Water Cooler

Section A on the Green RiverEngineering a more eco - friendly water cooler on the Green River


Trout Abundance on the Green River Depends on Food

Studies vary as to the limiting factor for trout abundance, sometimes citing habitat over food availability, but the Green River appears to be food limited (Filbert and Hawkins 1995). Deits of Green River trout of all sizes are consprised mostly of invertebrates (Vinson et al. 2006). Based on the Filbert and Hawkins study (1995) and the work by Vinson et al. on trout diets (2006), the abundance of trout in the middle Green River is controlled to a large degree by the abundance of macroinvertebrates.

One factor contributing to macroinvertebrates abundance in a river ecosystem is the energy flowing into that ecosystem. The addition of either energy or nutrients to a river system generally results in increased primary productivity (Forrester et al. 1999) subsequently resulting in higher trout densities. Construction of Flaming Gorge Dam resulted in decrease in sediment, an increase in available light, and bloosting the abundance of marcoinvertebrates. Vinson (2001) documented an increase in macroinvertebrates from 1000 organisms/meter squared to 10,000; resulting in a much larger food source for trout.

This high level of prey ultimately boosts the predator population which feeds on it (e.g., trout). As a comparison point, the most productive Wyoming trout streams have over 5,000 trout per square meter. As the populations height in 1987 to 1989, the Green River below Flaming Gorge Dam supported 20,000 to 22,000 salmonids per mile. The count has come down recently due to reduced stocking, with current estimates ranging from 8,000 to 14,000 fish per mile.


Trout Species and Distribution on the Green River below Flaming Gorge

Trout didn't inhabitat the middle Green River to their present extent today - largely due to the construction of Flaming Gorge Dam in 1962. With the dam came drastically reduced water temperature, and far less silt, creating ideal habitat for cold-water loving trout.

Flaming Gorge Dam on the Green River in Utah

Today a self-sustaining population of brown trout can be found in these waters, while the rainbow and cutthroat populations are maintained through a large stocking effort.

In the middle Green River, only brown trout and mountain whitefish spawn successfully enough to naturally maintain their populations. The success of these two species is proof of adequate spawning habitat below Flaming Gorge Dam. So, the question becomes: Why don't rainbow and cutthroat trout spawn successfully here?

An 18 inch brown trout caught on the Green River in Utah

Roger Scheidervin (2006) hypothesized that the cause is rooted in the recent change in flows below the dam. Since 1992, when the river shifted to a flow regime with a large spike for spring floods and relatively constant flows for the rest of the year, brown trout populations have increased dramatically in this tail water (Scheidervin 2005). Since 2000, brown trout have increased from 40% of the tail water species composition to 65% (Schneidervin 2005).

A brown trout caught on the Green River in Utah

Both brown trout and whitefish spawn in habitat similiar to that used by rainbow and cutthroat. The major difference is their spawning season. Brown trout and whitefish spawn in the fall. These species fry emerge from the gravel up to several months before rainbow or cutthroat fry. Their earlier emergence likely allows brown trout and whitefish to develop and move to safer habitat. Consequently they avoid being swept downstream in the large spring flood flows. Studies show newly emerged trout benefit from lower water flows during the emergence period (Nehring et al. 1993), and need plenty of slower backwater habitat during the first couple of years of development (Moore and Gregory 1988).

In the middle Green River the stocking of new rainbow and cutthroat accounts for the vast majority of population growth. Roughly 10% of rainbow and cutthroat trout sample every fall are wild-born fish (Grass 2004). As of 2006, the Utah State Division of Wildlife Resources stocks 25,000 seven inch rainbow trout every spring.

Reference - Salmonid Abundance and Distribution in the middle Green River by Karrigan S. Bork