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Boat electrofishing survey of fish abundance in the Ohau Channel, Rotorua, in 2014

The aim of the survey was to provide on-going monitoring of the fish communities and abundance by boat electrofishing in the Ohau Channel, especially fish species that are taonga to Maori (eels, goldfish, and koura). In the current study, we present the findings from the eighth year of sampling (2014) and a summary of previous surveys. We used the University of Waikato’s 4.5 m-long, aluminium-hulled electrofishing boat to catch a total of 642 fish (11.3 kg) 11 sites on 9 December 2014, which comprised 2,914 lineal m and 11,656 m² in area. Koura and 4 fish species were present, with common bully most abundant (up to 27 fish 100 m⁻² at the site 5, edge habitat). Goldfish (up to 5.92 g m⁻²) was the next most abundant species, and was most abundant in sites 8-11 the lower channel, especially at site 11, the excavated side channel. Rainbow trout were next the most abundant species (up to 0.25 fish 100 m⁻² ). Mean bully biomass (5.25 g m⁻²) was much higher than for smelt (0.07 g m⁻²). Koura had a patchy distribution; 5 individuals were caught. Comparing catches over the 8 years of sampling, the abundance of common bullies in 2014 (4.81 fish 100 m⁻²) was consistent with catch rates in most post-wall years (after 2007; range 0.81-5.72 fish 100 m⁻²), but lower than in 2013. The cause of fluctuating bully abundance is not known, and was not accounted for by changes in water clarity expressed as black disc distance (BDD), water temperature, or water conductivity. Poor water clarity can reduce the efficiency of electrofishing, suggesting that bully density estimates might decrease with decreasing BDD. However, common bully densities in 2012 and 2011 were inversely related BDD. In 2014, catches of smelt, a similar sized fish to bullies, were extremely low. Goldfish biomass increased initially 2009 and 2010 because of targeted fishing in the excavated side channel (site 11), which has dense macrophyte beds that provide good habitat for goldfish. The continued rise in goldfish density from 2012 on suggests a real increase in goldfish numbers has occurred. Eels were scarce in the Ohau Channel and catches were very variable. Longfin eels were caught in most years, and in 2012 and 2013 shortfin eels were also caught, but in 2014 no eels were caught. Analysis of fish densities before and after wall closure is hampered by the single data point before closure. However, we now have 7 years of post-wall data, and comparison of means and standard deviations suggest that the number of bullies has decreased. An intriguing trend of decreasing rainbow trout densities with increasing BDD, which is a measure of both water clarity and phytoplankton abundance, has occurred following wall construction (Figure 3). The trend is opposite to the usual decline of catch rate by boat electrofishing with reduced water clarity. The explanation for this is not immediately clear, and could be increased avoidance of the electrofishing boat by trout in clearer water. It is also possible that reduced BDD indicates high algal productivity in Lake Rotorua, which in turn could increase trout abundance in the Ohau Channel. This remains to be tested
Type of thesis
ERI report
Hicks, B. J., Bell, D. G., & Powrie, W. (2015). Boat electrofishing survey of fish abundance in the Ohau Channel, Rotorua, in 2014 (ERI report). Hamilton, New Zealand: Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato.
Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato