American Plaice | Oceana Canada

Canadian Marine Life Encyclopedia

American Plaice

Hippoglossoides platessoides

Also known as

Dab, plaice, sole

Distribution

Continental shelves of the Atlantic Ocean from the Arctic Circle to Rhode Island

Ecosystem/Habitat

Soft sandy or muddy bottoms

Feeding Habits

Ambush predator

Conservation Status

Threatened

Taxonomy

Order Pleuronectiformes (flatfishes); Family Pleuronectidae (right-eye flounders)

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When American plaice emerge from their eggs, they have a normal fish-shaped body. Like other flatfish, they develop their unique shape over time.  As they grow, their left eye starts to migrate over to their right side and their body begins to flatten. By the time they reach the juvenile stage and settle on the ocean floor, they will have assumed the classic flatfish shape. Their flattened body allows them to lie flush with the ocean floor and swim on their side, undulating just above the ground in search of their favourite foods. As adults, American plaice are known to be fairly adaptable to changes in prey availability; however they commonly feed on worms, molluscs, sea urchins, starfish, crustaceans and small fish. 

American plaice are an oval-shaped, right-eye flounder. This means that both of their eyes are on the right side of their flattened body. Interestingly, there are also left-eyed flounders, such as the Pacific sanddab. The right side of American plaices, or top-side, are a reddish- to grayish-brown that is generally uniform in colour. As juveniles, they have three to five large, dark spots along each edge of their body, which eventually disappear as they mature. The left side, or underside, of their body is white. They have a rounded tail, a relatively large mouth compared to their head, and a light-coloured, practically straight lateral line which runs from their gills to their tail. The lateral line marks a system of sense organs in their body that are used to detect movement, vibration and changes in pressure. Their large mouth and almost straight lateral line are features that can be used to distinguish them from other similar looking flounders. American plaice can grow to be more than 60 centimetres in length.

Male and female American plaice reach sexual maturity at different periods in their lives. Females usually reach maturity around eight to 11 years of age, while males reach maturity around three to six years of age. Spawning occurs in the spring between April and June. American plaice are batch or serial spawners, which means that they spawn multiple times during the season. Throughout the spawning season, one female can release between 400,000 and 1.5 million eggs. The number of eggs a female can produce during the spawning period depends on her size, with larger, older females producing the highest quantities of eggs. This spawning and fertilization occurs near the ocean floor, but once the eggs are fertilized they rise up to the surface of the ocean. The eggs will float along the upper layers of the water column, where they will hatch and begin to feed on zooplankton and other microscopic organisms. As they grow, they will begin the metamorphasis into their flat, adult body shape and settle onto the ocean floor.  

American plaice were historically harvested all across the Atlantic coast of Canada, particularly by bottom longline fisheries. However, the introduction of otter trawls to the fishery in the 1960s quickly led to overfishing and greatly reduced their numbers. There is a moratorium on American plaice in all of the fishing regions surrounding Newfoundland and Labrador, but they are still caught in low numbers in the southern Gulf of St. Lawrence and as part of a multi-species groundfish fishery on the Scotian Shelf. Today, American plaice are typically harvested by trawls or seines, but small numbers are caught using longlines and gillnets.

The two major populations of American plaice in Canada, the Maritime and the Newfoundland and Labrador populations, were both assessed by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) as threatened in 2009. The small Arctic population was assessed by COSEWIC as data deficient in 2009 as well. The two major populations were designated as threatened due to the dramatic population declines caused by overfishing during late 1900s. The Newfoundland and Labrador population saw a decline of 94-96 per cent over a 30-50 year period, while the Maritime population saw an average population decline of 76 Per cent. Despite this sever population decline, American plaice have not been assessed or listed under the Species-at-Risk Act (SARA) in Canada.