The Environment Protection Authority (EPA) assisted Break O’Day Council by investigating a significant fish kill incident in the Scamander River in June 2015. The Council hosted a public information session in Scamander in August to follow up community concerns. The session was attended by the EPA and a detailed report on the results of the investigation was provided. A copy of the EPA Investigation Summary Report, which includes detailed reports on fish pathology, water sampling and analysis, observations and shoreline residue substance identification, is provided here and the key points from the report are listed below.
Dead fish (bream, trevally, mullet) observed in and on the banks of the Scamander River were reported to Break O’Day Council and the EPA on 19 May 2015. Examination of dead bream found nothing to indicate the cause of death (AHL Report 15/1571).
Reports of hundreds, possibly thousands, of dead bream of all sizes, in and on the banks of the Scamander River, were received from about 4pm 24 June 2015, and for the following few days. The EPA commenced an investigation into the event on 25 June 2015, to determine if the mass mortalities were a result of pollution or environmental harm, with the assistance of Council.
An inspection of the river was conducted, and water quality measurements, water samples and river shoreline residue samples were collected and analyzed. No evidence of pollution was observed during the inspection, which extended from the mouth of the river, east to the TasWater water treatment plant at Eastern Creek Road, and north to Trout Reserve (Figure, Sampling Locations - Scamander River, 26 June 2015).
Water quality parameters and analytes tested included temperature, dissolved oxygen (DO), pH, conductivity, nutrients, metals, and organochlorine and organophosphorous pesticides. No pesticides were detected, and levels of nutrients and metals were no higher than expected for an aquatic-estuarine environment (AST Report 71630).
Water quality data showed the Scamander River was stratified; deeper waters were warmer (eg. 17C at 3m depth), more saline and had lower DO levels, including readings of less than 3mg/L, below which is toxic to fish. Surface water readings included temperatures of less than 10C in places, and ice sheets were observed on the river (eg. ‘Hospital Corner’, ‘Dunns Arm’) around that time (Table 1, Hydralab Data).
Examination of dead bream again found nothing to indicate the cause of death (AHL Report 15/2005). Examination of the white shoreline residue found it to be silica, nontoxic and naturally occurring (AST Report 71726).
Stratification of waters in the lower reaches of the Scamander River is a natural response to seasonal weather, including high rainfall and cold temperatures. The most likely explanation is that the fish became trapped in the deeper, more saline waters and died from lack of dissolved oxygen. Decomposition of naturally sourced materials in the deeper saline waters may result in hypoxic conditions (depleted in dissolved oxygen to levels that are detrimental or fatal to aerobic organisms). There were 2 significant rain events in June that may also have contributed to the situation by transporting plant material from the catchment, which then sank into the bottom waters and, on decomposition, consumed DO, lowering DO even further. Circumstances were possibly exacerbated by exposure to thermal stress if the fish tried to escape the deeper, low-DO waters into the much colder surface waters.