Alum dosing effects on fish and aquatic invertebrates: Utuhina Stream 2025

Abstract

The Bay of Plenty Regional Council holds resource consent to alum (aluminium sulphate) dose the Utuhina Stream, an inflow to Lake Rotorua. Under circumneutral pH (pH 6–8) aluminium binds dissolved phosphate, reducing availability for phytoplankton and thereby inhibiting formation of algal blooms. Monitoring surveys for potential impacts on the fish and macroinvertebrate communities due to alum dosing on the Utuhina Stream are conducted annually. This report presents the results from the 2025 survey. Macroinvertebrates, fish and kōura (freshwater crayfish; Paranephrops planifrons) were sampled from one control (Site 1) and two treatment (Sites 2 and 3) reaches of the Utuhina Stream using sweep nets for macroinvertebrates and bank mounted electrofishing for kōura and fish. Common bully (Gobiomorphus cotidianus) were the numerically dominant fish species, constituting 97% of individuals caught, with juvenile trout and eels (Anguilla sp.) also captured. Kōura were present at all sites, but in low abundance compared to common bully. There was an improvement in the semi-quantitative macroinvertebrate community (soft-bottomed streams; MCI-sb) score at Sites 1 and 3 in 2025 compared to the previous years, with Site 1 and Site 3 returning to the good (MCI-sb >100) category. However, the MCI-sb score for the Site 2 reach declined in characterisation from good in 2024 to fair (MCI-sb 80–100) in 2025. Large interannual variation in MCI-sb scores is typical for these reaches of the Utuhina Stream, which have been attributed to flood-related disturbances of the stream bank morphology and in-stream vegetative cover (Ling 2021). Subsamples of up to 10 common bully and kōura from each site were frozen for later analysis of tissue aluminium concentrations. Common bully (flesh, gill, liver) and kōura (flesh, gill, hepatopancreas) tissue samples were triple digested using tetramethylammonium hydroxide, cold hydrogen peroxide and nitric acid. The samples were then diluted and aluminium concentrations determined by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Common bully sample numbers were limited at Site 3 due to small body sizes preventing extraction of tissues with sufficient mass for analysis, this prevented statistical comparison between upstream (Site 1) and downstream (Sites 2 and 3) of the alum dosing point. However, common bully tissue concentrations were generally equivalent to long-term mean values. Low sample size was also a factor limiting analysis of kōura at Site 1 due to the small number of individuals captured and small body size. Overall, there was little difference in tissue aluminium concentrations between upstream and downstream reaches of the alum dosing point. A notable difference was observed at Site 3 where 2025 kōura flesh concentrations (geometric mean 20.4 mg Al kg-1 wet weight) were significantly higher (ANOVA, P <0.05) than the long-term mean concentrations (geometric mean 3.2 Al kg-1 wet weight). However, both long-term and 2025 kōura flesh concentrations were well below levels likely to result in toxicological impacts or present a risk to human health via consumption.

The findings of the current report support previous conclusions that alum dosing of the Utuhina Stream does not have a notable effect on macroinvertebrate and fish communities. While there are occasional interannual and site differences, these can be primarily attributed to hydrological and habitat variability.