Fishing in the deep: A dive into deep-sea fisheries

Most of the things I post about on the Fisheries Blog are associated with art or fun fish facts. My scientific research on deep-sea fishes isn’t inherently ‘fisheries’ related as I study the systematics (evolution) of deep-sea fish groups. That being said, as someone who grew up in the midwest, I have had multiple experiences working for freshwater fisheries departments and have a deep appreciation for the work being done to sustain and manage fish in waterbodies throughout the country. In an attempt to tie some of my current research with my past fisheries experiences I am focusing this post on fisheries in the deep-sea. As marine fisheries stocks plummet, countries look elsewhere for fish. And by elsewhere I mean deep, real deep…..

The deep sea is host to an enormous amount of life. Most people imagine the deep-sea as being a cold, dark, and desolate place, and in some respects, they’re right. Plunge a camera 500 meters down and you won’t see all that much. That being said, the deep-sea is also the largest habitat on earth and is host to the deep-scattering layer, a daily migrating mass of organisms that also contains the largest populations of vertebrates on the planet. Fisheries efforts in the deep-sea have increased through time as populations of more easily accessible fishes have dropped due to historic overfishing and lack of population management.

Solid line: all species. Dashed line: demersal species only. Figure from Deep-Sea Fishes Biology, Diversity, Ecology and Fisheries (Priede, 217)

Fishing in the deep-sea is much more costly, as the gear associated with this work has to be sturdy enough to withstand the increased stresses of fishing so deep. By 1978, global deep-sea fisheries reach about 1 million tonnes and roughly 3.7 million tonnes by 2003. Although these numbers may seem large, they are only ~ 3.5% of the total annual world catch.

Dashed line: total marine species captured. Solid line: total deep-sea species. Figure from Deep-Sea Fishes Biology, Diversity, Ecology and Fisheries (Priede, 217)

There are already numerous types of deep-sea fishes that are harvested in fisheries activities every year. The methods and techniques to capture these fishes are similar to freshwater and near-shore fisheries. Baited traps and lines, static nets, pelagic trawling, and bottom trawling are all used to harvest deep-sea fishes, with catch abundances varying significantly by type of method. One of the most successful but also most destructive methods is bottom trawling.

Bottom trawling uses an otter trawl net, which includes a net with a cod-end, usually ground gear of some sort that allows the net to move over the bottom, and trawl doors. This method of fishing is indiscriminate. Fisheries know general areas of high species abundance for the species they are specifically targeting, but they also pull up immense amounts of non-targeted fish species and additional organisms like deep-sea corals and sponges. These methods not only harvest targeted and non-targeted fish species, but they are damage deep-sea ecosystems in which these fishes survive and reproduce, further decreasing the rate at which these populations can bounce back. 

Although not all deep-sea fisheries use bottom trawling, and otter trawling nets are becoming more sophisticated, many of the deep-sea fishes being targeted in these fisheries are demersal, or bottom dwelling species. Many of these species are slow growers, have long maturation rates, and low fecundity, all traits that make them vulnerable to overfishing. Some of the deep-sea fish species being targeted that may end up on your plate include:

  • Coryphaenoides rupestris (Roundnose grenadier)
  • Hoplostethus atlanticus (Orange roughy)
  • Merluccius merluccius (Hake)
  • Lophius piscatorius (Common monkfish)

Some of the deepest species being fished include:

  • Spectrunculus grandis (Pudgy cusk eel, 4300 mfbd) 
  • Gymnoscopelus bolini (Grand lanternfish, 4200 mfbd)
  • Dissostichus eleginoides (Patagonian toothfish, 3850 mfbd)
  • Isistius brasiliensis (cookie cutter shark, 3700 mfbd) 
  • Centroscymnus coelolepis (Portuguese dogfish, 3700)
  • Albatrossia pectoralis (Giant grenadier, 3500 m fbd) 
  • Coryphaenoides cinereus (Popeye grenadier, 3500 mfbd)

Some of the largest deep-sea species being fished include:

  • Somniosus microcephalus (Greenland shark, maximum length 7.3 m), 
  • Hexanchus griseus (Bluntnose sixgill shark, 4.82 m), 
  • Ruvettus pretiosus (oilfish, 3 m), 
  • Dissostichus eleginoides (Patagonian toothfish, 2.15 m)
  • Genypterus blacodes (pink cusk-eel, 2 m)

There’s been increasing interest in focusing on deep-sea fish species that have high abundance and where time at maturity is earlier. Many of these species live in the midwater, or deep-pelagic zones like the mesopelagic. One such group are the lanternfishes, the group I work on. Although extremely abundant, they don’t school, are difficult to ‘corral,’ and are known to be adept at avoiding nets. Fishing for these pelagic groups would require trawling for long periods of time at specifically targeted depths in order to accumulate the necessary amount of fishes for an adequate catch. 

Focusing on deep-sea fisheries may only supplement global fisheries catches for a small period of time, as many of these species are slow growers with low fecundity. Fisheries techniques are ever-improving, and many countries have efforts in place to better manage marine fisheries stocks. Hopefully, with time and global effort, we can find a balance between maintaining these marine populations while continuing to harvest without having to go too deep.

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