Cutthroat Trout Management Concepts Cutthroat Trout Management









Effects of Lake Trout on the Ecology of the Yellowstone Area

The Greater Yellowstone Area encompasses many lakes and streams. These bodies of water are not just important to the organisms, such as fish, that live in them. They are very important to the ecology of Yellowstone as a whole, and influence the entire ecosystem.

Lake Trout and Yellowstone Lake

Since the end of the last ice age, Yellowstone Lake has been a stable ecosystem with a limited number of trophic levels.  The few trophic levels in the lake food chain are what, under prevailing ecological theory, make the lake stable.  Adding a new top predator to the lake in the form of the piscivorous Lake Trout could severely disrupt this ecosystem.

Piscivorous fish, such as the lake trout, can clear a lake by indirectly driving phytoplancton production down by up to an order of magnitude (Townsend et al 2000).  This is done by decreasing the numbers of planktivorous fish such as the cutthroat, which increases the number of herbivorous insects and zooplankton, which results in a decrease in phytoplancton.  Yellowstone Lake is a clear, alpine lake already.  How will a possible decrease in planktonic biomass affect the lake ecosystem?  It is impossible to say for sure.  It could alter the biochemistry of the lake by increasing the amount of unused dissolved inorganic minerals in the water.  With no phytoplankton to “fix” these minerals into organic biomass that can be used by the lake ecosystem, these minerals may simply flow out of the lake as it is drained by the Yellowstone River.  This could create a nutrient poor system.


Lake Trout and Surrounding Streams

It is true that there are many Cutthroat that live in rivers and streams in the area, but effects of Lake Trout predation on Yellowstone Cutthroat are not limited to the Cutthroat populations that reside exclusively in Yellowstone Lake.  Many of these river-dwelling Cutthroat do not spend their time there over the harsh mountain winter.  In the fall they migrate into the more hospitable, deeper waters of the lake.  There, they too will encounter predation by Lake Trout.  This predation on the overwintering population will lower river Cutthroat numbers just as it lowers populations of cutthroat living in the lake.  The Yellowstone Lake crisis is really a Yellowstone Cutthroat trout crisis.  There are consequences for both river fish and lake fish.

Lake Trout and the Yellowstone Ecosystem

In the spring, Yellowstone Cutthroat trout migrate out of Yellowstone Lake and up into the rivers and streams that feed into it.  They do so to spawn.  Spawning trout are easily picked out of these narrow, shallow streams by other animals that feed on them.  Over 40 species depend on Yellowstone Cutthroat for food.  This includes the osprey, the bald eagle and the threatened grizzly bear, but also many less publicized animals.

To the grizzly bear, which is listed as threatened under the Endangered Species Act, spawning cutthroat can be a very important food source.  The cutthroat are spawning just as the bears are coming out of hibernation.  During this time more than 90% of the bears’ diet may be made up of trout (Mattson and Reinhart 1995).

As stated before it is not only the large, glamorized animals that rely on Yellowstone Cutthroat trout as a food source.  Shrews, mice, and even squirrels are known to utilize Cutthroat.  There are many animals in the Yellowstone ecosystem.  Some eat small trout, and some eat large ones.  Some only occasionally eat fish, and some depend on them heavily.  The animals that eat Cutthroat may also by food for yet another animal, and these animals too will be affected by Cutthroat loss.  A dead trout can move through the Yellowstone ecosystem in varied and complex ways.

If the Yellowstone Cutthroat falls many other species can be expected to fall with it.  The Lake Trout will not take the place of the Cutthroat Trout in the Yellowstone food chain.  Lake Trout live in relatively deep water.  They spawn in the lake, not in streams.  They are too deep and not available to terrestrial predators.  The osprey, which eats virtually nothing but Cutthroat Trout from Yellowstone Lake, will fail if the Yellowstone Cutthroats do.  So will the white pelican whose diet is almost 100% Cuttthroat (Schullery 1996).  This is a common theme with many animals in the Yellowstone ecosystem.