Turning effluent into food for fish

Nature doesn’t waste a thing, and a new initiative on a Katikati farm to convert dairy effluent into fish food aims to replicate that practice.

Jointly funded by the Bay of Plenty Regional Council and Ministry for Primary Industries’ Sustainable Farming Fund, and involving NIWA and DairyNZ, the trial converts nutrients from dairy effluent into algae and zooplankton to become feed for wild fish.


One green bottle of ‘good’ algae – Dr Rupert Craggs, a principal scientist and manager of NIWA’s Aquatic Pollution Group, explains the workings of the algae ponds on the Noble farm.

BOPRC rivers and drainage manager Bruce Crabbe says the idea was inspired by the work of freshwater scientist the late Charlie Mitchell of Raglan, who was well-respected for his 40 years of freshwater fisheries research.

While spreading dairy effluent on pastures is the accepted use, nutrient leaching is still an issue – and Bruce says investigating other uses for the effluent has merit. The potential implications of this project for farmers around the country are significant, he believes.

“A successful trial will provide options for resilient and sustainable farming systems that reduce nutrient loss, improve water quality and aquatic habitats, and provide additional income from aquaculture.”

Significant benefits
The trial, which began with construction of special algae ponds mid-2015, at a cost of about $100,000, is in its early stages but should it prove successful, could see dairy farmers, aquaculturalists and the environment benefit from a new use of existing technology.

“It is still early days for the project but if everything works as it should, we expect to see improved water quality in the drainage system and an increase in numbers of fish species such as tuna (eel), inanga (whitebait), and mullet.”

In late-February an open day to outline the trial results to date was held on the Matahui Rd farm owned by Ian Noble. He was approached to allow the trial on his property because it has on its boundaries the Aongatete Stream, two roads and kiwifruit orchards, making it effectively isolated from other dairy farms.

Bruce is pleased with the open day and the enthusiasm of visitors to the farm.

“The project is really just kicking off so we don’t have any long-term monitoring results or data to share. But having the ponds built and some of the associated infrastructure in place meant that we could have some great in-depth conversations about what the project expects to achieve and some of the blue-sky possibilities for the technology.

“Farmers showed a natural interest in the economic possibilities around creating a feed source or energy source from the ponds or selling the products produced. While the project isn’t focused on that aspect of the process, it was great to see visitors catch on to the possibilities that open up if we can show that the technology works in this trial.”  

On the Noble farm, sharemilker Andrew White milks 240 cows through a herringbone dairy and effluent is captured in an existing pond, before being irrigated on to pasture.

The project has involved the construction of two purpose-built high rate algal earth ponds into which some of the effluent is pumped. Under the oversight of NIWA, these ponds have been set up to encourage the growth of algae.


Two purpose-built high rate algal earth ponds on the Noble farm near Katikati are the first stage in a system to turn effluent into fish food. 

Green algae
Dr Rupert Craggs, a principal scientist and manager of NIWA’s Aquatic Pollution Group, told field day attendees that two small paddlewheels, one in each large U-shaped pond, keep the liquid moving which adds oxygen to the water and encourages the growth of ‘good’ green algae, while discouraging the growth of unwanted anaerobic algae.

During the process algae takes up ammonia, phosphorus and carbon, helping to improve water quality.

On a regular basis, the algae is pumped to small inverted funnel-shaped algal harvest ponds, where it is settled and harvested and then piped in slurry form to a separate zooplankton pond constructed within the farm drainage system.  

Charlie Young, of Raglan Eels, a colleague of Charlie Mitchell and the company’s director of business management, product development, tourism and marketing, was also among experts to speak at the field day.

Charlie Mitchell, he says was a kind of “mad scientist” like Doctor Emmett Lathrop ‘Doc’ Brown in the movie ‘Back to the Future’. Charlie was continually coming up with new ideas and finding innovative solutions to challenges, many of them driven by lack of funds.

Whitebait godfather
“He was the godfather of whitebait and the first person to successfully achieve life-cycle closings of any marine migratory native fish when he spawned and reared common whitebait, banded kokopu and giant kokopu in captivity. His frustration was with the cost of imported fish food to keep the project going.”

Charlie Young says he realised dairy effluent could form a source to grow algae, and then zooplankton and suggested the idea to Charlie Mitchell, who worked on developing the concept.

On the Noble farm, algae from the ponds are piped to an existing drain, which has been cleaned and widened to create the zooplankton pond where these crustaceans feed on the algae and provide food for wild fish.

Charlie says if the trial is successful, it may open opportunities for farmers to harvest eels and whitebait for sale, or to form partnerships with fishermen who have quota to do so, bringing in another source of revenue, while reducing nitrogen leaching and making good use of dairy effluent. It may also be possible for farmers to establish fish farming ventures alongside dairying.

David Guccione, of Bay of Plenty Polytehcnic, who also knew Charlie Mitchell well, was another of the experts taking part in the field day.


Bay of Plenty Polytechnic’s David Guccione outlines how the level control structure at the end of the zooplankton pond works.

Fish smorgasbord
He explained the workings of the control structure at the end of the zooplankton pond, where it meets the farm drain, which controls its water levels. Screens within the structure also allow zooplankton to pass through to the drain while stopping fish, other than the occasional eel travelling overland, from entering the zooplankton pond.

Fish-friendly floodgates, at the drain’s outlet to the Aongatete River, enable native fish species to enter the farm drain. “They will find this smorgasbord of food and will come to feed.”

David says the quality of the water entering the river is high, but also rich in nutrients which fish require. Providing extra food for native fish should help boost fish stocks, something David says will be of significant benefit, in the face of declining fish number.

Further open days will be held during the coming three years of the trial.


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