Pelagic zone

Demersal

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Guest post: Lauren Flynn, New Mexico State University

Example of a demersal catch, painted in the 17th century. Nature morte de poissons au bord d’un ravage by Jan van Kessel the Elder, Source: Wikimedia Commons.

From the Latin verb demergere, meaning ‘to sink’, demersal describes fish that live on or just above the ocean or lake floor.  It can also refer to a commercial fishery for groundfish and shellfish, which typically uses gear that is weighted so it sinks to the bottom.  Demersal fish are frequently captured by a trawl net that drags along the ocean or lake floor.  As a result, demersal fisheries tend to be less discriminate than pelagic fishing methods such as long lines or gill nets that target fish using specific bait, specialized hooks, or mesh size.  Bycatch can be an issue for demersal fisheries, but trawl net modifications using Bycatch Reduction Devices (BRD) help to increase capture of target species and reduce unintended catch.

Examples of demersal fish species are cod, haddock, and flatfish such as turbot and halibut.  Popular shellfish include crab, shrimp, and lobster.

For comparison, see pelagic.

Pelagic zone

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The pelagic zone is divided into layers based on light

The pelagic zone is divided into layers based on light

The pelagic zone is the region of a body of water (lake, river, or ocean) that is not associated with the bottom (see benthic zone) or shore (see littoral zone).  This habitat zone is truly a three dimensional habitat space.  Some fish that occupy the pelagic zone never encounter the bottom or shore throughout their lives.  Because the pelagic zone is a nutrient poor environment, large fish have two basic strategies to get meals – either swim long distances in search of nutrient-rich prey (like many oceanic sharks and tunas) or drift with currents and eat nutrient-poor prey (like the Ocean Sunfish Mola mola).  The pelagic zone is divided into zones based on light penetration from the surface:

  • Epipelagic (sunlight; 0-200m)
  • Mesopelagic (twilight; 200-1,000m)
  • Bathypelagic (no light; 1,000-4,000m)
  • Abyssopelagic (4,000m – ocean floor)
  • Hadopelagic (deep sea trenches)