METABOLIC COSTS AND PHYSIOLOGICAL CONSEQUENCES OF ACCLIMATION TO ALUMINUM IN JUVENILE RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) .2. GILL MORPHOLOGY, SWIMMING PERFORMANCE, AND AEROBIC SCOPE

Juvenile rainbow trout (Oncorhynchus mykiss, 5-13 g) were chronically exposed to sublethal Al (38 mug.L-1) in acidified soft water (Na+ = 85 , Ca2+ = 28 muEq.L-1, pH 5.2-5.4) for 36 d. Acclimation (increased res istance to challenge with 162 mug Al.L-1 Al at pH 5.2) occurred after 5 d and was associated with a fourfold increase in gill mucous cell de nsity and reduction in apparent lamellar surface area; initially eleva ted blood-water diffusion distances returned to normal after 34 d, but the reduction in apparent surface area persisted. Chronic exposure to acid alone (pH 5.2, same water chemistry) caused no morphometric chan ges but resulted in persistent impairment of U(crit) (critical aerobic swimming speed) by about 10%. This was due to increased oxygen requir ements at subcritical swimming speeds (loading stress) and was allevia ted when trout were swum at pH 6.5 (zero Al) on day 36. In trout preex posed to sublethal Al, U(crit) was chronically impaired by approximate ly 16% due to loading stresses and reduction in the maximum rate of ox ygen uptake, Mo2 max (limiting stress); U(crit) and Mo2 max remained d epressed even when fish were swum at pH 6.5 (zero Al). Reduced gill ar ea compromises the aerobic scope for activity but may be an unavoidabl e cost of acclimation to Al.