Electrofishing survey of the Manawatu, Whanganui, and Mokau rivers and Lake Rotorangi, Petea River

We used New Zealand’s first successful electrofishing boat to survey fish in three large North Island rivers and in Lake Rotorangi, hydro-electric impoundment on the Patea River. The primary objective of the fishing was to see if undesirable fish species such as koi carp (Cyprinus carpio haemaotopterus; Zhou et al. 2003), brown bullhead catfish(Ameiurus nebulosus), and rudd (Scardinius erythophthalmus) have become established in the Manawatu, Whanganui, Petea, and Mokau rivers. Koi carp are known to be present in several ponds within the Manawatu and Whanganui River catchments, and a juvenile rudd was caught by a whitebaiter near the mouth of the Mokau River. Lake Rotorangi has an established trout fishery, but European perch (Perca fluviatilis) have been illegally released into the lake, and are numerous near Stratford. This lake is also vulnerable to illegal releases of other fish species, so it was included in the survey. A secondary objective was a general survey of native and sports fish. Nine species of native fish and three introduced species were captured during the survey of 27 sites in four river systems. Fish habitat was generally poor, with little in-water obvious at the shoreline. Aquatic macrophytes were only in Manawatu River tributaries (the Koputaroa and Tokomaru streams). All three rivers had U-shaped channels with soft banks generally of clay and mud. Poor water visibility made observations difficult in the mainstreams of all the rivers. Common smelt were the most abundant and widespread fish species, and were present at all sites except those in the Tokomaru Stream and Lake Rotorangi. High water conductivity prevented effective electrofishing at Site 25 in the Mokau River mainstem, where the juvenile rudd was found. Some unexpected catches in this survey were stargazers in the Manawatu River at Site 1 and 3, 10 km upstream from the sea, and black flounder in tributaries of the Manawatu (Sites 4 and 5), up to 23 km inland. Black flounder were found in the Whanganui River at Site 13, 30 km from the sea. Rainbow trout were caught in the Manawatu River at several sites (Table 4, Figure 3). No recognised pest fish species were caught, but we cannot exclude the possibility that pest fish might occur at sites that were not fished. Despite these limitations, we are confident that our study has provided a good summary of the fish species present in the three rivers and Lake Rotorangi, and has provided estimates of minimum densities for the sites that were fished.