Aquaculture – an opportunity to diversify

Aquaculture production recently exceeded wild harvest of aquatic foods, and according to the FAO, consumption of aquatic food has risen steadily over the past 60 years to an average of 20 kg/person/year.

Aquatic animals are nutritious, there are a diverse range of species and product opportunities, and the feed conversion efficiency of ‘cold blooded’ aquaculture stock is generally superior to that of terrestrial agricultural animals. 

It’s attributes like these that have investors, landowners and farmers increasingly interested in the potential for aquaculture production as a method to diversify land and water use in NZ. 

As part of the Our Land and Water National Science Challenge, PAMU commissioned AgResearch and Plant & Food Research to consider how a Canterbury dairy farm might adapt their land and water resources to include aquaculture production. 

Despite abundant freshwater resources there is very little land-based aquaculture production in our country to use as case studies for diversification.

And while there are many international examples of integrated agri-aquaculture systems, it’s difficult to directly apply overseas models at home, for the same reasons many farming systems tend not to be directly transferrable between nations. 

There are numerous biological, geographic, environmental, economic, political and cultural factors which means there needs to be some home-grown adaptation and innovation to make integrated agri-aquaculture systems work for our corner of the world.

Aquaculture will thrive if we can do it more efficiently and with a lower carbon and nutrient footprint than everyone else, and if we create products with unique selling points and market value that cannot be replicated elsewhere.

Using the water twice – fertigation using aquaculture waters

One of the best ways to develop low nutrient footprint production is to integrate aquaculture outputs into the wider farming ecosystem. 

By designing systems that utilise the effluent water from aquaculture systems for agricultural fertigation is a great example of integrated production. 

Aquaculture effluent waters tend to have fewer dissolved organics than dairy farm effluent, and the nutrient profile is generally well suited to rapid uptake by plants. In addition, having aquatic stock reared in tanks, ponds or raceways means all nutrient outputs are contained and readily transportable in effluent waters.

The challenge to making aquaculture fertigation work well is having sufficient storage capacity for effluent water so that it can be used when needed.  

Figure 1 gives an overview of the concentration and types of nutrients in aquaculture waters compared with those from farm dairy effluent and dairy cow urine. The total N concentration of effluent from aquaculture sources is much lower than dairy effluent point sources. 

The nutrient composition of aquaculture effluent differs from that of dairy effluent in some ways (specifically in the calcium and nitrate concentrations), but is similarly dominated by organic nitrogen, ammonia, and phosphorus, providing opportunities for fertigation.

The organic nitrogen component of aquaculture effluent is mostly comprised of suspended solids such as uneaten food, faecal material and microbial flocculent, and if necessary these can be concentrated and removed via filtration, flocculation, foam fractionation and hydrodynamic concentration.

The ability to capture and re-direct all effluent discharges from aquaculture systems gives farmers the potential to fully control and integrate nutrient flows from aquaculture into agriculture.