| When
most people think of fish and wildlife migrations, visions
of huge herds of caribou moving across the tundra, salmon
jumping up waterfalls or flocks of geese in V-patterns comes
to mind. However, Louisiana has some tremendous fish migrations
which take place year round. Many Louisianans know these
migrations as “fish runs”, not to be confused
with a disorder of the bowels caused by over consumption
of seafood.
The vast estuaries found in Louisiana’s coastal zone
are some of the world’s most productive ecosystems.
Hundreds of estuarine dependent species use Louisiana’s
coastal marshes during some part of their life cycle. Shrimp,
crabs, spotted seatrout, redfish and flounder are some of
the most heavily harvested of Louisiana’s estuarine
dependent species.
Finfish
species, like spotted seatrout and redfish, are probably
hard for most folks -- even Louisianans -- to think of as
migratory, because they are always present in coastal estuaries
in good numbers. The reason for their consistent presence
in estuaries is their large and age varied population.
Spotted
seatrout in Louisiana spawn from April to September. During
this period, when individuals are ready to spawn, they will
migrate from the upper, fresher areas of the estuary to
more saline waters, even if that requires them to migrate
offshore. This occurs throughout the spawning period, with
fish at different stages of spawn throughout the estuary,
so it’s not a huge synchronized, infrequent event.
However, peak spawning activity is in May-June.
Redfish, on the other hand, spawn primarily in offshore
waters near passes and inlets in late August through November.
The eggs hatch and the juveniles remain in the estuary until
they reach sexual maturity around 4 years old. At maturity,
they migrate offshore and become part of the brood stock
which are found in huge schools offshore. Redfish migration
is also difficult to discern because of its huge population
and varied age classes.
The
southern flounder has a true en masse migration pattern
in Louisiana. In the fall of the year, female flounder form
large staging schools near mouths of waterways entering
coastal bays. These females will move to offshore waters
in huge spawning schools with the passage of the first few
cold fronts of the season. Once offshore they will join
up with males, which primarily stay offshore. The eggs are
carried by the tides inshore where the larval fish undergo
a metamorphosis where their right eye moves to its left
side and the fish assumes a horizontal posture primarily
near bottom. After spawning, the females begin their migration
back into the estuaries in early spring where they will
disperse until the next fall.
Shrimp have a very short life cycle and migrate into and
out of estuaries in just a few weeks in some cases. Mature
adults spawn offshore in high salinity waters, where eggs
are released and are carried inshore by tides. The larval
and juvenile shrimp feed and avoid predators in the grass/water
interface of marshy areas. When they reach sub-adult size
they migrate offshore to join the spawning stock. This happens
almost continuously from early spring with the brown shrimp
migration through December, when the last of the white shrimp
trickle out of the estuary. Obviously, there are peaks during
this period, too.
Blue
crabs undergo yet another migration pattern. Female crabs
migrate to high salinity waters to release their eggs into
the water column. The larval and juvenile crabs are carried
into the estuary by the tides where they feed and seek shelter
from predators. Male crabs stay primarily in brackish waters
of the estuary, while females migrate to brackish areas
for mating nearly year round, then to high salinity areas
for spawning of eggs. The peak for spawning activity in
Louisiana is May-June and to a lesser extent September-October.
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