RESPONSES OF JUVENILE RAINBOW-TROUT UNDER FOOD LIMITATION, TO CHRONICLOW PH AND ELEVATED SUMMER TEMPERATURES, ALONE AND IN COMBINATION

Rainbow trout were exposed (90 days) in synthetic soft water to sublet hal low pH (5.2) and a simulated climate warming scenario (+2 degrees C above the control summer temperature range of 16.5-21 degrees C), al one and in combination, under conditions of limited food (similar to 4 % dry body weight day(-1)). Weight specific oxygen consumption rates ( MO2) were similar to 55% of MO2(max), in contrast to similar to 75% of MO2(max) found in trout fed an unlimited ration. This is likely due t o a reduction in food quantity and thus feeding activity. However, the trout exposed to low pH at control temperatures exhibited higher conv ersion efficiencies and increased growth. In contrast, trout exposed t o +2 degrees C had reduced growth rates. No ionoregulatory disturbance occurred in any treatment, suggesting that this ration was sufficient to provide a replacement salt load in the diet. Energy budgets indica ted that the limited ration resulted in a lowered optimum temperature for growth, with a greater proportion of the energy intake dissipated for metabolic expenditure, resulting in reduced conversion efficiencie s. A fourfold reduction in faecal and unaccounted energy losses indica ted higher absorption efficiencies than in satiation-fed trout. (C) 19 98 The Fisheries Society of the British Isles.