Just as the waste nutrients of terrestrial fertilizers can stimulate harmful blooms of microalgae when they wash into aquatic environments, the nutrient-rich wastewater from open-system aquaculture operations can stimulate similar harmful eutrophication events, when not properly managed. The increasing implementation of indoor recirculating aquaculture systems (RASs) has been used as a solution to mitigate this issue for both marine and freshwater aquaculture. But it’s challenging to remove the excess nutrients – which would harm cultured fish – from the water before recirculation.
Whilst RASs can effectively eliminate environmental and public health concerns over farm nutrient emissions into the aquatic environment, the quality of recirculated water must be precise in order to maintain fish health. This problem has been addressed in the past by using biofilters – living microbial colonies that remove excess nutrients, such as nitrogen and phosphorous – however, an interdisciplinary research team at the Technical University of Denmark has shown that the sustainability of these systems can be taken one step further.
The team used a combined approach of using microalgae to bioremediate the recirculated water, and bio-harvesting of these algae by zooplankton to maximize resource recovery, with the zooplankton being a suitable live feed source, particularly for fish larvae.
Like terrestrial plants, marine microalgae require nutrients such as phosphorous and nitrogen – two of the most significant nutrient emissions from finfish aquaculture – to grow, and researchers have exploited this trait to remove excess nutrients from recirculated, nutrient-rich water. Using the algae species Chlorella vulgaris and Scenedesmus dimorphis, the researchers managed to extract 100 percent of the phosphorous and 70 percent of the nitrogen from recirculated water sourced from a freshwater RAS operation.
Although the use of microalgae to extract excess nutrients from RAS facilities is not a novel technique, the challenge of this approach lies in the harvesting of the algae after growth. The researchers overcome this challenge, however, simply by exploiting the natural behavior of the zooplankton species Daphnia magna – the humble water flea, which grazes on freshwater algal communities. After harvesting of the algae by D. magna, all phosphorous and 90 percent of excess nitrogen had been assimilated by the water fleas, showing promise of this approach for efficient nutrient removal prior to water recirculation, and the potential for commercialization of this approach.
“To harvest the Daphnia we just used sieves, and we could even use this method to separate adults from neonates – this is the best approach. This is much easier than harvesting algae, which often requires more time, energy and resources,” explains Professor Borja Valverde-Perez, the corresponding author of the study.
The researchers have proposed that the D.magna populations fed on experimental diets of microalgae grown on recirculated water may provide a suitable feed substitute for fish meal, with the fleas having a suitable protein content of 30 percent dry weight, and favorable fatty and amino acid profiles, including high levels of lysine and methionine , which are often the most limiting of the essential amino acids.
The researchers believe that D.magna fed on RAS-grown microalgae would be a suitable feed for omnivorous fish and shrimp. Not only would this feed source have the potential to reduce costs to aquaculturists on a commercial scale, but would also reduce pressure on wild fish stocks, which are a key source of fish meals used in commercial feeds.
“I understand it is a good live feed, and is better than using the algae itself as a feed, due to better palatability and digestibility. As a live feed, Daphnids are particularly good for fish larvae, as the fish have to swim to catch them, which keeps the fish larvae active,” says Professor Valverde-Perez.