The Current Magazine Spring 2018 - Page 26

By DR. ROB LUSARDI

CalTrout/UC Davis Wild &

Coldwater Fish Research Lead

WHAT THE SCIENCE SAYS

When I say “salmon and trout habitat”, what comes to mind? Temperature? Flow? Riparian cover? Large woody debris? Cobble substrate? Undoubtedly these all represent important types of habitat for trout and salmon. But what about food? Food (e.g., stream insects) and its role in salmonid habitat has largely been ignored. There may be good reason for this. Many streams and rivers are nutrient poor and relatively unproductive, suggesting that food is typically hard to come by. However, there are several exceptions to this. Delineating and restoring productive rivers, tributaries, and habitats that produce abundant food resources is, at least in part, critical to restoring robust and resilient salmonid populations throughout California. If these habitats are indeed rare, that’s all the more reason to conserve them. Rare habitats can improve life history diversity and species resilience to change (e.g., climate change).

Very few papers in the scientific literature point to food as an important characteristic of juvenile trout and salmon habitat. Overwhelmingly, physical habitat such as stream temperature or flow are identified as the primary factors affecting juvenile salmon or trout during rearing. For instance, Welsh et al. (2001) found that juvenile coho salmon sought colder water when weekly maximum averages approached ~62 °F on the Mattole River in Northern California, and that temperature threshold is broadly applied to other watersheds throughout the state. Yet, Bisson et al. (1988) found strong rates of juvenile coho production in Washington streams exhibiting temperatures above ~77 °F. The authors speculated that the abundance of invertebrate food played an important role in those observations. In another study, Railsback and Rose (1999) found that food, not temperature, limited the growth of rainbow trout during summer. So, how do we reconcile these results? Watersheds are an amalgamation of context specific habitats. Importantly, ample food can change the playing field and allow salmonids to either occupy or take advantage of slightly warmer water habitats and, in some cases, improve growth rates.

Volcanic spring-fed rivers are a good example of productive river ecosystems that are of high conservation value. Lusardi et al. (2016) found that spring-fed rivers in the upper Sacramento drainage, on average, exhibited food densities 7-fold greater than adjacent runoff rivers in the same river basin and up to 16-fold greater during winter (Figure 1). Lusardi et al. (in prep) also showed that juvenile coho salmon could compensate for warmer water temperatures and exhibit high growth rates when they took advantage of high densities of aquatic invertebrates in the Shasta River, a tributary to the Lower Klamath River. Recent work by Jeffres et al. (in prep) compared O. mykiss growth rates between the Shasta River (spring-fed) and Scott River (runoff) and found that Shasta River fish exhibited daily growth rates 20% greater than those from the Scott River and attributed the difference to a combination of food and water temperature during rearing.

In this column we

highlight important

scientific publications

by CalTrout staff and

others that expand upon

our understanding of the management or science regarding trout, steelhead, and salmon in California.