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Aquatic plants: unsung but prime salmon habitat

Posted on November 13, 2014 by

Photo by Carson Jeffres/UC Davis
A Chinook salmon in Big Springs Creek near Mount Shasta. Photo by Carson Jeffres/UC Davis, 2012

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. 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 streams grow faster than those reared in streams fed by snowmelt and rainfall runoff, such as those in California’s redwood forests.

Aquatic macrophytes in the spring-fed Shasta River. Photo by Robert Lusardi
Aquatic macrophytes in the spring-fed Shasta River.Photo by Robert Lusardi/UC Davis

Surprisingly, they suggest that trout benefit not only from the stable flow and temperature regimes of spring 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 debris provides structure, velocity heterogeneity and refuge from predators [1]. It also has been shown to increase habitat area and reduce competition between fish [2]. Similarly, biologists have long recognized pools as superior salmonid habitat for providing predator and thermal refuge, velocity heterogeneity and improved food resources [3].

UC Davis salmon-growth experiment on Big Springs Creek near Mount Shasta. Source: Robert Lusardi
UC Davis salmon-growth experiment on Big Springs Creek. Photo by Robert Lusardi/UC Davis

Spring 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 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, the 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. The trout gorged on seemingly unlimited food while exerting minimal energy.

Juvenile coho salmon in Big Springs Creek, by Carson Jeffres/UC Davis
Juvenile coho salmon in Big Springs Creek, by Carson Jeffres/UC Davis

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 [4]. Other studies have shown that macrophytes reduce competition between individual fish through visual isolation and increase fish density [5].

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.

Underwater tour of Big Springs Creek, by Carson Jeffres/UC Davis
Underwater tour of Big Springs Creek, by Carson Jeffres/UC Davis

It’s no surprise that spring 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 [6].

Pacific salmon recovery efforts in the Lower Klamath River drainage often focus on spring systems such as the Shasta River because of 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, a doctoral candidate in ecology at the UC Davis Center for Watershed Sciences, researches 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 in monitoring the restoration of Big Springs Creek.

Citations

[1] Crook, D.A. & Robertson, A.I. (1999) Relationships between riverine fish and woody debris: implications for lowland rivers. Marine and Freshwater Research, 50, 941-953.

[2] Sundbaum, K. and I. Naslund. 1998. Effects of woody debris on the growth and behaviour of brown trout in experimental stream channels. Canadian Journal of Zoology-Revue Canadienne De Zoologie 76:56-61.

[3] Nielsen, J.L., Lisle, T.E. & Ozaki, V. (1994) Thermally stratified pools and their use by steelhead in Northern California streams. Transactions of the American Fisheries Society, 123, 613-626; Rosenfeld, J.S. & Boss, S. (2001) Fitness consequences of habitat use for juvenile cutthroat trout: energetic costs and benefits in pools and riffles. Canadian Journal of Fisheries and Aquatic Sciences, 58, 585-593.

[4] Champion, P. D. and C. C. Tanner. 2000. Seasonality of macrophytes and interaction with flow in a New Zealand lowland stream. Hydrobiologia 441:1-12.

[5] Eklov, A.G. & Greenberg, L.A. (1998) Effects of artificial instream cover on the density of 0+ brown trout. Fisheries Management and Ecology, 5, 45-53.

[6] National Research Council (NRC). 2004. Endangered and Threatened Fishes in the Klamath River Basin: Causes of Decline and Strategies for Recovery; Wales, J. H. 1951. The decline of the Shasta River king salmon run. Bureau of Fish and Wildlife. California Division of Fish and Game