INFLUENCE OF SALINITY ON COPPER AND AZIDE TOXICITY TO LARVAL TOPSMELT ATHERINOPS-AFFINIS (AYRES)

Performance of a 7-d growth and survival toxicity test protocol using larval topsmelt, Atherinops affinis (Ayres), was evaluated for copper chloride and sodium azide at representative estuarine salinities. Results showed that topsmelt are amenable to toxicity testing at estuarine salinities ranging from 5 to 34 parts per thousand since control survival was 100% in all toxicity tests. Sensitivity to both toxicants increased at lower salinities, with the LC5Os for copper ranging from 205 mu g/L at 34 parts per thousand to 44 mu g/L at 10 parts per thousand, and those for sodium azide ranging from 54 mg/L at 34 parts per thousand to 7 mg/L at 5 parts per thousand. Larval tissue osmolality decreased with increasing copper concentration relative to control fish. Copper uptake was not affected by changes in salinity. This suggests that increased sensitivity to copper was due, in part, to the increasing physiological challenge of osmoregulation. It is also possible that cupric ion concentration increased at lower salinities, resulting in reduced larval survival. it is hypothesized that increased sensitivity to azide at lower salinity was due to the interaction between azide toxicity and increasing osmotic challenge. A second experiment with azide showed that larval acclimation time did not affect the interaction between salinity and azide toxicity.