Study
after study of flowing streams and river systems have demonstrated
the value of coarse woody debris (CWD, or fallen trees)
to fish. In salmon streams and rivers and slow-moving bayous,
CWD provides more habitat complexity, and generally, more
fish.
This
puts fish populations directly in the crosshairs of some
important uses of waterways: navigation and drainage. Particularly
in low-gradient waterways (like many larger flowages in
Louisiana), trees can reduce the capacity to move water
away from inhabited or farmed land. In areas where flooding
from a small bayou filled with trees is getting into homes
and businesses, you can bet that “clearing and snagging”
and dredging will be used to solve the problem. In extreme
cases, that bayou then becomes nothing more than a drainage
ditch with the minimal fish populations that you would expect
in a drainage ditch.
In pond
and lake systems, studies have also proven the value of
trees to fish. Areas with more woody structure have better
fishing and hold more fish, particularly the more popular
species.
Anglers
know that trees in the water are good spots to cast, but
may not realize how much those trees add to the health of
the whole system. A recent study in Wisconsin demonstrated
this in a dramatic way. The researchers actually cleared
more than 75 percent of the CWD from one arm of a lake that
has two similar branches. One photo showed students in the
water cutting and pulling trees; two are using an old-fashioned
two-man crosscut saw, a skill not often gained in college.
The
de-snagged side was left with 80 logs per shore mile; the
control side had 213. And a block net was positioned between
the arms. Fish sampling showed no differences before the
experiment but dramatic effects afterwards.
Largemouth
bass in the de-snagged arm ate fewer fish and more terrestrial
prey (frogs, snakes, mice and insects) and grew more slowly.
Most of the fish consumed by bass in this lake were yellow
perch, which use CWD as feeding and spawning areas and as
refuge from predation. After de-snagging, no significant
perch reproduction occurred in the treatment arm, while
the control arm had successive spawns. Numbers of perch
in the de-snagged arm remained low.
In our
waters, the bass/sunfish relationship is most equivalent
to the bass/perch one in Wisconsin. While bluegills don’t
use CWD for actual spawning substrate, the other responses
should be very similar. Although the study lake did not
contain much aquatic vegetation, these researchers suspect
that in many situations, weed beds probably serve similar
roles as CWD.
Any
efforts that people make to preserve most of the CWD in
systems will pay off in fish. While most folks don’t
want piles of decaying trees in front of their lake home,
many can live with a few trees in the water on parts of
their shoreline.
Trees
should always be left in the water on undeveloped shorelines.
On private water bodies, trees can be added (though small
ponds benefit little). Most studies have shown that heavy
hardwoods give the most lasting effects, holding more three-dimensional
surface area for the longest time. Sac-au-lait fishermen
like to add fresh vegetation periodically, because minnows
are drawn to it.
In streams
and bayous, any compromises on snagging and dredging will
also make for more fish. In some cases, a bayou will supply
adequate drainage without being turned into a straight pipe.
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