Doormats
— that’s what big flounders are called. While
Louisiana has many species of flounder, the fishery is centered
around the southern flounder, Paralichthyes lethostigma. Concern
over the biological health of the stock of this fish has led
to both recreational and commercial harvest restrictions.
Unfortunately, flounder are a poorly-researched fish in Louisiana.
One study
done by Louisiana State University scientists does give some
information on the biology of this popular fish. The researchers
obtained 1,259 southern flounders from recreational fishermen
and commercial fish dealers. Each fish was weighed and measured,
the spawning condition noted, and they were aged by counting
the rings in their otoliths (ear bones).
The results
were interesting. First, male and female flounders were quite
different, biologically. Males live to a maximum of four years
old, compared to eight years for females. In spite of the
fact that males grew faster than females, the shorter life
span of males meant that females grew much larger than males.
The most frequently found size for females in the study was
just under 16 inches long, compared to 11 inches for males.
Females grew to a maximum size of over 30 inches (a real doormat)
compared to under 17 inches for males.
The research
also indicated that males are much more likely to be found
in offshore waters than females, except during spawning season
when both sexes are present. Female flounders begin moving
from bays and estuaries in the fall out to offshore waters,
where spawning takes place in December and January.
Females
were sampled during the spawning season. Nine of 11 one year
old and 123 of 128 two year old females were mature, which
indicates that 50 percent of the females may reach maturity
before their first birthday. (It should be noted that the
researcher cautioned that more research is needed in this
area.) At 20 inches and 2 1/2 pounds, all females are mature.
During the two-month spawning season, females were found to
spawn every 3.6 to 6.4 days, producing an average of 44,225
to 62,473 eggs per spawn.
Their
doormat appearance disguise hunting agility. Flounder are
well adapted for ambushing quick-moving prey such as fish
or shrimp. The flattened shape allows them to become nearly
invisible on the bottom. Their brains have large optic lobes
to serve their large eyes, and they have large mouths and
strong teeth. Typically, they remain motionless on the bottom
and wait for their prey to come within striking distance before
attacking. While waiting, flounder show rapid eye movements
as they track their prey.
Research
indicates that flounders will eat from 4 to 8 percent of their
body weight in food each day. Feeding activity is heaviest
at water temperatures of 61 to 77°F and during the three-day
period following a first quarter moon and the three-day period
before a new moon.
Flounders
eat a wide variety of food items including shrimp, mullet,
anchovies, croakers and menhaden (pogies). One research project
in Texas reported southern flounders to be the dominant fish
predator of brown shrimp during the spring in Galveston Bay.
The researcher also noted a set increase in the predation
rate of brown shrimp in muddy water. This may have been due
to the feeding advantage of muddy water or to a change in
shrimp behavior. Flounders feeding on fish seem to prefer
smaller ones. Unlike most predatory fish, which eat larger
fish, as they get larger, flounder just eat higher numbers
of small fish.
Download:
flounderdoormats.pdf
(358KB)
