New Zealand Journal of Marine and Freshwater Research abstracts
Control of macrophytes by grass carp (Ctenopharyngodon idella)
in a Waikato drain,
New Zealand
R. D. S. Wells
National Institute of Water and Atmospheric
Research Limited
P.O. Box 11 115
Hamilton, New Zealand
email: r.wells@niwa.co.nz
H. J. Bannon
B. J. Hicks
Centre for Biodiversity and Ecology Research
Department of Biological Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
email: b.hicks@waikato.ac.nz
Abstract Hornwort (Ceratophyllum demersum L.)
and other aquatic macrophytes have historically been mechanically removed
from the Rangiriri drain and Churchill East drain to maintain drain efficiency.
As an alternative control method for the high plant biomass that accumulates
at the end of summer, the effect of stocking diploid grass carp (Ctenopharyngodon
idella L.) on the aquatic vegetation was evaluated in these Waikato drainage
systems. At the start of the trial, both drains had a low diversity of aquatic
macrophytes, and of the nine species (including the emergents), seven were
exotic. Two months after grass carp were released to Churchill East drain
(the treated drain) the four submerged and floating macrophyte species became
scarce in the main drain. Over the same period, these species increased in
biomass in Rangiriri drain (the untreated drain), where hornwort became dense
and surface-reaching and remained so for the duration of the trial. However,
grass carp did not control submerged vegetation in smaller side drains or
the shallow, upper parts of the main drain, or the marginal sprawling species
and emergent species. The cost of leasing the grass carp was similar to the
cost of clearing the drains mechanically, but grass carp provided continuous
weed control. However, subsequent to this trial, 62 dead grass carp were found
in Churchill East drain in February 2001, and weed cover subsequently increased.
This illustrates that grass carp management in New Zealand agricultural drains
can be problematic due to periodic fish kills.
Keywords diploid grass carp; Ctenopharyngodon idella;
macrophytes; Ceratophyllum demersum; Glyceria maxima; biodiversity;
weed control
M02004 Received 22 January 2002; accepted 24 July 2002; Published 20 March
2003
New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37:
85-93
0028-8330/03/3701-0085 $7.00 © The Royal Society of New Zealand
2003
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