Deep sea is just a shortcut for "all that enormous part of the Biosphere we barely know anything about". The reasons are our physiological limitations, and the lack of interest by most humans. Deep sea started at some 30 m for centuries. Now the limit is somewhere around 200 m, because that is were you need a big boat and just can't make it alone. This is convenient because it coincides with near total darkness, as well as with the average edge of the continental platform. But there is nothing too scientific in "deep sea" --rather, it's a bold acknowledgment of ignorance and inability.

I think that you can use the term "deep-sea" for these species at those depths. Among the Mediterranean, this term can be used both for batyal and abyssal plans, sensu Pérès & Picard (1964), referring to the bottom marine environment that starts from the top of the continental slope, roughly where vegetable life is no longer present...

Hi Pierre,

This is a good question, actually I had the same doubt recently. I am not sure if there is a definition widely accepted to define deep sea. However, during the implementation of the MSFD in Spanish water 200 m has been chosen as the threshold to define the deep sea (http://www.magrama.gob.es/es/costas/temas/estrategiasmarinas/IV_D1_Noratlantica_tcm7-207268.pdf, it is spanish I am afraid...) and most recently this threshold has been chosen like limit for the deep sea for baning trawling in deep sea water in the EU. So, even when I am not an expert on deep sea, in my opinion you can perfectly speak about deep sea for these depths.

Cheers

Deep sea is just a shortcut for "all that enormous part of the Biosphere we barely know anything about". The reasons are our physiological limitations, and the lack of interest by most humans. Deep sea started at some 30 m for centuries. Now the limit is somewhere around 200 m, because that is were you need a big boat and just can't make it alone. This is convenient because it coincides with near total darkness, as well as with the average edge of the continental platform. But there is nothing too scientific in "deep sea" --rather, it's a bold acknowledgment of ignorance and inability.

That part of marine realm from where there is no light available ( from zero light compensation depth) right up to the bottom of an ocean or sea.

Thank you all for your answers.

While asking my questions, I was thinking about others criteria, linked with hydrological parameters (depth of occurrence of Western Mediterranean Deep Waters) or large decrease of nutrients but this is nice to have others' opinion. I think the concept of no light and consequently no benthic photosynthetic organisms would be a good criterion to define these zone, a bit more rigorously than just "the place it's complicated to sample in" !

Jose Manuel, I have no problem for understanding Spanish documents, but unfortunately, the link you quote appear broken...

We have the same issue with cold-water corals, many people refer to certain localities as deep-sea corals where for some species that have been found at 50m through to several thousand metres. The definition generally stands as the vast majority of cold-water coral records are found below the photic zone.

I think it is fine to refer to your fish species as being "deep-sea", as you did collected them from deeper water. If the species in question were also found at the surface, then you have simply species that have a very wide depth range.

The approximation of vertical density distribution in the ocean can be discribed as a three layer structure.The surface layer is the region from the sea surface having a temperature of about 10C. The transition region where the temperature decrases from10C to 4C is called the thermocline.The DEEP SEA REGION is the region below the thermocline. The temperature and salinity are the main factors controlling density of the different watermasses.

Sorry Pierre, try now:

http://www.magrama.gob.es/es/costas/temas/estrategias-marinas/IV_D1_Noratlantica_tcm7-207268.pdf

Deep sea is simply defined as the zone of the sea water column closest to the continental deep, where no photosynthetic organism exists and life there is predominantly chemotrophic.

Thank you all for your answers, references and links. This would be really helpful !

This depends on who you ask as well as what you are studying (e.g. currents, chemistry, biota). I have found a variety of different criteria used in the literature. Since I study deep-water sharks, I follow the IUCN Shark Specialist Group's definition: "The World Conservation Union (IUCN) Shark Specialist Group defines deep-water chondrichthyans as those “whose distribution is predominantly at, are restricted to, or spend the majority of their lifecycle at, depths below 200 m” (Kyne and Simpfendorfer, 2007)". Since many fish species are capable of deep DIVES but do not necessarily reside in the "deep sea", this definition had to be somewhat flexible.

Hi there,

As people above have mentioned there can be differing interpretations for the 'deep-sea'. Generally speaking the deep-sea starts at the continental shelf-break ~200 m water depth. Here's a good general (and recent) reference: http://www.biogeosciences.net/7/2851/2010/bg-7-2851-2010.html

Hi there,

There is a general agreement (ICES, FAO) to consider true deep sea species those at which the mode of the abundance on the distribution by depth is around 400 meters depth. Most of the species under this definition are considered as low/late life history characteristics (slow growth, late maturity, etc). However, you find many resources within these criteria for which the species distribution covers a range of depth from shallow to very deep (depending on the life history stage). So, a practical management definition is to consider as deep-sea the species found down 200m but with a distribution down to 400m (usually until 600m). This is mainly a fishery definition because the fleet coverage of different assemblages by depth.

I agree with Mario - generally those off the continental shelves, that is deeper than 200 m. There is an issue around the shelf slope of course where many fisheries take place but I think the EU and most others consider these deep-sea fisheries.

I do not agree with the suggestion that deep-sea is below the thermocline - this can occur at a variety of depths as shallow as 20 m or so depending on the mixing regime.