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Ocean Ranching

A type of fish farming where juvenile fish are released into the ocean to grow unprotected and unassisted to be harvested in the future.

Oceanodromous fish

Migratory fish that move within salt water only.

Operculum

The bony covering that protects the gills.

Ornamental fish

The small fish, such as koi, which are typically grown to be kept in aquariums or small landscape ponds.

Osmosis

The diffusion of liquid that takes place through a semipermeable membrane between solutions starting at different osmotic pressures, and that tends to equalize those pressures.  Water always moves toward the more concentrated solution, regardless of the substances dissolved, until the concentration of dissolved particles is equalized, regardless of electric charge.

Ovaries

The female reproductive organs.

Overfishing

Detrimental practice of removing more of a natural aquatic species than what natural reproduction can support.

Oxygen Depletion/Low Oxygen

A condition, normally occurring at night, in which oxygen dissolved in pond water has been depleted mainly because of the decomposition of organic matter and respiration of organisms in the pond.