Appendix 3: Methods for Assessment of Stream Freshwater Ecology Boffa Miskell Ltd | Waihi North Project: Freshwater Ecological Assessment | To calculate the amount of enhancement required to mitigate the impacts of streamworks an environmental compensation ratio (ECR) was calculated. The environmental compensation ratio utilises the SEV score to calculate a ratio for the minimum area to be restored as mitigation for unavoidable stream loss. The ECR has the underlying principal of ‘not net loss’ and is based upon ‘no net loss of area-weight stream function’. A minimum ratio of compensation of 1:1 is required. The formula for calculating the ECR is as below: • ECR = [(SEVi-P – SEVi-I)/(SEVm-P – SEVm-C)] x 1.5 • SEVi-C & SEVi-P are the current and potential SEV values respectively for the site to be impacted. • SEVm-C & SEVm-P are the current and potential SEV values respectively for the site where environmental compensation is to be applied. • SEVi-I is the predicted SEV value of the stream to be impacted, after impact. • 1.5 is a multiplier. The ECR calculation requires the prediction of a ‘potential’ and ‘impact’ SEV scores. The potential scores for impact sites assume that best practise enhancement works have been undertaken. The prediction of the impact scores assume that the proposed streamworks have been undertaken. The generally accepted SEV score for culverts is 0.2. The predicted potential and impact scores do not include biotic functions (invertebrate fauna intact and fish fauna intact) as they are too difficult to predict. The ECR considers that environmental compensation ratios greater than 1 are valid because of: • The ecological risk factors associated with the cumulative loss of streams and the steady change in areal distribution of high-quality stream reaches; • The long time-lag before full benefits of environment compensation (i.e. from riparian planting) accrue to the mitigated sites; and • The overall difference between the expected and actual success of stream restoration methods. Biological Indices Macroinvertebrate Community Index The Macroinvertebrate Community Index (MCI) score is a biotic index that can be used as an indicator of stream water quality. It relies on the fact that biological communities are a product of their environment – with different organisms having different habitat preferences and pollution tolerances (Stark & Maxed 2007). The MCI involves assigning tolerance values to all taxa based on their tolerance to pollution. Taxa that are characteristic of pristine conditions score higher than taxa that are predominantly found in polluted conditions, where 0.1 is the lowest and 10 is the highest. The final MCI scores are calculated using presence-absence data, with the scores range from 0 to 200, with streams with no taxa present scoring 0 and streams in exceptionally pristine conditions scoring 200 (Stark 1993). For all streams surveyed the MCI-sb scores were utilised owing to the predominantly sandy/silty stream beds present across all sites.
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