Utuhina Stream 2006-2020: In-stream alum dosing effects on fish and aquatic invertebrates

Abstract

This report presents the results of an on-going assessment of the fish and aquatic macroinvertebrate communities of the Utuhina Stream, Lake Rotorua, from 2006 to 2020, and an assessment of the bioavailability of aluminium in fish and kōura to satisfy Bay of Plenty Regional Council’s annual resource consent conditions 9.6, 9.8 and 9.7, respectively, for consent 65321 for the discharge of alum to the Utuhina Stream. Macroinvertebrates, fish and kōura (freshwater crayfish) were sampled from one control and two treatment reaches of the Utuhina Stream annually. Common bully (Gobiomorphus cotidianus) is the dominant species in the fish community of the Utuhina Stream. Kōura (Paranephrops planifrons) and juvenile trout were always present at all sites but variable in abundance. Differences in species abundance from year to year are most likely due to flood-related disturbances to stream bank morphology and in-stream vegetative cover or physical displacement of fish. No obvious effects of alum dosing on stream fish or macroinvertebrate communities were observed between the upstream control site and sites downstream of the alum discharge. Several other fish species were occasionally captured during sampling and the regular occurrence of juvenile koaro from 2016 to 2019 is possible evidence of this taonga species becoming established in the Utuhina. Analysis of stream macroinvertebrates also showed no consistent differences between the upstream control site and the sites downstream of the alum dosing. Overall, all sites were characterised as fair to good quality for a soft-bottomed stream. Some evidence of aluminium bioaccumulation was seen in some tissues of common bully (gills and liver) in some years, resulting from continuous alum dosing of the Utuhina Stream, but there was no evidence of bioaccumulation of aluminium in the tissues of kōura. Alum exposure in these species does not appear to affect their health or abundance in the stream. iii Overall, continuous alum discharge does not appear to negatively impact the ecology of the lower Utuhina and improvements in the ecological condition of the Utuhina Stream will be achieved by ongoing riparian restoration and mitigation of the impacts of flood flows.