Effects of exposure to sublethal levels of copper on growth and health of sea farmed rainbow trout

Executive summary

Macquarie Harbour is an almost entirely land-locked estuarine harbour with an excellent potential for fish farming and already has a number of established aquaculture sites. However, there is concern about the potential effects of pollution from the King River on the fish farming in Macquarie Harbour. The operation of the Mount Lyell Mine at Queenstown over the last 100 years resulted in significant pollution of the harbour with copper and to a lesser extent other toxic metals. Even though the new operator of the mine, Copper Mines of Tasmania Pty Ltd, has constructed a tailings dam, acid drainage from historic mining operations remains a major environmental concern, and there are elevated levels of copper in parts of the harbour, particularly around the mouth of the King River. In these areas, fish may be exposed to harmful levels of copper. Two species cultured in the harbour are rainbow trout and Atlantic salmon. Both species are hatched and reared in freshwater hatcheries for about a year before being moved to sea-cages where they grow to marketable size.

Copper is one of the most toxic heavy metals to fish. Though numerous studies have been carried out on the effects of copper on fish, most of them focused on the early life stages of fish in freshwater. Little is known about the potential effects of exposure to copper on salmonid fish during grow-out in sea-cages under the brackish water conditions that exist in Macquarie Harbour.

The aim of this project was to determine effects of copper on growth and health of rainbow trout in brackish water. A wide range of tests was chosen to establish the effects. The effects on growth were determined by measuring wet weight gain during the exposure time. Additionally, sensitive biochemical tests were carried out to find any growth effects which would not be obvious in weight changes after a few weeks. These tests included protein content in white muscle and the RNA:protein ratio in white muscle. The health of the fish was measured by testing effects on non-specific immune response (phagocytosis assay), on specific immune response (antibody production in response to vaccination), on the relative proportion of different types of white blood cells, on the structure of fish organs, on stress response and on osmoregulation.

The exposure to copper affected survival of rainbow trout. Mortalities were associated with concentrations of 20 µgL^-1 of ASV-labile copper (a measure of biologically available copper) or 60 µgL^-1 of total copper and occurred after more than two weeks of exposure. The growth of the fish was affected by six weeks exposure to about 2.7 µgL^-1 of ASV-labile copper in one tank, but this concentration of copper did not affect the growth of fish in a second tank. Exposure to 8 µgL^-1 or more of ASV-labile copper consistently affected growth. Although there was no statistically significant effect, the results suggested that antibody production against bacterial pathogens would be affected at all tested copper concentrations (exposure for seven weeks to average concentration of 3.29 µgL^-1 or greater of ASV-labile copper). There did not seem to be any consistent relationship between exposure to copper and non-specific immune response of rainbow trout, measured by phagocytic activity of white blood cells. A statistically significant reduction in the percentage of circulating lymphocytes (white blood cells responsible for specific immune response) was observed in the fish exposed to copper for seven weeks. Elevated levels of sodium and potassium in the blood of fish exposed to higher concentrations of copper for seven weeks suggested osmoregulatory problems. The fish exposed to copper seemed to have altered structure of their gills, in particular an increased number of mucous cells, an increased number of chloride cells and an increased thickness of respiratory epithelium were observed. There was a great difference between individual fish for most of the measurements.

Although this study investigated the effects of copper on rainbow trout, the results cannot be easily extrapolated to the situation in Macquarie Harbour because of differences in water chemistry. In both experiments the fish were exposed to copper only in the form of copper sulphate. The hydrochemistry of Macquarie Harbour is much more complex and although copper is the major obvious contaminant, fish may be also exposed to pH changes, other metals and changes in salinity. On the other hand, Macquarie Harbour water would most likely contain more organic matter and possibly humic acid or other compounds capable of binding copper and making it less available to the fish.

The adverse effects of copper on rainbow trout observed in this study and published literature indicate that caution should be applied if it is proposed to culture salmonid fish in those areas of Macquarie Harbour most affected by copper pollution. Although some of our results are not statistically significant and sometimes there is a lack of a dose response relationship, they should be treated as potential effects on rainbow trout exposed to copper in brackish water. Rainbow trout tested in this study seemed to be more susceptible to copper than reported in the literature, possibly as a result of salinity changes, which are not uncommon in Macquarie Harbour. Not only could the survival of the fish be affected by copper, but sub-lethal concentrations of copper may also reduce growth and lower resistance to infectious diseases or to salinity fluctuations.

The experiments tested the effects of copper only on rainbow trout. It should be noted that no experiments were done with Atlantic salmon, which is the second species cultured in Macquarie Harbour. Future research should include additional copper toxicity studies on rainbow trout and Atlantic salmon investigating potential effects of Macquarie Harbour water on the fish. The study described in this report could not address environmental factors other than copper level and salinity changes. Running the experiments in situ or using Macquarie Harbour water would be an advantage. Furthermore, research is necessary to determine the best analytical copper speciation method. As the effects detected in our experiments did not always relate to ASV-labile copper or total copper level in water, other measurements of copper speciation, such as free copper ion content in water should be used. Additionally, copper levels in Macquarie Harbour should be monitored on a regular basis, particularly in the vicinity of fish farms. Potential use of data loggers for water quality measurements, including copper concentrations, should be investigated.