Winter flounder (Pleuronectes americanus)

Description

The winter flounder (Pleuronectes americanus, formerly classified as Pseudopleuronectes americanus) is a flatfish with right-eye orientation. It has a small mouth with lower incisors and few upper teeth, an oval shape, and relatively thick body; its broad caudal fin is slightly larger than other Atlantic flounders (Buckley 1989). Winter flounder vary in color, and depending on the surrounding substrate, they can range from reddish brown to olive green or black, with a white underside (Buckley 1989). Average adult size is 270 millimeters at age three and 450 millimeters at age ten. Males tend to be 40 to 50 millimeters smaller than females, and have an average life span of eleven and twelve years, respectively (Grosslein and Azarovitz 1982).

Background

Distribution. Winter flounder are found from Labrador to Georgia, comprising numerous separate stocks. They inhabit bays, estuaries, and shallow coastal areas with substrates varying from soft mud to pebbles. In the New York Bight, they inhabit the shallows to 55 meters, and winter flounder eggs, juveniles, and adults are abundant in Raritan Bay (Walford 1971, Berg and Levinton 1985, MacKenzie 1990). Adults tend to have highest abundances in the winter and early spring in the Lower Hudson region and late fall and spring in the Lower Bay region.

Feeding. Winter flounder rely on vision to identify prey, and thus feed during the day. Fish lie motionless on the sediment surface, head raised looking for prey (Buckley 1989). Adults feed on small plant matter and benthic invertebrates such as crustaceans, cnidarians, nemerteans, polychaetes, molluscs, and ascidians (Grosslein and Azarovitz 1982, Buckley 1989). Larvae and juveniles consume amphipods, copepods, phytoplankton, small polychaetes, nemerteans, and ostracods, and prey densities strongly influence survival at these early life stages (Buckley 1989). The amphipod Ampelisca abdita was the dominant prey species found in stomach contents of juvenile winter flounder sampled in Jamaica Bay (Franz 1990).

Fishery. U.S. landings of winter flounder average 6,000 to 15,000 metric tons (MT), but only a small portion of these landings come directly from the New York/New Jersey Harbor area (Grosslein and Azarovitz 1982, Buckley 1989). This popular fish is marketed as "flounder" or "lemon sole" (large specimens). Grosslein and Azarovitz (1982) postulate that the recreational catch in the NY Bight area exceeds the commercial catch due to their almost year-round availability in the bays; Buckley (1989) contends the same for the New England area.

Life Cycle

Spawning occurs December to June, principally in upper estuaries. Female winter flounder lay 500,000 to 1,500,000 eggs, depending on maturity. These eggs are demersal and nondispersive, are laid on sandy substrates at depths of 2 to 80 meters, and incubate 15 to 25 days. Salinity and temperature influence timing of spawning and incubation (Buckley 1989). Larvae hatch at 3 to 3.5 millimeters and have a wide vertical band of pigmented cells that divide the postanal portion of the body (Buckley 1989). Within 12 to 14 days, the yolk sac is absorbed and the larvae are approximately 5 millimeters in length. Larvae remain on the bottom except for short, upward swimming bursts, and they resemble full adults after metamorphosis in 2.5 to 3.5 months (8 to 9 millimeters in length; Grosslein and Azarovitz 1982). Maturity in winter flounder is based on size; fish in warmer waters mature years before their northern counterparts (Buckley 1989). Juveniles remain in shallow waters near the areas they were spawned for a few years, living a benthic lifestyle and rarely leaving the substrate. Adult winter flounder spend the winter months in bays and estuaries and move offshore in summer (Grosslein and Azarovitz 1982).

Environmental Influences

Salinity and Temperature. Winter flounder are euryhaline, tolerating salinities ranging 5 to 35 practical salinity units (psu; Buckley 1989). Eggs and larvae are more sensitive to extreme salinities than adults, and survival at these early stages can be affected. Preferred salinity for spawning is 31 to 32.5 psu. Winter flounder are also susceptible to changes in water temperature, and human activities that create artificially high temperatures can have a large impact on the species (Grosslein and Azarovitz 1982). Winter flounder prefer cooler temperatures (below 15 degrees Celsius), but will remain active in waters reaching 22.5 degrees Celsius (Grosslein and Azarovitz 1982). At 23 degrees Celsius, fish become inactive and bury themselves in the mud; temperatures above 28 degrees Celsius are lethal to this species. Spawning occurs in waters reaching 0 to 3 degrees Celsius, and water temperature is an important factor influencing adult seasonal distributions, as they are less tolerant to warm water than juveniles (Buckley 1989). The stocks south of Cape Cod migrate offshore in spring and summer to find cooler waters (Grosslein and Azarovitz 1982).

Predation. Winter flounder are prey for numerous large, coastal and estuarine predators, including striped bass (Morone saxatilis) and goosefish (Lophius americanus). In fact, predation by fish, birds and some invertebrates is a main cause of mortality in larvae and juveniles (Grosslein and Azarovitz 1982).

Pollution. Winter flounder are particularly susceptible to pollution (Grosslein and Azarovitz 1982). The eggs are laid directly on the substrate and therefore any toxins in the sediment can affect their viability. This species' close association with the benthos also potentially exposes the fish to sediment toxins. Grosslein and Azarovitz (1982) noted that few larvae survived in polluted estuaries, and that winter flounder were entirely absent from polluted sections of NY/NJ Harbor. In particular, winter flounder experience increased mortality as a result of exposure to insecticides, especially DDT (Buckley 1989).

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