"December 2004"

The invasive alga Didymosphenia geminata is now abundant in the lower Waiau and Mararoa Rivers, Southland, both of which are popular fishing rivers. There is concern that it may have been transferred to neighbouring catchments, which also have important trout fisheries. 2. Sixty-two sites in the Upper Waiau, Von, Oreti, Aparima and Mataura Rivers were surveyed on 17-18 December 2004 to check for the presence of low levels of D. geminata. 3. The survey targeted river sites from which D. geminata would be most likely to spread into the river (i.e. fishing/recreational access points). The sampling method also targeted habitats most likely to be colonized by D. geminata. 4. Thirty-six drift-net samples provided information about algal drift between the sites. These samples were taken because, in rivers where it is present, D. geminata is known to form drifting fragments. 5. Approximately 550 sub-samples of stone scrapings and drift net samples were examined microscopically. No traces of D. geminata cells or stalks were found in any of the examined material. 6. A simple test using binomial (presence/absence) data was used to estimate the probability of finding D. geminata in the survey. Assuming independent sampling units (sites), random sampling, and uniform distribution of D. geminata with an expected occurrence in less than 5 of every 100 sampling units (if it was present in the rivers), a 62-site survey has a 95% probability of finding D. geminata in at least one sampling unit. For an expected occurrence of 2 in every 100 sampling units, the probability of finding D. geminata in the survey falls to 72%.. ... - Executive summary.

Electronic