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Boat electrofishing survey of common smelt and common bullies in the Ohau Channel

We conducted a boat electrofishing survey of the Ohau Channel, which flows from Lake Rotorua to Lake Rotoiti, on 13 December 2007. The purpose of the survey was to investigate the longitudinal pattern in densities of common smelt (Retropinna retropinna) and common bullies (Gobiomorphus cotidianus) along the Ohau Channel. We caught 1,267 fish comprising three native fish species and two introduced fish species in 1.58 km of fished distance at a total of 10 sites. Native species caught were the common smelt, common bully and longfinned eel (Anguilla dieffenbachii) and introduced species were rainbow trout (Oncorhynchus mykiss) and goldfish (Carassius auratus). Assuming that the bow-mounted anodes effectively fished a 4 m swath then the total area fished was 6,328 m2 (0.632 ha). Common smelt densities varied among the 10 different sites in the Ohau Channel ranging from 0 to 10.6 fish 100 m-2. Smelt density was higher at the upstream end of the channel near the weir at the Lake Rotorua outlet, decreasing with increasing distance from the weir. Smelt were found in the littoral zones but were not caught in mid-channel habitats. In the upstream reaches of the Ohau Channel, directly below the weir, a high number of juveniles (4.4 fish 100 m-2) were captured compared to the amount of juveniles captured at the other sites (0 – 1.2 fish 100 m-2). Common bully densities varied among the 10 different sites in the Ohau Channel ranging from 0.2 to 58.3 fish 100 m-2. No longitudinal pattern in the distribution of common bullies was evident along the channel. The highest densities were found halfway along the Ohau Channel where there was an abundance of dense macrophyte beds. Common bully densities were found to be much higher in the edge habitats with macrophyte beds compared to the mid-channel habitats and the willow edge habitat where there were relatively low densities. Size frequency data shows that there is generally a higher proportion of small bullies than larger ones suggesting that recruitment is occurring. Both adult and juvenile rainbow trout were observed in the Ohau Channel. Most of these individuals were found in the upstream section of the channel below the weir and ranged from a 75 mm juvenile to a fully grown adult about 500 mm long. Large longfinned eels were also captured and were only found in the downstream section of the Ohau Channel in willow-dominated edges. In the bottom third section of the channel, near the possible artificial embayment, goldfish were present.
Commissioned Report for External Body
Type of thesis
CBER Contract Report
Brijs, J., Hicks, B.J. & Bell, D.G. (2008). Boat electrofishing survey of common smelt and common bullies in the Ohau Channel. CBER Contract Report No. 66, client report prepared for Environment Bay of Plenty. Hamilton, New Zealand: Centre for Biodiversity and Ecology Research, Department of Biological Sciences, School of Science and Engineering, The University of Waikato, Hamilton.