American Plaice (Hippoglossoides platessoides)
American plaice (Hippoglossoides platessoides) belongs to the flatfish group of fishes, whose members are strongly compressed laterally and lie and swim on one side. When the young fish hatch from the egg, at or near the surface, they have the normal fish form. During development, as they settle to the bottom of the ocean, a change occurs in the body structure. The head becomes twisted so that the fish now swims and lies on its side. The upper side (which now has both eyes) is normally pigmented as compared to the lower side, which lacks pigmentation. Plaice almost invariably have their eyes on the right side of the body.
The fish has a large mouth that extends at least below the middle of the eye. The body is covered with relatively small scales, the tail fin is rounded, and the line that runs along the side of the body (the lateral line) is slightly curved just behind the gill openings. The colour is normally reddish to greyish brown on the upper-pigmented side and white on the lower side.
This species is distributed on both sides of the Atlantic, although the European type probably is a subspecies of the North American fish. In the Northwest
Atlantic they occur from West Greenland to the Gulf of Maine. The general distribution off the east coast of Canada is outlined in this figure.
As indicated, the occurrence of this species is widespread throughout the area and is probably the most abundant flatfish in the Northwest Atlantic. The area of highest abundance is the Grand Banks, especially the northern half.
While American plaice is considered to be a “cold” water species, it nevertheless appears to have a fairly wide temperature tolerance. It occurs in temperatures ranging from about -1.5°C to temperatures above 5°C and from inshore localities down to 700 m. However, the preferred temperature for this species appears to be in the range from just below 0°C to about 1.5°C and principally in the 90 to 250 m depth range. As a rule, plaice encountered in the deeper ranges are at higher temperatures. It is worth noting that the largest catches are normally taken between 125 and 200 m in temperatures of -0.5° to 1.0°C.
For most of their distribution, female plaice begin to spawn at eight or nine years of age when they are about 30 cm in length (depending on the location). However, most female plaice do not reach the spawning stage until at least 11 years of age (40-45 cm). Some males, on the other hand, become sexually mature at three years of age when they are 15-20 cm in length (depending on the localit
y). However, all male plaice are usually mature after the age of six (25-30 cm).
Plaice produce large quantities of eggs. Thus, a 40 cm plaice, on the average, produces 250,000 to 300,000 eggs and a 65-70 cm plaice produces nearly 1,500,000 eggs. Spawning and fertilisation of the eggs occur at or near the bottom and the fertilised buoyant eggs float to the surface layer where hatching occurs.
While no specific spawning grounds for this species have been recognised, certain localities offer environmental conditions such as bottom type, temperature and depth that are particularly favourable for spawning activity. American plaice are spring spawners, with spawning occurring at least as early as the first part of April on the Flemish Cap, and on the southern half of the Grand Banks, to late May or early June off Labrador. The length of time between fertilisation and hatching of the eggs varies considerably depending on the water temperature in the upper layers. Hence, developing eggs and larvae could drift a considerable distance before the young fish finally settle to the bottom.
Age and Growth
Plaice are relatively slow growing fish. Age is determined by counting rings on the ear bones (otoliths). The dark rings are probably formed when environmental conditions and food supplies are unfavourable and growth is slow. The lighter rings probably reflect more favourable conditions and periods of more rapid growth.
Because of differences in environmental conditions and food supplies throughout the region, there is considerable variability in the rate at which these fish grow. This figure gives a plot of the average length of American plaice for a number of localities in the Newfoundland-Labrador area. It can be seen that plaice on the northeast part of the Grand Banks at 10 years of age are, on the average, around 32 cm, whereas, on the southwest part of the Grand Banks they are approximately 45 cm in length. The growth curves shown in the figure are for female plaice.
Male plaice are slightly smaller than the females from about six years of age, and usually have a shorter life span. American plaice from the Gulf of St. Lawrence (Div. 4T) and the Scotian Shelf (Div. 4V, 4W, and 4X) have a growth rate slightly below the curve, for the southwest part of the Grand Banks. Plaice up to at least 25 years of age have been caught, but in heavily exploited stocks 20 years of age is usually the maximum recorded.