease-free, zero water-exchange culture systems always typically have excess, potentially toxic, nitrogen, producing valuable secondary crops from this waste has proven multiple benefits.

Tim Dobbins, Vice President of Education at Nelson and Pade, Inc., shows a tilapia grown in aquaponics at Nelson and Pade’s Demonstration greenhouse in Montello, Wisconsin

© Dr Bill McGraw


Aquaponics is currently one of the most popular forms of secondary crop production. Interest in zero water-exchange aquaponic systems is currently exploding and at the forefront is Nelson and Pade Aquaponics Inc, the largest supplier of aquaponic systems, training and supplies in the world, with 700 complete aquaponics units sold to customers from 27 different countries.

Rebecca Nelson and John Pade formed the company in 1994 after running a hydroponics company during the previous decade. Studying the work of Dr Jim Rakocy at the University of the Virgin Islands, they created their own system designs, establishing a commercial US aquaponics industry while disseminating valuable information for this burgeoning new science by publishing the bimonthly Aquaponics Journal. The company is currently working with the University of Wisconsin-Stevens Point in creating courses that teach all the fundamentals as well as advanced training.

There are several important advantages to aquaponics which have made it very successful. Having regular daily harvests to continually supply fish and vegetables directly to market is a big plus, as aquaponics facilities are typically in a greenhouse or indoors and regularly accessible year-round. Unlike traditional hydroponics, there is minimal need for any ion supplementation (calcium, magnesium and potassium). Aquaponics uses a minimal amount of water compared to traditionally grown agriculture crops and there is no need for any fertiliser, as all plant nutrition comes from fish waste. Aquaponic systems typically are continually disease free as long as biosecurity and good management practices are maintained.

The biggest challenges to aquaponics companies in the US are obtaining funding and dealing with regulation. Start-up costs are typically high and obtaining loans for a newly formed operation is often difficult as aquaponics is a relatively new commercial industry. In regards to the newly developed 100 tonne salmon and trout aquaponic system in Wisconsin, which is touted to be the largest aquaponics facility in the world, Nelson had this to say: “The company has a 4 acre facility which reportedly cost $100 million? This extremely high start-up cost is not typical and we have seen failures when others have over-spent up front – at the end of the day the return on fish and vegetables has to account for that.”

Tilapia are well suited to farming in conjunction with greens in aquaponic systems

© Dr Bill McGraw

Regulations in the US for establishing an aquaponics facility are cumbersome. However, in regards to profitability Nelson states: “Aquaponics has the biggest commercial potential of any RAS polyculture or multi-trophic aquaculture system, as plant crops produced from fish waste are very valuable in the local market and they grow much faster than fish.”

In Wisconsin alone, aquaponics provides millions of dollars of fresh organic fish and vegetables shipped directly to local markets. The major fish and vegetable species are tilapia and fancy lettuce which sell, respectively, for $4 per pound and $4 per head. Total variable costs are less than $1 per pound of product and the total time to pay off all investments is between three and five years. The current demand for fresh products grown in aquaponic systems is bigger than what the current industry can produce and, according to Nelson, the market potential is the easiest challenge to conquer. Fresh products from aquaponics can be found throughout the state, with over a dozen commercial systems operating with the Nelson and Pade designs.

The most important skills to consider for a successful commercial aquaponics are variable. Business acumen in establishing reliable markets for organically produced products, while being knowledgeable in the science of water quality are two important attributes. According to Nelson, the management of solids produced from the animals is one specific process that needs to be designed correctly and mastered, while another key element is learning how to maintain the proper bacteria associated with nitrification, denitrification and mineralisation, in order to maintain the proper nutrition – created from fish waste – for the plants. Daily operations require periodic checks on dissolved oxygen and various forms of waste nitrogen and on pH. In the end there is a complementary balance that must be achieved between the fish and vegetable sections of the system.

Algae and seaweeds in ponds and tanks

There are multiple driving forces promoting secondary crop production in aquaculture systems. The limit of fishmeal used in aquaculture feeds sourced from the ocean has been estimated to be about 5 million tonnes according to IFFO, the Marine Ingredients Organisation. Yet, according to many sources, aquaculture production is expected to double during the next two to three decades to meet the protein demands of a rapidly growing human population. Complicating this problem are the increasing catastrophic weather events associated with global climate change, contributing to a lack of continual predictable harvests from many areas of agriculture. This situation has spurred the development of a variety of alternative protein sources for use in aquaculture feeds such as insects, algal and biofloc meals and byproducts of aquaculture and fisheries processing. Another important point often overlooked is the issue of biosecurity, paramount in many areas of agriculture to prevent the vertical transfer of disease from the use of same-species byproducts in feeds that have not been properly sterilised.

Research on RAS and ponds alike has shown that algae naturally produced in aquaculture ponds and tanks from feeding a primary crop can feed an equal amount of secondary crop biomass such as tilapia. As feed is typically the biggest variable cost in aquaculture production, it makes economic sense to utilise algae, which is a natural sink for nitrogen. Nitrogen is the most expensive element in aquaculture feeds and the one that forms the most toxic compounds in intensive systems (ammonia, nitrite, nitrate). Research has shown that intensive systems can have 70 percent of the total nutrient input from feed stored in suspended algal biomass. In turn, 60 percent of the total nitrogen input can be utilised by integrating a variety of secondary herbivorous and plant crops in a multi-trophic system. Moreover, plants and algae-eating bivalves have been documented to grow 50 percent faster when fed aquaculture effluents compared to the same species grown in the wild. The amount of secondary crops that can be grown from aquaculture waste, such as oysters and macroalgae, has repeatedly been documented to be five times that of the primary target crop.

The author has expereince of growing oysters (C. virginica) in floating raceways inside 1,000m2 shrimp ponds

He noted that "these animals grew much faster than those found in the wild, with no feed and very limited labour costs involved" © Dr Bill McGraw

The first experience I had with the utilisation of algae to produce a secondary crop was during my graduate studies where juvenile oysters (C. virginica) were grown in floating raceways inside 1,000m2 shrimp ponds. These animals grew much faster than those found in the wild, with no feed and very limited labour costs involved. In turn, as the oysters removed algae from the pond, they removed excess nitrogen and phosphorous while helping to maintain faster-growing and more nutritious algal populations, which would provide multiple benefits in terms of shrimp nutrition. Algae is a valuable component in the diet of shrimp and one proven to be boon for faster growth.

This multiple benefits of a secondary crop species would inherently improve the economics of any aquaculture operation. Published research has shown that the yearly revenue of aquaculture ventures can be increased by more than 20 percent through direct sales and complementary efficiencies, as well as the benefits outlined above and elsewhere. As diversification is always beneficial in farming practices, investing limited capital to produce a secondary valuable crop to increase revenue – while boosting the growth of the primary crop at the same time – makes good business sense.

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