A stepwise-reduction-cold vapor atomic absorption spectrometry (CVAA) method has been further developed. The method is relatively simple and quick and allows measurements of both inorganic and organic mercury in the same small biological sample. It provides an effective technique for studying biomagnification of mercury in a marine grazing food-chain (algae/mussels/fish). In laboratory experiments with mussels exposed to both inorganic mercury (965 ng I-') and organic mercury (methylmercury) (35 ng I-'), and fed algal cells at a natural low concentration from a chemostat for 80 d, uptake of both mercury species was linear. Organic mercury was taken u p 15 times more readily than inorganic mercury. Flounders force-fed with food contaminated with equal concentrations of organic and inorganic mercury for 46 d readily accumulated organic mercury in blood cells, liver, kidney and muscle-tissue while inorganic mercury was only accumulated in measurable amounts in Liver and kidney. During an elimination experiment (48 d) with both force-fed and starved flounders (loaded with mercury), organic mercury concentration did not decrease in the muscle-tissue, but significantly decreased in the liver, kidney and blood cells, while inorganic mercury tended to increase in the liver. Thus organic mercury may be biotransformed to inorganic mercury in the liver. Relatively low concentrations of organic mercury in the gall bladder indicate that enterohepatic recirculation may not play a similar role in fish as reported for mammals. Higher mercury concentrations in the proximal part of the intestine compared to the distal part show, however, that some mercury may be secreted via the bile and reabsorbed, but no differences between fed and starved fish were found Though only about 1 % of the total mercury in chronically polluted waters may be in the organic form, an enhanced uptake rate of organic mercury in mussels leads to differential partitioning of the 2 mercury forms. Since fish such as flounder may primarily feed on juvenile bivalves about 75 % of the total mercury ingested by fish may be in the inorganic form. The efficient accumulation of organic mercury and the lack of elimination results in an increasing organic mercury concentration with both age and trophic level (i.e. biomagnification) in the marine grazing food-chain.

Bothriocephalus acheilognathi Yamaguti, 1934, a tapeworm known to be pathogenic to some fish species, has become established in the endangered humpback chub ( Gila cypha Miller, 1964) in Grand Canyon, USA, following the tapeworm’s introduction into the Colorado River system. The potential impact of this tapeworm on humpback chub was studied by exposing the closely related bonytail chub ( Gila elegans Baird and Girard, 1853) to the parasite under a range of conditions that included potential stressors of humpback chub in their natal waters, such as abrupt temperature change and a limited food base. Survival of infected fish under low food rations was considerably lower than that of control fish, and mortality of infected fish began 20 days earlier. Growth of infected fish was significantly reduced, and negative changes in health condition indices were found. No significant negative impacts were revealed from the synergistic effects between temperature shock and infection. Bothriocephalus acheilognathi does present a potential threat to humpback chub in Grand Canyon and should be considered, along with conventional concerns involving altered flow regimes and predation, when management decisions are made concerning conservation of this endangered species.