Silver carp for weed control

Silver carp (Hypophthalmichthys molitrix) were brought to New Zealand in the 1960s to assess whether they could be used to control phytoplankton. 

They are now bred and grown on fish farms licenced by the Ministry for Primary Industries. They are very unlikely to reproduce in New Zealand water bodies because of their very specific breeding requirements. 

If you want to use silver carp you must apply to move freshwater species. Possessing them without approval carries a penalty of $5000. 

Why use silver carp for weed control 

Silver carp are used overseas to reduce blue-green algal blooms. They are sometimes used in conjunction with grass carp to control the algal blooms that can occur after aquatic plant removal. 

The use of silver carp in New Zealand has been limited and there has been no evidence they have achieved a decrease in algal numbers. 

Silver carp eating habits 

Silver carp are filter feeders that consume large quantities of phytoplankton (and some zooplankton). They can filter 10-16% of their body weight in phytoplankton per day at moderate water temperatures. 

Silver carp cease to feed below 10^◦C and are most active at temperatures over 20^◦C.  

Measures for containing silver carp 

It is extremely important to contain silver carp in the water body you release them to. 

Security measures are a balance between allowing fish passage for native species and preventing the escape of silver carp. Generally a screen mesh size of 35 mm is used to contain grass carp of 250 mm (fork length). However silver carp are slimmer than grass carp so if a screen of 35 mm mesh is used, silver carp need to be at least 310 mm (fork length). 

Mesh screens of 35 mm do not usually interfere with the migration of native fish. 

See fish passage management in New Zealand for more information on fish passage management for our native fish. Email the DOC Hamilton Office if you would like more information on the measures you will need to take. 

Applying to use silver carp 

See apply to move freshwater species for more information. 

More information 

Some issues in risk assessment reports on grass carp and silver carp (PDF, 101K) 1999