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.
Floodplain habitat has also been shown to be extremely productive. Katz et al. (2017) found that juvenile Chinook reared on experimental floodplains in the Central Valley exhibited extremely high growth rates (average=0.76 mm/day), which was attributed to the abundance of large bodied zooplankton. Corline et al. (2016) found that these experimental floodplains exhibited densities of zooplankton up to 1000 times greater than those found in the river (e.g., Figure 2). Other studies in the Central Valley have come to similar conclusions. Mostly notably, Jeffres et al. (2008) also found differences in juvenile Chinook growth rates between those reared in a natural floodplain and in-river enclosures and attributed those differences, at least in part, to abundant food resources. Though less studied, freshwater lagoons can also be extremely productive habitats. Recent work by Osterback et al. (2018) found that both steelhead and coho occupied a freshwater lagoon during periods of relatively high water temperature and low dissolved oxygen, yet exhibited high growth rates during these periods. The authors proposed that elevated growth rates were a function of high rates of food availability in the lagoon.
To be sure, sufficient cold water is absolutely vital for the long-term persistence of salmonids. Warmer water temperature can increase stress, agnostic behavior, and increase disease virulence. However, rare habitats that are rich with food likely confer growth advantages to salmon and trout during rearing. Growth and size of juvenile fish is important because research has shown that fish entering the ocean at larger size generally have a better chance for survival and eventual adult return. Additionally, climate change predictions suggest earlier snowmelt runoff and an increase in stream water temperature, particularly in California. Under such conditions, food rich habitats are well positioned because coldwater fishes would likely be able to metabolically compensate for slight increases in water temperature with abundant food resources. Unfortunately, floodplains, lagoons, and other food rich habitats are some of the most threatened in California. Identifying, conserving, and improving access to these habitats is vital and will strongly aid in the recovery of imperiled salmonid populations.
Dr. Robert Lusardi is the California Trout-UC Davis Wild and Coldwater Fish Scientist.
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Fall River. Photo by Val Atkinson
Figure 2. Photograph of relative zooplankton densities between an experimental floodplain, adjacent slough habitat, and the Sacramento River. Photo: Miranda Bell Tilcock.
