Seasonal variation in mortality of brown trout (Salmo trutta) in an acidic aluminium-rich lake

We have studied the seasonal variation in aluminium toxicity in caged brown trout (Salmo trutta), during one year (October 1994 to September 1995) in a small acidic aluminium rich lake, Lake Nepptjern (ANC -22.6 μeq l-1, pH 5.2, total Al 400 μg l-1). Trout from two and three different year classes were exposed each month to the lake water for 48 h. Fish were placed in keepnets located in the middle of the lake, at 2 m depth. Fish mortality and water physico-chemistry were monitored during the exposures. The concentration of inorganic monomeric aluminium in the water was approximately 300 μg l-1 in average, and the water was acutely toxic to the fish. The observed mortality varied throughout the year, and was highest during spring and summer. During spring, the small fish were more sensitive to the toxic water than larger fish, while the opposite was the case during summer. Water temperature and fish length could explain most of the seasonal variation in mortality. Statistical analyses indicated that water acid neutralising capacity (ANC) and the amounts of total organic carbon (TOC) and silicon in the water also could explain some of the variation in mortality. Variation in other physico-chemical parameters, however, such as silicon, TOC and ANC could only explain the variation in mortality to a limited extent. The mechanism for the temperature dependent mortality is discussed, and we suggest that the dependence of water O2-solubility and fish metabolism upon temperature is of importance. The difference in mortality between small and large fish is discussed in terms of the gill area/body weight ratio, and it seems to be that small fish suffer more from diffusive ion loss having a larger relative gill area than larger fish. Large fish, on the other hand, have a lower relative maximum oxygen uptake than small fish and will suffer more under conditions where aluminium is accumulated on the gill surface.