GRASS CARP; A SOLUTION, OR JUST A NEW PROBLEM? There is much interest in New Zealand in the potential of the grass carp (Cyprinidae: Ctenopharyngodon idella) to control or even completely eradicate aquatic plant pest population in some instances. Interested parties include the Department of Conservation (DoC), the National Institute of Water and Atmospheric Research (NIWA), local government councils, the Fish & Game Council, Maori Development associations, conservation groups such as the New Zealand Native Freshwater fish Society (NZNFFS) and Royal Forest & Bird Protection Society and of course the grass carp aquaculturalists.

Aquaculturalist Gray Jamieson with one of his grasscarp.

The different sides of the grass carp introduction debate typically follow with district council and drainage engineers being keen to use the fish, but with many conservation groups and the Fish and Game Council being strongly opposed. Arguments produced for the introduction of grass carp (largely to either heavily modified or artificial waterways) are that current methods for clearing these waterways (usually consisting of at least annual mechanical excavations topped up with herbicide applications) are dangerous, expensive and environmentally inadequate. Arguments against the introductions are the possibility of them breeding in the wild (very unlikely), escaping/ being transferred into other areas (more likely) and unknown impacts on both native plants, water fowl, sportfishing and freshwater fish communities. Given that areas with excessive aquatic plant pest growths are typically monocultures of the introduced species with no significant native plant growths and have very poor freshwater communities (being heavily compromised both by the excessive plant growths and by the highly intrusive methods of control) this latter problem is probably not a serious concern. It is also hard to do much more damage to the system than is already regularly done with the devastation of mechanical clearing and possible cumulative, persisting and more insidious effects of sprays. However, whatever the method chosen to control aquatic plant pest infestations is, the NZNFFS feels that it must be selected on a case-by-case basis (what is suitable for one water body does not mean this method is the best for another) and should ideally be targeted at the problem; not the symptoms. Both use of grass carp, drain clearing, and herbicides target the symptoms (rampant choking plant pest growth) when targeting the problem instead (why do we have these excessive weed growths?) is a more logical, and frequently more ecologically sensitive and sustainable approach. Illustrating this point, the problem with most of these aquatic weed infested areas is that they have very high nutrient levels, and also very high levels of incident sunlight causing high water temperatures and encouraging rampant plant growth. Whilst lowering nutrient levels such that plant growth becomes nutrient limited is largely a catchment scale initiative that is frequently not viable, simple strategies such as riparian planting to shade such rampant growths may be a simple yet highly effective option as most of these plant pest species concerned are shade intolerant. Further repercussions such as better habitat for both instream aquatic life such as native fishes and the invertebrates they fed on, and even enhances terrestrial life e.g. bird life etc. from both fallen and living woody material are likely added benefits. Only where the identified causes of the problem cannot be viably removed should symptomatic solutions be considered as a last resort. One of the dangers of highly effective solutions to any problems symptoms, is that the problem often does not have to be solved; instead they can be continually masked by removing the symptoms so that all appears well on first appearance, when in fact the problem remains or even develops further. Personally, my only real concern with the use of grass carp in New Zealand is that its use as a symptomatic solution may be given precedence over other more suitable (and problem oriented solutions) such as the aforementioned riparian planting, and that a ready solution at hand of the decision makers (all too frequently already with a lack of a sound ecological understanding of the processes concerned) will result in grass carp unduly being used in areas where more suitable and sustainable methods of control with further habitat enhancing benefits may have been more appropriate. This seems to be becoming the case already with the argument already polarising into simply either grass carp vs digging/ spraying when in reality the situation is far more complex and the range of management solutions, both tried and tested in New Zealand and overseas, far more extensive. So, whilst I do think that grass carp and such fishy biological controls can make a significant contribution to water ways management in New Zealand, I feel that these are not a final be-all and end-all solution to these human-induced dilemmas but should be considered in light of other available strategies (as should any management methodology) given the unique original and desired characteristics of the water body concerned. Back to THREATS REFERENCES CITED Collier, K. J.; Cooper, A. B.; Davies-Colley, R. J.; Rutherford, J. C.; Smith, C. M. & Williamson, R. B. 1995a: Managing riparian zones: a contribution to protecting New Zealand’s rivers and streams. Vol. 1. Concepts. Department of Conservation. Wellington, New Zealand. 39 p. Collier, K. J.; Cooper, A. B.; Davies-Colley, R. J.; Rutherford, J. C.; Smith, C. M. & Williamson, R. B. 1995b: Managing riparian zones: a contribution to protecting New Zealand’s rivers and streams. Vol. 2: Guidelines. Department of Conservation. Wellington, New Zealand. 142 p. McDowall, R. M. 1990: New Zealand freshwater fish: a guide and natural history. Heinemann-Reed. Auckland, New Zealand. 553p. Rowe, D. K.; Schipper, C. M. 1985: An assessment of the impact of grass carp (Ctenopharyngodon idella) in New Zealand waters. Fisheries Environmental Report No. 58. New Zealand Ministry of Agriculture and Fisheries. Rotorua, New Zealand 177p. Williams, M. J. 1984: The likely impacts of waterfowl on the introduction of grass carp to New Zealand waterways. New Zealand Service Technical Report No. 4. Department of Internal Affairs. Wellington, New Zealand. 27p.