Winter flounder

The name 'winter' flounder refers to their annual spawning migrations into nearshore waters in winter. Adults migrate in two phases; an autumn estuarine immigration prior to spawning, and a late spring/summer movement to either deeper, cooler portions of estuaries or to offshore areas after spawning. This pattern of seasonal distribution may change in colder waters at the northern extent of their range where winter flounder migrate to shallow waters in the summer and deeper waters in the winter. The annual spawning period varies geographically and although spawning periods overlap considerably, peak spawning times are earlier in southern locations.

During spawning, females release demersal (negatively or neutrally buoyant) adhesive eggs whose properties facilitate retention within spawning grounds. Many factors influence larval and juvenile growth and survival, including temperature, salinity, dissolved oxygen, and food availability. Nursery habitat for winter flounder larvae and juveniles is typically saltwater coves, coastal salt ponds, estuaries, and protected embayments; although larvae and juveniles have also been found in open ocean areas such as Georges Bank and Nantucket shoals. Larvae are predominantly found in the upper reaches of estuaries in early spring, moving into the lower estuary later in the season.

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.

Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Pleuronectiformes
Family: Pleuronectidae
Genus: Pseudopleuronectes
Species: P. americanus

By 1963 it had been determined that Winter Flounder moved offshore when the water got above 15°C and returned to the shallows when the water cooled. Usually this would result in a winter/deeper/offshore and summer/shoaler/inshore pattern. However the inshore summer water temperature does not always reach beyond 15°C, in which case the Winter Flounder would remain inshore. The preferred temperature range was determined to be a very narrow 12-15°C, across several populations throughout the NW Atlantic.

This means a narrow depth range, accounting for the all-or-nothing nature of Winter Flounder catches, and confounding interpretation of catch rate (catch per unit effort, or CPUE) trends once the distributional range was known to the fishing industry. Thus CPUE declined drastically in the early years of the fishery, but by later years the fish locations could be predicted accurately enough to compensate for population decline. The narrow temperature preference was later (1988) disputed, it being argued that the offshore movement in summer was for the purpose of feeding. In the end both temperature and feeding appear relevant.