No single test species can be expected to provide a comprehensive assessment for environmental conservation and protection.  However, with a suite of well-defined toxicity tests where a variety of species are tested and a range of end points are measured, the results can contribute to an holistic interpretation of potential toxic impacts of complex effluents on the receiving environment. Little information is currently available on the relative sensitivity of New Zealand native freshwater species to chemical contaminants. Recent reviews provide information on the major sources of contamination and aquatic ecotoxicological studies (Hickey 1995) and the sensitivity of freshwater species to toxicants (Hickey 2000).

The objective of this study was to develop standardised acute and chronic aquatic toxicity tests for heavy metals using two indigenous freshwater invertebrate species and two indigenous freshwater fish species, and derive tolerance levels for selected heavy metals.

Two native freshwater invertebrate species, a cladoceran (Ceriodaphnia dubia) and an amphipod (Paracalliope fluviatilis) were selected, being representative of lake and riverine environments respectively. The selection criteria included: 

• known contaminant sensitivity for these species groups
• available laboratory methods for acute and chronic tests with related species
• widespread occurrence in New Zealand and ecologically importance of the species in their environments.

The two native fish species selected were the common bully (Gobiomorphus cotidianus) and the inanga, galaxiid (Galaxias maculatus). These were selected based on widespread occurrence in New Zealand and ecological importance of the species.

Priority metals for toxicity testing were selected based on their predominant use for timber treatment. These include copper, chromium (VI) and arsenic (V), using the highest oxidation state, which is most stable for chronic test exposures. Large international databases are available for copper and chromium (VI) and very limited data for arsenic to facilitate comparison of the relative sensitivity of the native species. 

The sensitivity of the test species to individual chemicals is compared to published values for other native and overseas species and ranked to determine the relative sensitivity and for numeric guideline derivation.

Rainbow Trout has historically been the freshwater fish species most used in toxicity testing within New Zealand and internationally. This species is still relevant in assessing toxic impacts on freshwater fish in New Zealand especially in water systems that are part of trout fisheries. However, the toxic effects on native fish species also needs to be assessed. 

Acute and chronic toxicity test protocols have recently been developed for the common bully and inanga to assess short and long term impacts of toxicants on native freshwater fish species. In the acute test, juvenile fish are exposed for 96 hours to a range of toxicant concentrations in a static system (i.e. exposure solutions are not renewed). A toxic effect is determined by a statistically significant decrease in the survival of fish exposed to the toxicant relative to the survival of fish in a control (i.e. toxicant is absent).

In the chronic test, juvenile fish are exposed for up to 30 days to a range of toxicant concentrations in a flow-through system (i.e. exposure solutions are constantly renewed). A toxic effect is determined by a statistically significant decrease in the survival and growth of fish exposed to the toxicant relative to the fish in a control.

Summary - aquatic freshwater toxicity tests in New Zealand

• relative sensitivity of freshwater aquatic species

• freshwater invertebrates

• freshwater fish