Glossary of Fishery Terms - M

M

• Mariculture - a particular branch of aquaculture where marine organisms are cultivated in the open ocean, or an enclosure of the ocean, or in tanks, ponds or raceways filled with seawater. Examples are the farming of marine fish, prawns, oysters and seaweed.

• Marine mammal - mammals that are primarily ocean-dwelling or depend on the ocean for food, such as porpoises, whales, seals, walrus and polar bears.

• Marine protected area (MPA) - marine area with some level of legal restriction to protect living, non-living, cultural, and/or historic resources.

• Maximum economic yield (MEY) - the total amount of profit that could be earned from a fishery if it were owned by one individual. An open entry policy usually results in too many fishermen so profits are barely higher than opportunity costs. See economic rent.

• Maximum sustainable yield (MSY) - the maximum catch that can be taken from a species' stock over an indefinite period. Under the assumption of logistic growth, the MSY will be exactly at half the carrying capacity of a species, as this is the stage at when population growth is highest. The maximum sustainable yield is usually higher than the optimum sustainable yield. Studies have shown that fishing at the level of MSY is often not sustainable. See also long-term potential yield.

• Meristics - A series of measurements on a fish, such as scale counts, which are used to separate different populations or races of fish.

• Mesopelagic - ocean depths extending from 200 to 1000 metres (650 to 3280 feet) below sea level

• Migration - a systematic (as opposed to random) movement of individuals in a fish stock from one place to another.

• Minimum landing size - the smallest length at which it is legal to keep or sell a fish. Sizes vary with the species of fish and also vary in different places around the world.

• Population model - a hypothesis of how a population functions. It often uses mathematical descriptions of growth, recruitment and mortality.

• Mollusc - A group of freshwater and saltwater animals with no skeleton and usually one or two hard shells made of calcium carbonate. Includes oysters, clams, mussels, snails, conches, scallops, squid and octopus.

• Morphometrics - Measurements which characterise the form, shape and appearance of an animal or plant. Difference in morphometrics, such as colouration, can be used to distinguish different stocks of the same species.

• Mortality - Mortality is a death rate from various causes, such as the proportion of a fish stock dying annually. See also natural mortality and fishing mortality.

• Mud flat - are coastal wetlands that form when mud is deposited by the tides or rivers, sea and oceans. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries.