Flounder, Paralichthys lethostigma (meaning parallel fish that forgot its spots), are one of the more peculiar fish native to the Eastern and Gulf Coasts of the United States. As juveniles and adults, they can survive in bodies of water ranging from freshwater rivers and lakes to bays and offshore.
Article by Shane Bonnot, CCA-Texas Coastal Advocacy Director (From the April/May Currents)
The flounder’s ability to tolerate varying salinities has allowed them to be widely distributed and highly abundant throughout our coastlines, particularly during seasonal spawning migrations. Their range begins in northern Mexico and extends all the way to through the Albemarle Sound in North Carolina, with a slight absence of presence around the tip of Florida. This wide distribution, coupled with their fighting ability and quality of flesh, make them one of the most prized catches among coastal anglers.
While they spend most of the year inshore feeding on invertebrates and smaller fish, adult Southern Flounder migrate to offshore waters each year during the late fall and early winter to depths of over 100 feet to spawn (some flounder have been found at depths greater than 350 feet), releasing on average 45,000 eggs per pound of body weight.
After the eggs are fertilized in the water column, they float to the surface and, depending on the water temperature, will hatch after an approximate 48-hour incubation period. A colder incubation temperature would equate to a longer incubation. After hatching, the larvae (measuring ~0.1 inches), will feed off their yolk sac for nearly a week, during which time the young fish develop their eyes located on opposite sides of the head. The fish will begin feeding on prey items after the mouth parts and digestive tract have formed, and for the most part, they look like a “normal” fish larvae at this point of their life.
Over the next approximately 30-40 days (again, depending on water temperature), something truly remarkable happens. Losing their aura of ordinary larval fish, southern flounder will undergo a metamorphic process in which the skull bones bend and shift positions as the right eye migrates to the left side of the head. During this process the fish will swim at a slight angle, and pigmentation on the left side (top side) of their body begins to develop.
Once the right eye is in its final resting position, the fish settles onto the right side of its body and becomes a true flatfish for the remainder of its life (interestingly, a small percentage of Southern Flounder larvae undergo left eye migration, which makes one wonder if eye dominance plays a role in metamorphosis). It is just after metamorphosis when the flounder can tolerate variable water conditions, allowing them to thrive in bays and estuaries as they migrate from offshore.
Flounder fry at 30 days
Imagine the energy demand required and resulting stress for an eyeball to move across a head, shifting skull bones in the process. Everything has to be in sync to pull off that feat. Unfortunately for flounder there are factors outside of their control that highly influence successful metamorphosis. One of the most important abiotic factors that drives flounder development in offshore waters is temperature. In the Gulf of Mexico, young Southern Flounder larvae thrive at 62-65°F and can tolerate temperatures ranging from 59-73°F.
After the flounder complete metamorphosis, that tolerance increases dramatically, but during the first few weeks of a flounder’s life it is absolutely critical that water temperatures remain stable in the upper 60s. Winter water temperatures above that narrow tolerance have devastating effects on Southern Flounder larvae survivability, and subsequently recruitment into the bays the following spring.
