Olympia, WA, USA
Ecology : Bass are bad news for lake trout
Ecologists have been inspired by the old adage "You are what you eat" to
develop a reliable way of detecting and predicting potential damage and loss
of diversity in natural ecosystems after invasion by a non-native species.
M. Jake Vander Zanden of the Department of Biology at McGill University,
Montréal, Canada, and colleagues have been looking at changes in the diet of
the native Canadian lake trout (Salvelinus namaycush) that have been forced
on it by invasion of its home by two non-native fish species: the smallmouth
bass (Micropterus dolomieu) and rock bass (Ambloplites rupestris). Over the
past 100 years, these fish have been introduced deliberately into North
American lakes to 'improve' the fishing, and they are spreading. A
deleterious effect on lake trout has been suspected but controversial. In
their natural environments both lake trout and bass are so-called 'top'
predators, eating small fish and sitting at the top of the food chain. Small
fish normally make up the bulk of the lake-trout diet, supplemented by
smaller animals living in the surface layers of the lake - the zooplankton.
Originally, there was no thought that the introduced bass would compete with
the trout for food. Lake trout were considered to live out in the colder,
deeper waters of the lake and to catch their small fish prey there. Bass, on
the other hand, live and feed in the warmer shallow waters around the lake
shore. No apparent problem, therefore. But nature has a nasty habit of
confounding expectations, as Vander Zanden and colleagues can now confirm.
For in lakes where there are no small cold-water fish, lake trout do prey
largely on warm-water fish that live near the lakeshore. If these lakes are
invaded by bass, the invaders come into direct competition with the lake
trout - and win.
As they report in Nature: Vander Zanden and colleagues found that increased
predation due to smallmouth bass and rock bass has led to a severe decline
in the diversity and abundance of small fish prey in the shallow waters of
several lakes. They suspected that this would lead to direct competition
with the lake trout, toppling it from its perch as top predator and
downgrading its diet. This would have serious implications both for lake
trout populations and for the continued biodiversity of the lake ecosystem.
But actually proving this needed more than just careful observation.
And this is where "You are what you eat" comes in. The researchers were able
to prove the change in the lake trout's diet by looking at the carbon
isotope composition of trout muscle. This varies depending on whether the
fish has been feeding mainly on small fish prey or on zooplankton. In lakes
with no bass, they established that the lake trout diet is around 60% small
fish, with the remainder made up by zooplankton. But in lakes where bass are
well established, the lake trout were getting only 22% of their dietary
carbon from fish and were having to get the rest from zooplankton.
This dietary change is bad news for the continued survival of large
populations of lake trout. It also perturbs the intricate web of who eats
what in the lake, as the zooplankton provides food for many other species,
including some of the small fish on which the bigger fish prey.
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