Glossary of Fishery Terms - D

D

• Danish seine - a widely used commercial fishing technique which uses a small trawl net with long wire warps. The seine boat drags the warps and the net in a circle around the fish. The motion of the warps herds the fish into the central net. Danish seining works best on demersal fish which are either scattered on or close to the bottom of the sea, or are aggregated (schooling). See also purse seine.

• Dead zone - an area in an ocean or large lake where oxygen levels are extremely low, often due to eutrophication. Dead zones have been increasing since the 1970s.

• Deep ocean currents - currents in the deep ocean, also known as thermohaline circulation or the "conveyor belt", are driven by density and temperature gradients. They can be contrasted with surface ocean currents, which are driven by the wind.

• Demersal zone - the zone at or near the bottom of a sea or lake. Inhabitants of the demersal zone feed off the bottom or off other demersal fish. See also pelagic zone.

• Demersal fish - fish that live in the demersal zone. Examples are cod, flounder and snapper. Compared to pelagic fish, demersal fish contain little oil. See also bottom feeder.

• Demersal trawling - trawling on or near the bottom of a sea or lake. See also bottom trawling.

• Depletion - reducing the abundance of a fish stock through fishing.

• Delisted - a species which is no longer listed under the ESA. See also recovered species.

• Diatoms - minute planktonic unicellular or colonial algae.

• Downwelling - A downward movement (sinking) of surface water caused by onshore Ekman transport, converging currents or when a water mass becomes more dense than the surrounding water.

• dorsal - relating to or situated near or on the back.

• Dredging - dredge designed to catch scallops, oysters or sea cucumbers are towed along the bottom of the sea by specially designed dredge boats.

• Driftnet - a gillnet suspended by floats so that it fishes the top few metres of the water column. Drift nets can be many kilometres long. Because drift nets are not anchored to the sea bottom or connected to a boat, they are sometimes lost in storms and become ghost nets.

• Dropline - a fishing line with one or more hooks, held vertically in the water column with weights and generally used on the continental shelf and slope. Several droplines may be operated by a vessel, either on manually or mechanically operated reels.