Bass
fishermen have long recognized some differences between largemouth
bass from low-salinity marshes and those from purely freshwater
habitats. Although some changes have occurred in recent years,
brackish marsh bass are usually considered to be more plentiful,
but smaller than freshwater bass. Also, bass anglers frequently
described them as having a more stocky body build or being
“chunkier” than other bass. From these differences,
the question often arises as to whether marsh bass are genetically
different or whether the differences are due to habitat.
In the
late 1980s, biologists at the LSU School of Forestry, Wildlife
and Fisheries looked closely at brackish marsh bass to see
if they could answer some of these questions. They conducted
both lab and field studies. For the lab study, 24 fish were
collected from brackish water canals and marshes near Cut
Off, Louisiana. Salinities in this area averaged two parts
per thousand (ppt). For comparison, full strength seawater
is about 35 ppt salinity. Another 24 fish were taken from
Ben Hur Lake, which is freshwater at 0 ppt. All the bass were
held for two weeks to settle down and actively begin feeding,
before any experiments began. The freshwater fish were held
at 0 ppt salinity and the marsh bass were started at 2 ppt,
with salinity gradually being reduced to 0 ppt. Each fish
had its own tank. The bass were then assigned equally to one
of four categories of salinities; 0 ppt, 4 ppt, 8 ppt, or
12 ppt. Salinities were increased gradually by 1 ppt per day,
until the final salt concentration was reached. The whole
time, the bass were fed live golden shiners in an amount equal
to 2% of the weight of each bass. Each fish was weighed before
and after the 20-day experiment to determine growth rates.
All the
bass, both marsh and freshwater, that were held at 12 ppt
stopped feeding within one week and died before the experiment
ended. No difference in growth rates occurred in the marsh
bass held at 0 ppt, 4 ppt and 8 ppt and the growth rate for
freshwater fish at 0 ppt and 4 ppt was the same. However,
the growth rate of freshwater fish held at 8 ppt was lower
than for those held at 0 ppt. When the two groups were compared,
marsh bass outgrew freshwater bass at 8 ppt and freshwater
bass grew faster than marsh bass at 0 ppt. At 4 ppt, growth
rates were the same.
In the
field studies, 133 marsh bass were collected from the same
marshes over 12 months. These fish were compared to 115 freshwater
bass collected from False River. The results of detailed measurements
agreed with fishermen’s observations. Bass from brackish
marshes do have a stockier body build. Length at each age
for the first 4 years of their life was then “back-calculated”
for both groups of bass. The analysis showed that at each
age, freshwater bass were longer than marsh bass, especially
for those age one and two.
Examination
of the stomach contents of the marsh bass collected showed
a high percentage of invertebrates (animals without backbones)
compared to the typical diet of adult largemouth bass. Grass
shrimp were the most common food item by count and brown shrimp
were the most food item by weight. The most common fish eaten
were gulf killifish (cacohoes), silversides (glass shiners),
and menhaden (pogies). Fish are generally considered to be
a superior food item to invertebrates for producing growth
in predator fish such as largemouth bass.
The biologists
noticed large numbers of gulf killifish and menhaden in the
open waters of the canals, yet the marsh bass preferred to
stay in underwater grass beds that held less prey fish. They
suggested that marsh bass stayed in the grass beds, even though
the food supply was poorer, because the grass beds provided
protection from even larger predators such as alligators,
redfish and alligator gar. They noted that 40 percent of the
bass they collected from the marsh had open wounds or scars.
In marshes, largemouth bass are in the middle of the food
chain rather than near the top, as they are in most freshwater
habitats.
They concluded
that the differences in growth and body shape of marsh bass
are due to salinity, food availability and the risk of being
eaten by larger predators. Overall, the marsh environment
was considered excellent for largemouth bass.
Download:
marshbass.pdf
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