The Effect of Deposited Fine Sediment on Summer Survival and Growth of Rainbow Trout in Riffles of a Small Stream

—Elevated fine-sediment inputs to streams can alter a variety of conditions and processes, including the amount of fine sediment stored in riffles. We sought to measure the influence of deposited fine sediment on the survival and growth of juvenile rainbow trout Oncorhynchus mykiss (106–130 mm fork length) using a field experiment that included 18 enclosures in riffles of a small northwestern California stream. The experiment included six replicates of three levels of deposited fine sediment (low, background, and high) that embedded riffle cobbles at 0, 50, and 100%, respectively. Only 1 of 12 fish survived in highsediment enclosures, while survival of fish in lowand background-sediment treatments equaled or exceeded 50%. Lowand background-sediment treatments could be distinguished from each other by a difference in fish growth: fish in the low-sediment treatment gained mass, on average, while all surviving fish in the background-sediment treatment lost mass. In addition to providing relatively high survival and growth benefits for juvenile rainbow trout, low-sediment experimental units were colonized at significantly higher rates by other vertebrates, particularly coastal giant salamanders Dicamptodon tenebrosus. The amount of stored fine sediment in small streams may substantially influence the total amount of habitat available to vertebrates at the watershed scale. Human activities such as road building, agriculture, and timber harvest can elevate the input of fine sediment into stream channels. This sediment may be periodically transported as suspended load or bed load, or stored in the channel. All three of these outcomes may have important consequences for stream biota. For example, elevated suspended load increases turbidity, which reduces capture success for drift feeding fish (Sweka and Hartman 2001), while higher bed load transport may affect the density of benthic invertebrates (Culp et al. 1986) and the reproductive success of some fishes (Montgomery et al. 1996). During periods of low to moderate streamflow when the capacity for sediment transport is low, stream biota may be most affected by fine sediment stored in the channel. Stored fine sediment can affect fish in several ways. Channel aggradation can reduce surface streamflow by increasing flow through the streambed, thereby reducing fish survival (May and Lee 2004) and growth (Harvey et al. 2006). Deposited sediment can also reduce the availability of benthic invertebrates vulnerable to fish predators due to species-specific substratum preferences and increase agonistic interactions among fish due to reduced visual isolation. Suttle et al. (2004) linked both of these mechanisms to a negative relationship between the growth of age-0 steelhead (anadromous rainbow trout Oncorhynchus mykiss) and the embeddedness of the substratum in artificial channels placed in pools. The composition of the substratum can also influence predation risk for fish, with consequences for both growth and survival. Fischer (2000) measured 30% higher respiration rates for burbot Lota lota on pebble substratum compared with cobble substratum, even in the absence of predators. Fish may forego feeding opportunities if feeding requires substantial exposure to predators. Coarse substratum is more likely to provide microhabitats offering profitable feeding opportunities with relatively low predation risk. Substrate-dependent predation risk can also directly influence survival (e.g., White and Harvey 2001). The potential value of coarse substratum in reducing predation risk by providing cover is indicated by its increased use by fish when predators are present (Vehanen and Hamari 2004). Relatively coarse, unembedded substratum may be particularly important in shallow habitats in small streams because predation risk can be depth-dependent (Harvey and Stewart 1991), and low streamflows may provide little surface turbulence to conceal fish. In this study, our objective was to measure the effect of deposited fine sediment on the survival and growth of juvenile rainbow trout in shallow habitat in a small stream during summer, using a field experiment that incorporated the potential effects of deposited fine sediment on both energetics and predation risk. *Corresponding author: [email protected] Received April 7, 2008; accepted July 24, 2008 Published online April 20, 2009 434 North American Journal of Fisheries Management 29:434–440, 2009 American Fisheries Society 2009 DOI: 10.1577/M08-074.1 [Article]