| Anyone
who has fished in Louisiana backwaters knows that there
are “types” of water that look different and
hold different fish populations. A project conducted in
the Atchafalaya Basin by Louisiana State University researchers
showed just how well these appearances correlate with water
chemistry and the use of different habitats by different
species of fish.
Water
in the main channels and bayous tends to be muddy and brown
and flowing rapidly. This environment is characterized by
high levels of dissolved oxygen (D.O.) and little oxygen
“differential,” meaning the moving water is
well mixed, so that D.O. at top and bottom is about the
same. Brown water in the Basin is prime habitat for blue
catfish, spotted bass, longear sunfish, freshwater drum
(gaspergou) and bullhead minnows.
When
river water stays for any length of time in the deeper and
slower-moving lakes and bayous, there is a tendency for
sediment to settle and phytoplankton to grow. This green
water is often well oxygenated but stratified with higher
D.O. near the surface. Green water sites often have lots
of floating vegetation, and hold the widest variety of fish
species. Numbers of bluegill, shad, white bass, largemouth
bass, redear sunfish (chinquapin) and black crappie (sacalait)
are all significantly higher in green water.
Flooded
wooded swamps tend to develop black water, which is often
oxygen-poor and lower in pH (more acid) and lower in conductance
(dissolved salts and minerals). A distinct group of fishes
often inhabits black water sites: warmouth (goggle-eye),
spotted gar, pirate perch, black bullhead, and flier. These
species may be better adapted to low oxygen conditions,
but when D.O. gets really low, they often move to nearby
oxygenated waters to survive.
The
researchers noted that oxygen depletion is likely the most
important factor in fish distribution and abundance in the
Basin. High flow conditions in the early spring promote
flushing of decomposing vegetation and give the best water
quality (and probably fish production) for the rest of the
year.
Source:
Rutherford, D.A., K.G. Gelwicks, and W.E. Kelso. 2001. Physiochemical
effects of the flood pulse on fishes in the Atchafalaya
River Basin, Louisiana. Trans. Amer. Fish. Soc. 130:276-288.
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