134 critical summer temperatures were within the optimum range of fish species recorded, noting the absence of inanga, torrent fish, koaro and smelt within the Ohinemuri River at Waihi. The in-river temperatures were less favourable for macroinvertebrate species, approaching or exceeding upper incipient lethal temperatures for some species commonly occurring within the river. However, as outlined above, the consent monitoring of the ecological values of the Ohinemuri River has shown that most of the macroinvertebrates have continued to thrive in the river. In-river temperatures during the critical summer period were mostly below the upper critical temperature of 25°C (for upland streams) with recorded exceedances occurring on only two occasions. Despite these temperature statistics, we emphasise that our assessment of the Ohinemuri River at Waihi has shown that the ecological values (as demonstrated by fish, macroinvertebrate and periphyton communities) has remained stable and persistent. Whilst there is the potential of direct lethal impacts on the aquatic biota and/or impairments to productivity, recruitment or functionality, there is no evidence to suggest that this has occurred. We note that there is a temperature differential between the upper and lower reaches of the Ohinemuri River at Waihi, with the in-river temperature consistently slightly higher in the lower reaches at Waihi. We have noted that a >2⁰C has occurred on 21 occasions between upstream and downstream sites respectively (amounting to some 17% of the existing critical summer temperature record) over a 15-year period of spot temperature records (acknowledging that time of day of spot temperature recordings can be highly variable). In-river temperature increases >3⁰C between upstream and downstream of the discharges has occurred on only six occasions over the critical summer period during that same time period. Daily CRI temperatures recorded continuously over 2019-2020 also showed only two occasions where CRI temperature exhibited a >3⁰C change from the uppermost (site OC2) and lowermost (site OH6) sites at Waihi. The number of >2⁰C changes between upstream and downstream were much greater. We emphasise that while temperature increases occur throughout the reach of the Ohinemuri River at Waihi, the in-river temperatures upstream are already at or above the temperature thresholds set out in AC (2012). We consider that although temperature has the potential to result in adverse effects on the aquatic values there is no evidence to suggest that temperature has in fact resulted in any adverse effects on the aquatic ecological values (neither direct nor indirect effects) of the Ohinemuri River at Waihi. We also note that are a wide range of factors that influence in-river temperatures including depth and clarity of the river water, width of the river, river flows, the degree of shading, the influence of other tributaries and other water sources. As a consequence of the noted changes in temperature and likely other factors influencing this, we have recommended that a condition of consent that requires the collection of temperature data to inform an understanding of the temperature changes that occur in the Ohinemuri River in the vicinity of the treated water discharge.
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