Flatfish, or flounders, are bottom dwelling fishes primarily associated with soft substrates (mud and sand). They are unique among fish in being asymmetrical, both eyes lying on one side of a highly flattened body. They begin as fusiform (symmetrical, typical fish shape) larvae but early in life they start swimming on one side, and the eye on the underside migrates to the upper side. Flatfish lie on the bottom on the blind side, the eyes directed upwards and protruding sufficiently to give a lateral view of their surroundings. Principal food items include crustaceans, molluscs, polychaete worms and small fishes, which they obtain by foraging and ambush. Often perceived as purely sedentary, they can actually be quite active swimmers. Some species rise off the bottom to feed in the water column at night, or to catch currents for transport to another location. Flatfish exhibit various short- and long-distance movements to feeding or spawning grounds, and seasonal movements to deeper or shoaler waters of preferred temperature.
The species addressed here are Yellowtail Flounder (Limanda ferruginea), Witch Flounder (Glyptocephalus cynoglossus), American Plaice (Hippoglossoides platessoides) and Winter Flounder (Pseudopleuronectes americanus). This section focuses on populations of these species on the Scotian Shelf and in the Bay of Fundy (Northwest Atlantic Fisheries Organization Divisions 4V, 4W, 4X and 5Y in fisheries management parlance) that have supported commercial fisheries since the 1950's. Northwest Atlantic fisheries on these four species overall (including Greenland, USA, Newfoundland, and the Gulf of St Lawrence) constitute about 10% by weight landed of all the non-invertebrate fisheries in these waters. Scotian Shelf and Bay of Fundy flatfish represent very roughly 10% (Witch, Winter), 15% (Plaice), and 20% (Yellowtail) of Northwest Atlantic flatfish landings. Most flatfish are marketed in the USA.
Fishery management units for flatfish comprise the Eastern Scotian Shelf (4VW) and Western Scotian Shelf - Bay of Fundy (4X/5Y), the two zones intended to separate any discrete populations across the four different species of flatfish (American Plaice, Witch Flounder, Yellowtail Flounder, Winter Flounder). These species are managed as flatfish stocks without distinguishing the component species. This grouping of species can extend to species identification of the catches themselves, such that Unspecified Flounder can represent major portions of reported landings, but identification has improved in recent years. Part of the problem with identification is use of local names for species that may not be familiar to people processing the landings data (e.g. Dabs for Plaice, Blackback for Winter Flounder, Greysole for Witch Flounder, Lemon Sole for Yellowtail Flounder). The management units seem appropriate to separate populations of Yellowtail and Winter Flounders, while American Plaice and Witch Flounder are continuously distributed throughout the two management zones. Science advice for resource management is based on whichever species within each management complex exhibits the worst status. However no explicit decision rules or reference points exist to define precautionary quotas.
Among fish in general, flatfish species usually exhibit little relationship between spawner abundance and subsequent recruitment, and the least variability in recruitment, but pre-recruit abundance can often serve to predict recruitment. It is speculated that the highly two-dimensional nursery habitat requirement of most flatfish species (the tops of banks) consistently imposes an upper limit on recruitment, such that it can usually only be proportional to the size of the nursery area, and has very little potential for expansion. But there remains some concern that we have not ruled out the possibility of a relationship with some aspect of spawning fish condition as important (something like provision of nutrients to the eggs overriding any potential for spawner abundance to matter). Whatever the reason, it confounds attempts to predict stock trajectories based on Spawning Stock Biomass (SSB), historically a cornerstone metric for gauging stock status.
Types of Flatfish
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