Reproduction of common carp in South Australia, shown by young-of-the-year samples, gonadosomatic index and the histological staging of ovaries

Young-of-the-year (YOY) samples, gonadosomatic index (GSI) and the histological staging of ovaries were used to monitor the reproduction of common carp (Cyprinus carpio L.) in the lower River Murray, South Australia, from August 2001 to December 2002. Spawning occurred initially over 9 months from late September 2001 to May 2002, the longest period recorded in Australia. It recommenced in September 2002 and continued until at least December, when sampling ended. Contrary to previous reports, hatch-dates estimated from otolith analyses revealed that in each year, spawning was continuous from onset until completion, and that there were two peaks in YOY production between mid-October and December 2001 and mid-January and mid-March 2002. Over the entire period, there were at least 29 discrete spawning events at two locations about 30 river-km apart, most of them synchronous. GSI and histological evidence indicated spawning over seven months, including two months where the hatch-date data failed to identify any reproductive activity. Thus, the benefits of combining analyses of YOY and ovary samples are apparent here, where reproduction is protracted and there is potentially low YOY survivorship in some months, and where the local ecology of the target species is not well-understood.B.B. Smith & K.F. Walke