Detection of conspecific alarm cues by juvenile salmonids under neutral and weakly acidic conditions: Laboratory and field tests (Oncorhynchus mykiss, Salvelinus fontinalis)

A variety of fishes possess damage-released chemical alarm signals, that play a critical role in the detection and avoidance of potential predation threats. Recently, it has been demonstrated that the ability of fathead minnows (Pimephales promelas) and finescale dace (Phoxinus neogaeus) to detect and respond to conspecific alarm signals is significantly reduced under weakly acidic conditions (pH 6.0). Rainbow trout (Oncorhynchus mykiss) and brook charr (Salvelinus fontinalis) possess an analogous alarm signaling system. It is unknown, however if the trout alarm signaling system is likewise affected by relatively small changes in pH. I conducted laboratory and field trials to examine the potential effects of acute exposure to weakly acidic (pH 6.0) conditions on the detection and response of conspecific alarm signals by juvenile trout. My laboratory results demonstrate that while juvenile rainbow trout exhibit significant increases in antipredator behaviour under normal pH conditions (pH 7.0–7.2), they do not respond to the presence of conspecific chemical alarm signals (i.e. response is not different from controls) under weakly acidic conditions. Similarly, natural population of brook charr failed to detect conspecific alarm cues in a weakly acidic stream (mean pH 6.11) while they responded to these cues in a neutral stream (mean pH of 6.88). These data suggest significant, sub-lethal effects of acid precipitation on the ability of prey fishes to detect and respond to conspecific alarm cues in natural waterways.