Measurement of median lethal dose as a rapid indication of contaminant toxicity to fish

A new method was developed to rapidly measure the toxicity of contaminants to fish over 96 h. The method is the measurement of median lethal doses by intraperitoneal injection (IP-LD50). Contaminants are dissolved in 5% ethanol in saline or in cod-liver oil and injected at a rate of 1.0 ml per 100 g of fish. The results of parallel bioassays to measure toxicity by oral intubation (OI-LD50) or aqueous exposure (LC50) were closely linked to IP-LD50 values. The coefficient of determination (r2) for a linear regression between IP-LD50 and OI-LD50 values was 0.99. In contrast, the r2 between IP-LD50 and LC50 values was lower (0.44) and the regression curvilinear. However, r2 improved to 0.82 when LD50 values were divided by the logarithm of the octanol-water partition coefficient (log P). Therefore, some of the differences in toxicity between chemicals may be due to interactions between their lipid (or water) solubilities and their rate of uptake by fish. The IP-LD50 values of five chemicals varied with the carrier used; toxicity was lower for the oil carrier compared to injections with 5% ethanol in saline. The magnitude of the difference varied with the chemical tested, so that the two measures of toxicity did not appear to be related. However, the relationships again improved when the IP-LD50 (oil) was divided by log P. The measurement of IP-LD50 values was faster, simpler and less expensive than traditional LC50 bioassays. The use of IP-LD50 values to screen chemicals for their relative toxicity to fish will save time and money without any real loss in the relevance of conclusions. We propose their use for research on environmental quantitative structure-activity relationships.