Use of Benthic Prey by Salmonids under Turbid Conditions in a Laboratory Stream

—The negative effect of turbidity on the reactive distance of salmonids has been well established. However, determining the consequences of this relationship for overall feeding success remains problematic, as successful foraging by salmonids across a broad range in turbidity has been observed under a variety of conditions. Previous laboratory and field observations suggest that benthic feeding by salmonids in flowing water affects the turbidity dependence of foraging success. Two experiments were conducted in a laboratory stream to quantify benthic feeding success of salmonids across turbidity treatments ranging from 0 to 400 nephelometric turbidity units (NTU). In one experiment, cutthroat trout Oncorhynchus clarkii and coho salmon O. kisutch were offered the same nonliving prey in the drift and on the stream bottom; in the second experiment, cutthroat trout were offered only live oligochaetes moving along the stream bottom. In the first experiment, benthic feeding success of both cutthroat trout and coho salmon at 100 NTU was at least 70% of their feeding performance in clear water (i.e., 0 NTU), whereas neither species fed at 400 NTU. In the second experiment, benthic feeding success of cutthroat trout at 150 NTU was about 35% of their performance in clear water but dropped to near 0% at 200 NTU; no feeding was observed at 400 NTU. Passive integrated transponder tag detections suggested that cutthroat trout activity increased over the range of 0–150 NTU. Although drift and benthic foraging success decreased over the examined turbidity range, the results suggest that both cutthroat trout and coho salmon are capable of feeding from the benthos in relatively turbid, flowing water. The importance of this ability for salmonid populations will be influenced by the turbidity regime, food availability patterns, and hydraulic complexity of the systems they occupy. Various human activities can alter the suspendedsediment regime of surface waters, thereby creating a variety of consequences for aquatic food webs. One specific link of special concern is the effect of elevated turbidity on the foraging success of animals (e.g., salmonids) that partially rely on visual cues for feeding. Support for this concern comes from experiments documenting negative relationships between turbidity and (1) the distance at which salmonids react to prey in the water column (e.g., Barrett et al. 1992; Gregory and Northcote 1993; Vogel and Beauchamp 1999; Sweka and Hartman 2001a); (2) prey consumption by salmonids (DeRobertis et al. 2003); and (3) salmonid growth (Sigler et al. 1984; Shaw and Richardson 2001; Sweka and Hartman 2001b). Under some conditions, however, salmonids appear able to at least partially mitigate the strong effect of turbidity on the visual detection of prey. Several laboratory experiments involving various floating, planktonic, and benthic prey have revealed no strong relationship between salmonid foraging success and turbidity (Sweka and Hartman 2001b; DeRobertis et al. 2003; Rowe et al. 2003) or have reported greater foraging success in elevated turbidity than in clear water (Gregory and Northcote 1993). Most laboratory observations of nonsignificant or positive turbidity effects on fish foraging success have come from experiments with standing water. However, Sweka and Hartman (2001b) observed that brook trout Salvelinus fontinalis in a laboratory stream with modest water velocity (6 cm/s) were able to maintain foraging success on floating prey over a turbidity range from 0 to 40 nephelometric turbidity units (NTU); however, greater activity associated with foraging at the higher turbidity levels resulted in lower growth rates. Field observations of gut fullness (Arndt et al. 2002; White and Harvey 2007) have not detected strong differences in foraging success for salmonids in streams during relatively turbid (;25–75 NTU) versus relatively clear water conditions, but variation in food availability probably influenced these results. Previous research has left unanswered questions about the foraging abilities of salmonids in turbid, flowing water. Laboratory observations of successful benthic feeding in highly turbid, standing water (e.g., Rowe et al. 2003, who observed rainbow trout Oncorhynchus mykiss feeding on benthic chironomid larvae at turbidity levels up to 320 NTU) and field observations of feeding from the benthos under turbid stream conditions (Tippets and Moyle 1978) suggest that the ability of salmonids to feed from or near the * Corresponding author: [email protected] Received February 27, 2008; accepted June 13, 2008 Published online December 22, 2008 1756 Transactions of the American Fisheries Society 137:1756–1763, 2008 American Fisheries Society 2008 DOI: 10.1577/T08-039.1 [Article]