size
Size plays an important role in the survival of released fish, largely because of predation. Juvenile winter flounder are food for a host of estuarine animals including sand shrimp, herons, egrets, cormorants, striped bass, summer flounder, and—most recently—the European green crab (Carcinus maenas), an invasive species that has become their top predator in coastal N.H.

UNH research has demonstrated that cultured winter flounder need to be at least 30 millimeters in length, or the size of a small potato chip, before release. (The fish we release are typically 40 millimeters and four months old due to gender considerations.) They usually reach this size by three months, by which time they can swim fast enough to have a chance of evading predators, and yet are roughly the same size as wild juvenile flounder near the release site.

sex
Wild populations of young winter flounder tend to be half male and half female. To support this balance, we make every effort to insure that we release an equal number of male and female fish. If the sex ratio of our cultured fish is not 1:1, they are not released into the wild.

Like many fish, the sex of winter flounder may be determined by a combination of genetics and environmental cues. For some flatfish, increases in water temperature during juvenile incubation leads to a prematurely male population. This phenomenon could pose significant problems in cultured flounder destined for stock enhancement.

Our research has determined that the reproductive tissue of winter flounder in the hatchery differentiates when they are less than 41 millimeters in length, or about four months old. Currently, we are exploring how environmental conditions like temperature, density of fish in tanks, and light exposure can be manipulated to insure that the crop has an equal balance of males and female.