By Robert Lusardi and Ann Willis
For decades, California’s management and restoration of salmon and trout populations have focused on principles rooted in coastal redwood streams, mostly fed by rainfall runoff. These concepts portray ideal salmonid habitat as deep pools, shallow riffles and “large woody debris,” such as fallen trees and limbs.
Recent studies on spring-fed streams challenges this mindset. The findings strongly suggest these streams should play a larger role in the recovery and management of sensitive cold-water species, particularly salmonids.
Researchers at the UC Davis Center for Watershed Sciences found that trout in spring-fed streams grow faster than those reared in runoff streams in the same watershed.
Surprisingly, they suggest that trout benefit not only from the stable flow and temperature regimes of spring-fed streams but also from another dominant – yet much underappreciated – habitat feature: aquatic plants.
Known as macrophytes, these rooted vascular plants provide similar benefits as pools, and large woody debris.
As fish habitat, this woody debris provides structure, velocity heterogeneity and refuge from predators . It also has been shown to increase habitat area and reduce competition between fish . Similarly, biologists have long recognized pools as superior salmonid habitat for providing predator and thermal refuge, velocity heterogeneity and improved food resources .
Spring-fed streams generally lack the high volume flows of runoff streams that transport woody debris and scour pools. As a result, scientists and resource managers have paid less attention to the physical habitat dynamics of spring-fed systems than to other beneficial factors such as streamflow and water temperatures.
Macrophytes grow in many spring-fed streams and are largely a product of naturally occurring nutrients, stable flow and temperature, open canopy and low gradient. These plants provide many of the same benefits to trout and salmon of the more classic redwood streams type habitat.
On the Shasta River, a large spring-fed tributary to the Lower Klamath River, the UC Davis researchers conducted an experiment to understand the potential benefits of macrophyte habitat on juvenile steelhead trout.Presented with multiple habitat types, steelhead overwhelmingly selected macrophyte habitat during spring and summer foraging. The researchers found that macrophyte habitat provided abundant food – the plants also are important for invertebrates – and refuge from high-velocity currents. This suggests that the use of macrophyte habitat allowed trout to gorge on seemingly unlimited food while exerting minimal energy.
Unlike more classic habitat forms, the effects of macrophytes on trout and salmon operate at larger spatial scales. Additional research on the Shasta River and elsewhere has shown that macrophytes increase stream water depth and wetted habitat area, and can reduce water temperature through shading . Other studies have shown that macrophytes reduce competition between individual fish through visual isolation and increase fish density .
These broader benefits may be particularly important for salmonid populations in California during late summer and early fall, when flows in runoff streams typically decline and water temperatures rise.
It’s no surprise that spring-fed streams such as Hot Creek in the eastern Sierra historically boasted some of the highest trout densities in California. Likewise, Shasta River historically supported 50 percent of the Klamath Basin’s Chinook salmon, even though it accounts for only 1 percent of the basin’s annual streamflow .
Pacific salmon recovery efforts in the Lower Klamath River drainage often focus on spring-fed systems such as the Shasta River because they provide flow stability and optimal thermal habitat for rearing salmonids.
While many factors contribute to fish production, macrophyte habitat has received far less attention for its beneficial effects on trout and salmon. Fully understanding such species-to-species interactions as those between plants and fish is important and will assist salmonid conservation planning and recovery.
Robert Lusardi is a post doctoral scholar in ecology at the UC Davis Center for Watershed Sciences and a California Trout-UC Davis Wild and Coldwater Fish Researcher. He studies stream ecology and food web dynamics of volcanic spring-fed ecosystems in Northern California. Ann Willis, an engineer who coordinates research programs at the Center, has done extensive fieldwork monitoring the restoration of Big Springs Creek.
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