Boat electrofishing survey of fish abundance in the Ohau Channel, Rotorua, in 2017

Abstract

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 (tuna, or eels, morihana, or goldfish, and kōura, or freshwater crayfish). In the current study, we present the findings from the eleventh year of sampling (2017) 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 1,110 fish and 3 kōura (18.2 kg in total) from 11 sites on 5 December 2017. The sites comprised 2,909 lineal m and 11,636 m2 in area. Kōura (freshwater crayfish) and 6 fish species were present, with common bully the most abundant species (up to 29.6 fish 100 m⁻² at the site 8, edge habitat). Goldfish (up to 14.2 100 m⁻²) was the next most abundant species, with most goldfish at sites 8 and 11 in and around the side channel. Common smelt were next the most abundant species (up to 11.4 fish 100 m⁻²). Mean density of bullies (7.8 fish 100 m⁻²) was much higher than for smelt (1.69 fish 100 m⁻²). Kōura had a patchy distribution with only 3 individuals were caught at two sites. Comparing catches over the 11 years of sampling, the mean abundance of common bullies in 2017 was consistent with densities in most post-wall years (after 2007), but lower than in 2007 before wall closure (Newman-Keuls multiple range test, P = 0.034). 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, but low BDD did not correspond with low common bully densities. In 2017, smelt abundance had recovered somewhat from the low catches from 2014 to 2016. Goldfish biomass increased initially (2009-2010) because of targeted fishing in the side channel (site 11), which has dense macrophytes and offers good habitat for goldfish with no flow. However, the continued rise in goldfish density from 2012 on suggests a real increase in goldfish numbers. In contrast to most previous years, no eels were caught in 2017. Analysis of fish densities before and after wall closure is hampered by the single data point before closure. However, we now have 10 years of post-wall data, and comparisons of means and standard deviations suggest that the number of bullies have been lower since 2008 with the exception of 2015, when bully densities were the same as 2007, i.e., before wall closure. The large number of small juvenile bullies (<30 mm) suggests that recruitment is taking place in the channel, so fluctuating bully abundance is likely to be controlled by factors other than wall closure.