2909 17-Mile Drive © 2000 by Leo Poppoff

Pebble Beach, CA 93953

\(831\) 375-3302

November 16, 2000

Basin Watch

by

Leo Poppoff

HOW MACKINAW AFFECT LAKE TAHOE'S FOOD WEB

Resort owners protested. They didn't want voracious monsters from the
Midwest in Lake Tahoe. Anglers were happy with Tahoe's world famous
cutthroat trout fishery and didn't want Mackinaw messing it up.

But, Fish and Game experts had discovered that Tahoe's native Lahontan
cutthroat trout liked to hang around the edges of the lake. The middle
of the lake was empty and unproductive, the experts concluded. Mackinaw,
the lake trout from Lake Michigan, loved deep water. So, the fishery
folk reasoned, why not add Mackinaw to Lake Tahoe and double the fishing
experience?

Still, locals were making a big fuss. So, Mackinaw weren't planted in
Tahoe. Instead, in 1895, or thereabouts, they were hauled up the hill to
Tallant Lakes \-- and forgotten.

Mackinaw are not considered to be true trout, even though they're
commonly called "lake trout." They, and the eastern brook trout that
were planted in Tahoe's streams, are in the char family. Both Mackinaw
and brookies are aggressive, have large appetites, and have been blamed
for the demise of native cutthroats \-- or at least helping it along.

Around 1920, a strange fish caught in the Tallant Lakes was identified
as a Mackinaw (lake trout). Later, anglers reported that cutthroat trout
caught along Tahoe's west shore had torn fins and injuries. And before
long, Tahoe anglers were hooking big lake trout. By mid-century,
cutthroats had disappeared and lake trout became Tahoe's prime sport
fish. Trophy Macks, thirty-some inches long and weighing as many pounds,
were pulled out of the lake by expert anglers with special gear that
plumbed the depths of the lake.

Fish and Game experts believed they could improve the lake's fishery by
importing more food for Mackinaw. Kokanee (landlocked salmon),
Bonneville cisco, and mysids (tiny shrimp) were brought to the lake for
this purpose. Mysids were to eat algae and detritus, ciscos were to
fatten up on mysids, kokanee were to grow large on a diet of cisco, and
lake trout were to grow huge on a diet of kokanee. But, cisco didn't
survive. Mysids couldn't find enough algae, so they started dining on
kokanee's favorite zooplankton. Kokanee survived, but didn't become a
reliable food source.

In those days, folks weren't sensitive to the value of ecosystems. So,
the effect of importing exotic species wasn't a concern. Fish and game
managers seemed to believe they could mix and match at will.

Over the years Lake Tahoe's aquatic ecosystem got screwed up. Cutthroat
trout and other native fishes disappeared. Mysid (opossum shrimp)
populations mushroomed as they decimated populations of zooplankton. In
turn, mysids became important vittles for Mackinaw.

Researchers suspect that planting new species in lakes can seriously
affect the speed of eutrophication of the lake. But, how much of Tahoe's
problem can be blamed on new aggressive aquatic critters? Scientists
from Utah State University (G.P. Thiede, D.A. Beauchamp, and C.K.
Gemperle) and UC-Davis' Tahoe Research Group (B.C. Allen) are trying to
find out. They studied the effect that the Mackinaw's prodigious
appetite has had on populations of other aquatic life. And the results
are impressive.

In a recent progress report, the scientists published the following.
"From spring 1992 through winter 1995, mean total annual consumption (by
Mackinaw) amounted to 460 metric tons (tonnes) of mysids, 140 tonnes of
native fishes (suckers, minnows and sculpin), 6.7 tonnes of kokanee, 22
tonnes of lake trout, and 100 tonnes of other prey, mostly signal
crayfish." A metric ton is 2200 lbs. Researchers also found that
"consumption of kokanee varied both seasonally and annually. Predation
removed an estimated 22,900 adult kokanee, or 58% of potential kokanee
spawners in autumn 1992 and 33,700 kokanee, or 21% of the potential
spawners, in autumn 1994."

Consumption of mysids was similar in all three years, despite variations
in mysid populations. Lake trout ate about 15% of the mysid population
per year, on average. The big Macks also removed 20 to 48% of what's
left of native fish.

Researchers concluded that lake trout can have significant effects on
kokanee and native forage fishes. But, lake trout can't control mysid
populations, though mysids were their main food source. "Lake trout
appear to be responsible," researchers report, "for a large proportion
of native mortality, possibly contributing to substantial declines in
native fish abundance during the past 30 years." And, by eating their
young, lake trout have a significant effect on their own population.

It also appears that mysids have a very important role in Tahoe affairs.
This research shows that they're the main food source for lake trout.
Yet, lake trout haven't made much of a dent in mysid populations. And,
as we learned in an earlier article, Utah State and UC-Davis scientists
found that mysids also might be responsible for moving nutrients more
quickly to the bottom of the lake. That's quite a record for a
frail-looking little immigrant.