Hicks, Brendan J.; Tana, Ray; Bell, Dudley G.
Hicks, B. J., Tana, R., & Bell, D. G. (2013). Boat electrofishing surveys of fish populations in the Ohau Channel in 2011 and 2012. ERI report 26. Prepared for Bay of Plenty Regional Council. Hamilton, New Zealand: Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato.
The aim of the survey was to provide on-going monitoring of the fish communities and abundance 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 fifth and sixth years of sampling (2011 and 2012) in view of previous surveys using boat electrofishing in the Ohau Channel.
We used the University of Waikato's 4.5 m-long, aluminium-hulled electrofishing boat to catch a total of 399 fish (29.4 kg) from 10 sites in 2011 (2,420 linear m, 9,680 m² area) and 301 fish (12.8 kg) at 11 sites in 2012 (3,625 linear m, 14,500 m² area). Koura and 7 fish species were present in both years, with common bully the most abundant in 2011 and common smelt the most abundant in 2012. Common smelt were more abundant in 2012 (131 fish) than in 2011 (39 fish). Goldfish were most abundant at sites in the lower channel, especially at site 7.
As reflected in total numbers, common bullies had the highest densities of any fish species in 2011 (up to 7.4 fish 100 m⁻²), the majority of which were taken from edge habitats at sites 4, 5 and site 7. In 2012, common bully abundance was much reduced. However, despite the lower bully densities in 2012, mean bully biomass was higher in 2012 (1.00 g m⁻²) than in 2011 (0.04 g m⁻²) because bullies were smaller in 2011.
Common smelt had variable densities, the largest number of which was found in and around the excavated side branch at site 7 edge habitats (up to 14.7 fish 100 m⁻²) below the weir (site 1).
Mean density of rainbow trout in the Ohau Channel was similar in both years (0.27 fish 100 m⁻² in 2011,0.24 fish 100 m⁻² in 2012). Mean biomass of rainbow trout was much greater in 2012 (0.69 g m⁻² in 2011, 11.2 g m⁻² in 2012). Goldfish densities were greater in 2012, partly because goldfish at site 7 were targeted in 2012, and mean biomass was correspondingly greater (0.48 g m⁻² in 2011, 47.7 g m⁻² in 2012). Site 7 had 282 g m⁻² of goldfish biomass in 2012.
Catch per unit effort (CPUE) for common bullies in 2011 was twice that in 2012, reflecting lower densities in 2012. Common smelt CPUE in 2012, however, was 5 times greater than in 2011. Despite the much higher biomass of goldfish in 2012, CPUE was much the same.
The increased goldfish biomass in 2012 arose because of targeted fishing in the excavated side branch, which clearly offers good habitat for goldfish. In 2012, the first shortfin eel encountered in these surveys was caught. The abundance of common bully appears to have a clear trend of reducing abundance since these surveys started in 2007. The cause of this is not apparent, and not accounted for by water clarity expressed as black disk (BD) distance or water conductivity (Table 7). Poor water clarity can reduce the efficiency of electrofishing, but BD was greater in 2012 than in 2011. It is possible that smelt abundance has declined too, but the relatively inefficient sampling resulting from boat electrofishing must be considered. Independent verification is required.