Linking Habitat Quality with Trophic Performance of Steelhead along Forest Gradients in the South Fork Trinity River Watershed, California

Abstract

We examined invertebrate prey, fish diet, and energy assimilation in relation to habitat variation for steelhead Oncorhynchus mykiss (anadromous rainbow trout) and rainbow trout in nine low‐order tributaries of the South Fork Trinity River, northern California. These streams spanned a range of environmental conditions, which allowed us to use bioenergetics modeling to determine the relative effects of forest cover, stream temperature, season, and fish age on food consumption and growth efficiency. Evidence of seasonal shifts in reliance on aquatic versus terrestrial food sources was detected among forest cover categories and fish ages, although these categories were not robust indicators of O. mykiss condition and growth efficiency. Consumption estimates were generally less than 20% of maximum consumption, and fish lost weight in some streams during summer low‐flow conditions when stream temperatures exceeded 15°C. Current 100‐year climate change projections for California threaten to exacerbate negative growth patterns and may undermine the productivity of this steelhead population, which is currently not listed as endangered or threatened. To demonstrate the potential effect of global warming on fish growth, we ran three climate change scenarios in two representative streams. Simulated temperature increases ranging from 1.4°C to 5.5°C during the summer and from 1.5°C to 2.9°C during the winter amplified the weight loss; estimated average growth for age‐1 fish was 0.4‐4.5 times lower than normal (low to high estimated temperature increase) in the warm stream and 0.05‐0.8 times lower in the cool stream. We conclude that feeding rate and temperature during the summer currently limit the growth and productivity of steelhead and rainbow trout in low‐order streams in the South Fork Trinity River basin and predict that climate change will have detrimental effects on steelhead growth as well as on macroinvertebrate communities and stream ecosystems in general.

Linking resource availability and habitat structure to stream organisms: an experimental and observational assessment

“…The trophic significance of this taxon is likely due to their global distribution, high species and functional diversity, and productivity (Armitage et al 1995). Predators, such as juvenile coho in our study, may also preferentially consume chironomid pupae and adults because these life stages have energy-rich reproductive structures providing more energy per individual consumed relative to immature life stages: McCarthy et al (2009) showed that the energy density of winged insects (combined Ephemeroptera, Plecoptera, Trichoptera, and Diptera adults) was 1.53 higher than the energy density of Diptera larvae.…”

Potential Effects of Changes in Temperature and Food Resources on Life History Trajectories of Juvenile Oncorhynchus mykiss

“…Similarly, growth of Sockeye Salmon O. nerka in an Alaskan watershed increased with warming temperatures apparently owing to increased food availability (Schindler et al 2005). However, other empirical evidence has shown negative (McCarthy et al 2009) or no effects (Boughton et al 2007) of increased temperature on the growth of juvenile Pacific salmon and Steelhead in warm water, California streams. A likely reason for the inconsistency among our simulations and some empirical evidence is because of the thermal regime of the study watershed.…”

Causes and consequences of invertebrate drift in running waters: from individuals to populations and trophic fluxes

“…Although drift-foraging fish may have considerable dependence on terrestrial invertebrates (e.g., Wipfli 1997), drift of benthic origin constitutes the primary prey source for many, such as stream-rearing salmonids (e.g., Nielsen 1992), which are often food-limited (Boss and Richardson 2002;McCarthy et al 2009;Wipfli and Baxter 2010). Drift availability is therefore of great importance to drift-feeding fish production, and the magnitude of drift flux to fishes has been directly linked to increased growth (Keeley 2001;Kiffney et al 2014;Weber et al 2014), abundance (Fausch et al 1991), survival (Rosenfeld et al 2005), and movement patterns (Hansen and Closs 2009).…”