Aquatic Sciences 57/4, 1995 1015-1621/95/040350-10 $1.50 + 0.20/0 © 1995 Birkh~iuser Verlag, Basel Survival of crucian carp, Carassius carassius, exposed to a high low-molecular weight inorganic aluminium challenge Antonio B. S. Po1601, Sigurd A. Oxnevad 2, Kjartan Ostbye 2, Ronny A. Andersen 2, Deborah H. Oughton 3 and Leif A. VOllestad 2 Division of General Physiology, University of Oslo, R O. Box 1066 Blindern, N-0316 Oslo, Norway 2 Division of Zoology, University of Oslo, R O. Box 1066 Blindern, N-0316 Oslo, Norway 3 ISEM, Agricultural University of Norway, R O. Box 26, N-1432 As-NLH, Norway Key words: Crucian carp, Carassius carassius, A1 toxicity, surface waters acidifi- cation. ABSTRACT Crucian carp were exposed to three combinations of pH and inorganic aluminium concentrations for 25 days. Mortality, plasma chloride and haematocrit, and gill aluminium content were investi- gated. No mortality was observed in any of the three exposures. Crucian carp survived acidic Al-rich water (pH 5.17) with a high concentration of low-molecular weight inorganic A1 species (242 pg1-1) for 25 days. These fish had lowered plasma chloride levels and high amounts of aluminium on their gills compared to fish exposed to acidic Al-poor water (pH 5.16 and 12 pgl -~ Ali) and control water (pH 6.29 and 4 pg1-1 All). Haematocrit was the same in all three exposures. Because aluminium was not acutely toxic to the anoxia tolerant crucian carp, the present results give support to the hypothesis that A1 polymerization with subsequent hypoxia is of high importance for the mechanism of acute A1 toxicity to fish. Introduction The most serious effect of surface water acidification in the northern temperate regions seems to be the decline of natural fish populaitons (Schofield, 1976; Muniz and Leivestad, 1980; Muniz, 1984). It is now evident that aluminium mobilized from edaphic to aquatic environments is the principal toxicant, killing fish in acidified fresh waters (Burrows, 1977; Schofield, 1977; Dickson, 1978). Low-molecular weight inorganic forms of aluminium, often referred to as inorganic monomeric aluminium (Driscoll, 1984), are believed to be the most toxic aluminium species (Driscoll et al., 1980; Fivelstad and Leivestad, 1984). It has been suggested that the process of A1 polymerization, in which these low-molecular weight A1 species act as precursors, is a emchanism for the acute toxicity of aluminium to fish (Lydersen et al., 1990; Po16o et al., 1994; Po16o, 1995). It is hypothesized that aluminium poly- merizing onto the gill surface leads to severe clogging of the interlamellar spaces, and consequently to hypoxia (Po16o, 1995).