Black back
The winter flounder (Pseudopleuronectes americanus), also known as the black back, is a right-eyed ("dextral") flatfish of the family Pleuronectidae. It is native to coastal waters of the western north Atlantic coast, from Labrador, Canada to Georgia, United States, although it is less common south of Delaware Bay. It is the most common near-shore (shallow-water) flounder in the waters from Newfoundland down through Massachusetts Bay, reaching a maximum size around 61 cm in length and 2.25 kg in weight. The species grows larger on Georges Bank, where they can reach a length of 70 cm and weight of 3.6 kg. Although winter flounder historically supported large commercial and recreational fisheries, biomass and landings have decreased since the 1980s.
Larval Winter flounder are known to feed on nauplii, harpacticoids, calanoids, polychaetes, invertebrate eggs, and phytoplankton. As they grow, copepods, amphipods, and polychaetes all become more important elements of their diet. Adult winter flounder are opportunistic sight-feeders that target annelids, crustaceans, bivalves, and fish depending upon prey availability.
Winter flounder lay up to 3.3 million demersal, adhesive eggs that are retained within their spawning grounds. Depending on temperature, larvae of approximately 3 mm in length hatch in two to three weeks. These larvae are planktonic at first, but transition to a bottom-oriented lifestyle over time. Around five to six weeks after hatching, the left eye of the larval flounder begins to migrate to the right side of the body. The “flounder-like” juveniles then settle onto the seafloor and move into saltwater coves, coastal salt ponds, estuaries, and protected bays, where they may grow up to 100 mm within the first year. Adult winter flounder may grow as large as 70 cm and reach ages of 15+ years. Growth varies across their distribution, with northern populations generally growing slower than those at the southern end of their range.
Winter flounder reach sexual maturity at about two to three years of age and a total length of 25–30 cm, with variance across their range. Mature adults (excluding those on Georges Bank) generally migrate in two phases, where adults enter shallow estuaries in the late fall and early spring to spawn in late winter or early spring before returning to deeper parts of the estuary or offshore waters. However, these migration patterns may be altered or individuals may stay inshore year round if temperatures remain at 15 °C or lower or if enough food is available. Genetic analyses and tagging studies indicate that there is little mixing between the Winter flounder populations in the Gulf of Maine, South of Cape Cod, and on Georges Bank.
