As part of a program to study the impact of aromatic hydocarbons on tropical marine species, this paper presents results of comparative work on the effects of benzene and naphthalene to the white mullet Mugil curema and the «corocoro» Orthopristis ruber, two important fish species of the Venezuelan coasts. To this end, lethal and sublethal effects were measured by estimating the rate of excretion and respiratory activity (oxygen consumption, VO,) in order to establish an oxygen: nitrogen (O:N) index which served for comparison between species and chemical compounds. Acute toxicity estimates for benzene and naphthalene yielded 96h-LC,; values of 7.19 mg/1 (95% confidence limits 6.39-7.89) and 1.79 mg/1 (95% confidence limits 1,32-2.13) mg/1 respectively for M. curema, while for O. ruber values of 5.70 mg/1 (95% confidence limits 4.34-6.30) and 3.08 mg/1 (95% confidence limits 2.74 - 3.33), respectively were calculated. This indicates a lower acute toxicity for benzene than naphthalene on both species. With regard to physiological endpoints, overall respiratory rates of fish exposed to either benzene, naphtalene or controls indicate higher values for M. curema than for O. ruber. Organisms exposed to benzene at concentratoins of 0.01 mg/1 of benzene, VO, for M. curema and O. ruber yielded 0.62 and 0.36 mg O,/g/h respectively, while a concentration of 0.1 mg/1 resulted on VO,’s of 0.95 and 0.44 mg O_/g/h respectively. Naphthalene at a concentrations of 0.01 mg/ gave VO,’s 0f 0.62 and 0.39 mg O./g/h for M. curema and O. ruber, respectively, while at a concentration of 0.1 mg/1 the same calculations resulted on VO, values of 0.94 and 0.64 mg O,/g/h. Control VO,’s indicated a value of 0.79 mg/g/h for M. curema and 0.29 mg O /g/h for O. ruber. No significant differences between the effect of benzene and naphthalene
on the oxygen consumption were found. Ammonia excretion rate for juvenile of M. curema exposed to benzene concentrations of 0.01, 0.1 and 1 mg/l yielded values 0f 0.23+0.07;0.26 £0.13 and 0.51 £0.24 mg NH -NH,/g/day, respectively; these values were lower than control (0.81 £0.09 mg NH,-NH_/g/day). M. curema exposed to naphthalene yielded values of 0,47 +0.09; 0.25+0.04 and 0.41 £0.21 mg NH4-NH_/g/day. While for O. ruber values of 0.32 £ 0.05; 0.30 + 0.07 and 0.31 £ 0.06; and 0.39 + 0.03; 0.41 + 0.06 and 0.30+0.06 mg NH4-NH,/g/day exposed to benzene and naphtalene concentrations respectively, compared with controls (0.92£0.23 mg NH, -NH,/g/day). In addition, values of O:N ratios higher than 7 indicate an increased dependence of both species to carbohydrates and/or lipid reserves to obtain energy in order to compensate metabolic costs.