Juvenile grass carp in the Waikato River, May 1997

1. Introduction A small-sized grass carp (43 cm long) was caught in Lake Whangape, lower Waikato River basin, in May 1997. This fish was too small to be a recapture from the escapement from trial sites in 1984 (fish which are now mostly longer than 70 cm), so could have been either a recent escapee, or the result of natural reproduction in the river. Either way, the finding of this fish indicates a potential problem. The Department of Conservation asked the National Institute for Water and Atmospheric Research to examine the wild-caught fish along with fish captured from recent release sites and culture ponds, and provide a report on the likely origin of the fish captured from Lake Whangape. As all grass carp stocked into waterways recently have been diploid, the origin of the wild-caught grass carp cannot be differentiated on the basis of genetic ploidy. However, stocked fish, from artificial spawning in February 1995, were released into the Mangawhero and Eastern Drains (see Map 1) at between 17. Since then, there has been escapement of stock from the Eastern Drain as a result of damaged screens (Appendix, Pullan 1997). Thus, the wild-caught juvenile could be an escapee from these releases. Alternatively, if the wild-caught grass carp was a wild fish it would have been spawned in the spring of 1995. Weather patterns of high rainfall and warm temperatures, and consequently good spawning conditions (Rowe & Schipper 1985), did occur in the spring of 1995. This report compares the growth patterns on scales and otoliths of the wild-caught juvenile grass carp with those from fish both kept in ponds and stocked in the catchment, to determine whether this fish could be an escapee or the progeny of natural reproduction in the Waikato. 2. Age and growth patterns 2.1 METHOD Five juvenile grass carp from the Mangawhero Drain and three from the holding ponds at Warkworth were provided by the Ministry of Fisheries for comparison with the wild-caught juvenile. Several scales were removed from the upper mid-flank of each specimen and stored in a paper envelope. Pairs of sagittal otoliths were extracted from carp heads and stored. Both scales and otoliths were oven-dried at and to avoid explosion of the otoliths during the burning part of the preparation process. Scales were viewed under a microfiche reader, and several photo impressions taken for each specimen including the wild-caught fish. Otoliths were fractured …