Recirculation – the way forward for paua
Science Centres: Fisheries
Phil Heath discussed the potential of recirculation systems for paua aquaculture in Fisheries & Aquaculture Update No. 6. We now present some findings from NIWA’s own recirculation trials.
Studies in the UK and Ireland have clearly indicated that there is potential for farming European abalone in recirculation systems. NIWA has taken the initiative in evaluating this technology for use with New Zealand’s native abalone or paua.
In November 2002 a small recirculation system was established at the Mahanga Bay aquaculture facility in Wellington. The design was based on systems that I had used to grow abalone in Europe. The size of each component in the system was calculated to hold 20 kg of abalone and was related to measurements of ammonia production from paua already held at Mahanga Bay. New water (about 9% of the system volume) was added to the system every second day, after cleaning, and this equated to one complete water exchange every 11 days. The system was initially stocked with 5 kg of animals; the stock had increased to 15 kg by April 2003.
This recirculation system trial was designed to serve a dual purpose: to test recirculation technology and to test the effectiveness of size sorting as a stock management tool. The size sorting trial, which will be discussed in a later article, required the different size groups to be separated. To achieve this, the stock were held in baskets, and although this was effective for separating the stock it probably restricted the water flow to the animals more than commercial tray culture techniques.
To compare the effectiveness of recirculation and flow-through systems, an identical paua culture system was established using flow-through water at the rate of two complete exchanges per hour. The animals in the two systems received the same amount of food and the same cleaning and handling treatments. The main difference lay in water quality. Seawater temperature in the flow-through system was ambient and fluctuated between 15.0 and 19.1 oC during the experiment. In the recirculation system, it was maintained at between 17.8 and 18.2 oC. The pH in the flow-through system remained above 8.0, whereas in the recirculation system it dropped rapidly to below 7.7, and required constant monitoring to prevent shell erosion. Total ammonia levels in the recirculation system remained below 0.3 mg N per litre throughout the experiment. Ammonia was always below detectable limits in the flow-through system.
The growth of the paua in each system was measured every month. Growth rates were always higher in the recirculation system than in the flow-through system, surprisingly even in summer when ambient seawater temperatures matched, and occasionally exceeded, those in the recirculation system. Initially the overall growth rate in each system was lower than the 2 mm per month that should be achieved on a commercial farm. However, growth rates within the recirculation system have been on average over 50% higher than those in the flow-through system, and have recently exceeded 2.5 mm per month.
These results indicate that paua held in a recirculation system can perform significantly better than in flow-through systems. Our initial feeling is that this is due to the more stable temperature regime within the recirculation system. Before we can be fully confident that recirculation systems offer a real production advantage over flow-through systems for paua farming, we need to establish the best way to manage pH fluctuations and shell erosion.
For those new to the industry or interested in learning more about paua farming and water quality, NIWA will be running a two-day paua and water quality training course on 16 and 17 September 2003. Contact Mike Tait (m.tait@niwa.co.nz) for further details.