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The use of fine mesh monofilament mist nets to control rudd (Scardinius erythrophthalmus) from a small lake complex, in the Waikato region, New Zealand.

Grant Barnes¹, David Speirs¹, Keri Neilson², Rachel Kelleher³

¹Environment Waikato, PO Box 4010, Hamilton, New Zealand.
²Science & Research, Department of Conservation, PO Box 112, Hamilton, New Zealand.
³Waikato Area Office, Department of Conservation, PO Box 20025, Hamilton, New Zealand.
Email: grant.barnes@ew.govt.nz

The Rotopiko peat lake complex (South, East and North) has one of the best native macrophyte assemblages in the Waikato Region. Rudd (Scardinius erythrophthalmus), recently introduced to these lakes, threaten the submerged plant communities, with evidence implicating them in the collapse of aquatic vegetation in other shallow North Island lakes. We tested fine mesh monofilament gill nets to determine whether rudd could be controlled or eradicated by sustained fishing, whether this was cost effective, and provided ecological benefits with minimal non target impacts.

Different mesh sizes, orientations and spacings were used over 4 three-week periods during September & March of 2001 & 2002. Ecosystem response was measured with seasonal qualitative assessments of macrophyte and fish abundance and diversity, and annual sampling of benthic invertebrate communities.

Rudd were captured in both littoral and pelagic zones, with capture rates declining exponentially after the onset of each netting episode. Effective mesh size varied seasonally and decreasing distance between nets significantly increased capture rates in small mesh nets only. Nets were effective at reducing rudd densities, although a substantial effort was required. Various mesh sizes were required to target the full range of size classes present. Net location appears critical for capture of small rudd. Non target impacts were negligible, whereas large numbers of introduced goldfish (Carassius auratus) and brown bullhead catfish (Ameiurus nebulosus) were also removed. Eradication of rudd using mist nets alone seems unlikely except in the smallest waterbody, with additional control techniques being considered. Ecosystem response is still being evaluated.

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