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Abstract

A physiological model was developed to estimate fish body toxicant load based on information regarding the chemical exposure regime, fish body weight, lipid content and oxygen uptake. Three organic compounds of different hydrophobicity, 1,2,4,5- tetrachlorobenzene (TeCB), 3,4,5,6-tetrachloroguaiacol (TeCG) and 4,6-dichlorobenzenediol (DBD), were chosen as the test chemicals to carry out a series of investigations before an overall model was assembled. The primary focus of the model was to incorporate physiological components into traditional compartmental models in order to avoid the difficulties associated with complex conventional physiological models. The approach taken was to predict rate constants based on fish oxygen consumption, a parameter speculated, and subsequently shown, to be closely correlated to toxicant transfer in fish. A significant correlation was found between the toxicant uptake process, as characterized by the uptake rate constant (k1), and fish oxygen consumption, regardless of fish size and species. Moreover, the correlation was improved when fish toxicant body load was expressed on a percent body lipid basis. Similarly, fish toxicant depuration tests showed that there also existed a significant relationship between the toxicant depuration rate constant (k2) and fish oxygen uptake regardless of the differences in the chemical octanol/water partition coefficients (Kow ) . The finding that the chosen test compounds did not interfere with fish oxygen consumption after prolonged sublethal exposure justified the use of oxygen uptake as an indicator for fish toxicant transfer and, as equally important, the utilization of a large fish oxygen consumption database (OXYREF) in the proposed chemical modeling. A series of feeding experiments were also carried out and it was concluded that fish toxicant transfer across the gills plays a dominant role in the toxicant accumulation and depuration of non-metabolized chemicals in fish. Uptake of these toxicants in the food was negligible in determining body burden. In view of the above findings a general model was tested in which OXYREF was used to predict fish toxicant body burden. Based on the quantitative analysis, it was shown that the model was reliable and accurate in estimating fish body burden of a number of non-metabolized aquatic toxicants. Values calculated using this model agreed with most determinations reported in the literature. Despite the restrictions and preconditions associated with this physiological model, its main advantage over other compartmental or physiological models lies in the fact that the prediction is based on the actual physiological processes, and fish oxygen consumption rate is far easier and accurate to measure than other physiological parameters even in the absence of the OXYREF. This modified model possesses some functional reality which enables more realistic predictions, making it useful for aquatic environmental risk assessment.