Glossary of Fishery Terms - O

O

• Ocean basin - geologically an ocean basin is a large geologic basin which is below sea level.

• Ocean currents - Oceanic currents can be divided into surface and deep ocean currents. Surface currents are generally wind driven and develop typical clockwise spirals in the northern hemisphere and counter-clockwise spirals in the southern hemisphere. Surface currents can operate to a depth of 400 meters and apply to about ten percent of water in the ocean. Deep ocean currents are driven by density gradients in water due to temperature (thermo) and salinity (haline) differences. This thermohaline circulation, occurs at both deep and shallow ocean levels and moves much slower than tidal or surface currents. Upwelling and downwelling areas in the oceans are areas where significant vertical currents of water are observed. Ocean currents can be contrasted with the tidal currents that occur in coastal areas.

• Ocean surface waves - are surface waves that occur on the free surface of the ocean. They usually result from wind, and are also referred to as wind waves. Some waves can travel thousands of miles before reaching land.

• Ocean Tracking Network - a research effort using implanted acoustic transmitters to study fish migration patterns.

• Oceanodromous - fish that migrate only within salt (ocean) waters.

• Oceanography - the branch of earth sciences that studies the ocean, including marine organisms and ecosystem dynamics; ocean currents and waves; plate tectonics and the formation of underwater topography; and movements of various chemical substances and physical properties within the ocean and across its boundaries.

• Otoliths - calcareous deposits or bones found in chambers at the base of the skull in fish. Sectioned, these bones often show rings or layers which can be used to determine age.

• Otter trawl - An otter trawl is a demersal trawl which uses large rectangular otter boards to keep the opening of the trawl net from closing. Otter trawls are towed by a single trawler.

• Overfishing - occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans.

• Oxytetracycline - an antibiotic which is injected in a fish to leave a mark on its skeletal structures such as otoliths. When the fish is later recaptured, the mark left can be used to validate age estimates.