Waste into whitebait

Nine years of research on a coastal property near Raglan Harbour could point the way toward solving two apparently unrelated problems - New Zealand's depleted whitebait stocks and the impact of an increasing volume of waste effluent from farms and agricultural industry.

Since 1997 Charles Mitchell has invested thousands of dollars of time and his own money into researching the aquaculture of whitebait, developing a 'laboratory' of six original design ponds naturally powered by gravity flow and tidal energy to manage about 8500 tonnes of water.

The ponds, which are also home to eels and mullet, use natural tidal triggers for whitebait spawning activity and egg deposition.

He remains the only person in New Zealand to artificially breed whitebait and is now routinely spawning large shoals of whitebait.

Whitebait are one of the highest value seafoods on the New Zealand market, fetching up to $250 per kilogram in some seasons and seldom selling for less than $100 a kilo.

Over 50,000 adult whitebait (inanga) brood stock were spawned repeatedly between May and October in 2005, resulting in some 250 million whitebait larvae being released into Raglan Harbour from June until September.

Whitebait grow rapidly in the sea, making a 430-fold weight gain in three to five months, before returning to coastal waterways.

"We can reliably breed them in bulk with low labour inputs and enough whitebait return to the ponds each year from fresh hatched larval releases to provide a population growing at around 25% per annum. But the big barrier to a commercially viable whitebait industry is the very high natural mortality rate of larvae."

Mr Mitchell says that in the wild fishery, over 99.5 % of whitebait larvae die between hatching and returning from the sea.

"What we are looking for are means of intervention and domestication that will break that pattern. If we could improve survival to 1 % or even 2 % our population could rapidly shift from biologically sustainable to economically sustainable"

Research at Raglan suggests that both improving harbour water quality and increasing the amount of feed available in the first few weeks of life is critical to boosting whitebait survival rates and ultimately catch levels.

To achieve this goal Mr Mitchell has been trialing conversion of agricultural waste into ultra low-cost, high quality fish food to feed the inanga larvae after hatching.

With investment of around $80,000 over two years from the Foundation for Research, Science and Technology, he has been experimenting with waste yeast from a local winery, fish processing waste, chicken manure, horse manure, cow manure slurry and chopped clover as test diets for farmed zooplankton to feed baby whitebait.

Trolling the Internet for other feed ideas also has him considering using the algae slurry from advanced dairy effluent treatment ponds to grow the plankton.

"All kinds of organic waste can be used to produce food for plankton,"

says Charles, "because they feed on the bacteria and algae produced during the breakdown of wastes in water".

To develop captive strains of local planktonic organisms suitable for larval fish food, Mr Mitchell has used the Foundation investment to support a search of sewage effluent ponds, stagnant high tidal rock-pools and other fertile places for zooplankton species potentially valuable for aquaculture. So far three species have proven successful under intensive culture.

"With the food supplies flowing, this year we achieved a 25% increase in the length of the whitebait within four days before they ate everything and we had to release. This strategy has the potential to bypass the extremely high early larval mortality rate."

While there is a way to go before proof of concept is established, results to date leave Mr Mitchell optimistic. After seven generations, his captive breeding whitebait are growing larger and producing more than three times the number of eggs hatched by their counterparts in the wild.

Part of the research investment from the Foundation is to be used to refine a method of fish ear bone tagging to identify which whitebait returning to the ponds have originated from the farmed stock.

While developing a new commercial aquaculture industry is a top priority for Mr Mitchell, a fisheries consultant with 31 years experience, he's also aiming to improve the overall health of the whitebait fishery. He says catch quotas tend to be set on the numbers spawned in strong years, but these occur erratically, depending on weather conditions, meaning the population may be severely depleted in weaker years.

"Commercialisation must not mean over-exploitation of native stocks. My research has the potential to provide the know how and the techniques to grow the native inanga fishery within present intensive landuse and to ensure it is sustainable."

He sees being able to manage pollution at the same time as being another

plus:

"Waste can be turned into valuable products, depending on how you manage it. My research could offer the agriculture industry new ways to manage effluent while also supporting a high value fishery."

"This project is high risk, truly unknown and applied to today's issues.

Therefore support from the Foundation has been vital in helping me to carry on."

Ian Gray, a Research and Development Project Director working with Mr Mitchell, says the work being done near Raglan has exciting potential for other aquaculture industries and for environmental restoration.

"He has built up a huge body of knowledge about restoring aquatic environments using native species and natural forces, like tidal movement. It's a deliberately low tech approach with a lot of smart behind it which could really help develop New Zealand's so far untapped aquaculture potential while also benefiting the environment. The direct application of this integration between aquaculture and passive water movement systems is currently being evaluated for the restoration of a large and heavily degraded coastal water body in the South Island."