Cadmium accumulation, gill Cd binding, acclimation, and physiological effects during long term sublethal Cd exposure in rainbow trout

Juvenile rainbow trout, on 3% of body weight daily ration, were exposed to 0 (control), 3, and 10 mu g l(-1) Cd (as Cd(NO3)(2). 4H(2)O) in moderately hard (140 mg l(-1) as CaCO3), alkaline (95 mg l(-1) as CaCO3, pH 8.0) water for 30 days. Particular attention focused on acclimation, and on whether a gill surface binding model, originally developed in dilute softwater, coul d be applied in this water quality to fish chronically exposed to Cd. Only the higher Cd concentration caused mortality (30%, in the first few days). The costs of acclimation, if any, in our study were subtle since no signifi cant effects of chronic Cd exposure were seen in growth rate, swimming perf ormance (stamina and U-Crit), routine O-2 consumption, or whole body ion le vels. Substantial acclimation occurred in both exposure groups, manifested as 11- to 13-fold increases in 96-h LC50 values. In water quality regulatio ns, which are based on toxicity tests with non-acclimated fish only, this r emarkable protective effect of acclimation is not taken into account. Cd ac cumulated in a time- and concentration-dependent fashion to 60-120 x (gills ), 8-20 x (liver), 2-7 x (carcass), and 5-12 x (whole bodies) control level s by 30 days. Chronically accumulated gill Cd could not be removed by ethyl enediaminetetraacetic acid (EDTA) challenge. These gill Cd concentrations w ere 20- to 40-fold greater than levels predicted by the gill-binding model to cause mortality during acute exposure. In short-term gill Cd-binding exp eriments (up to 70 mu g l(-1) exposures for 3 h), gill Cd burden increased as predicted in control fish, but was not detectable against the high backg round concentrations in acclimated fish. In light of these results, Cd upta ke/turnover tests were performed using radioactive Cd-109 to improve sensit ivity. With this approach, a small saturable binding component was seen, bu t could not be related to toxic response in acclimated fish. Acclimated tro ut internalized less Cd-109 than control fish, but interpretation was compl icated by the possibility of radioisotopic exchange and specific activity d ilution in the large 'cold' Cd pool on the gills. We conclude that gill Cd burden is not predictive of mortality in acclimated fish, that the present gill modelling approach does not work in acclimated fish, and that longer t erm Cd-109 turnover studies are needed for this purpose. (C) 1999 Elsevier Science B.V. All rights reserved.