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The North Otago Irrigation Company this week received approval from the Department of Conservation to release carp into its Otago and Canterbury canals, near Smillies Rd, to control the growth of aquatic weeds.
North Otago Irrigation chief executive Robyn Wells said using grass carp was the ''most environmentally elegant'' solution to control weeds, which could block screens and cause problems with pumps.
Grass carp ''have extremely specific spawning and rearing requirements and the establishment of a wild population is extremely unlikely'', according to the National Institute of Water and Atmospheric Research.
The only other time grass carp were released in the South Island was 10 years ago at Lake Hood.
Ms Wells said the fish would be supplied and managed by New Zealand Waterways Restoration Ltd of Auckland, which would transfer the stock from an aquaculture farm in Nelson to the canals next month.
''Our first step now that we have approval is to implement the appropriate steps to ensure the carp stay within our canals and do not get pulled into our pumping equipment, by installing mesh on all inlets and outlets prior to the fish being released.
''Once released and established, the expectation is that the fish will reduce the weed volumes by two-thirds, which will mean that our employees will be moved to more cost-effective tasks.''