Effects of pre-acclimation to aluminium on the physiology and swimming behaviour of juvenile rainbow trout (Oncorhynchus mykiss) during a pulsed exposure

Anthropogenic acidification of the freshwater environment causes aluminium to be mobilised into the aquatic environment. When pH falls below 5.5, expo sure to aluminium concentrations as low as 12.5 mug.l(-1) can cause serious physiological disturbances in freshwater fish. However, under constant lab oratory exposures fish can acclimate and recover physiological status withi n 5-30 days. In reality, fish in the wild are likely to experience chronic sub-lethal exposure, with occasional elevations (pulses) to much higher lev els. The experiment described here investigated the effects of an environme ntally realistic, 4-day pulse exposure to a high level of aluminium (36 mug .l(-1)) in two groups of juvenile rainbow trout. One group was exposed to a lower level of aluminium (24 mug.l(-1)) for 16 days before and 10 days aft er the pulse ('aluminium-acclimated' fish). A second group was exposed to p H 5.2 alone for 16 days before and 10 days after the pulse ('aluminium-naiv e' fish). A third group exposed to pH 5.2 alone for 30 days (no aluminium a dded) acted as controls. Triplicate groups of 24 juvenile rainbow trout (2. 3-16.7 g) were randomly allocated to one of these three treatments. Swimmin g behaviour was monitored throughout and samples were taken on days 14, 20, 22, 26 and 30 for assessment of physiological status. No treatment effects were recorded in the control group (pH 5.2 alone). Fish in the 'aluminium- acclimated' treatment became hypo-active upon initiation of the exposure to 24 mug.l(-1) aluminium, but recovered after just 4 days of this exposure. Subsequent challenge on day 16 with the 36 mug.l(-1) aluminium 'pulse' caus ed these fish to became hypo-active again, but they recovered normal swimmi ng behaviour whilst still subject to the 4-day pulse. The 'aluminium-naive' fish also became hypo-active during the pulse exposure (36 mug.l(-1) alumi nium). However, they did not exhibit any recovery of swimming behaviour, ei ther during the pulse, or even 6 days after the cessation of the pulse, des pite a rapid depuration of gill aluminium load (within 2 days of the pulse finishing). Mortality was low in the aluminium-acclimated fish (4%) and sig nificantly higher in the aluminium-naive fish (26%). Haematological disturb ances were most extreme in the aluminium-naive fish and had not recovered t o control levels 6 days after the end of the pulse. This study provides new evidence, using behavioural responses, that previous exposure to low level s of aluminium may be an important factor abating the impact of aluminium o n fish in the natural environment. (C) 2000 Elsevier Science B.V. All right s reserved.