Vulnerable species with low resilience

Deep-sea species are characterized by slow growth, high or extremely high longevity, late sexual maturity and low fertility, biological characteristics which make them extremely vulnerable to exploitation and give them very low resilience.

Corals: the planet’s sentries

One fact to bear in mind: the diversity of corals in the deep sea is greater than in shallow water.

A second fact: much attention has been paid to corals that make up deep-sea reefs, but deep-sea coral diversity mainly consists of species that live alone or in isolated colonies.

We therefore tend to associate deep-sea corals (living at depths of 300 to 3 000 meters) uniquely with deep-sea coral reefs. It is true that these reefs are exceptional ecosystems in terms of their size, their biodiversity, their high rate of endemism, the increased biomass that develops nearby, and the habitat that they create for other species, particularly fish and invertebrates. Nevertheless, it should be remembered that only six deep-sea coral species (and two in particular: Oculina and Lophelia) are capable of making reefs, whereas around 3 300 others (without zooxanthellae) are dispersed around the deep sea.

Their skeleton is rigid, often brittle, and systematically crushed or torn out if it comes into contact with fishing gear (be it trawl, net or longline). Given their low individual mass, coral ‘catches’ often weigh little when they are made outside of coral reef areas.

The world’s oldest species

Deep-sea corals have among them the longest living organisms on the planet. Until very recently, we thought that they could reach the venerable age of 1 800 years. Little did we think that in 2009, age estimates for these champions of longevity would double. Radiocarbon dating methods have established that some species (Gerardia sp. and Leiopathes sp.) can live for over 4 000 years, making them the longest-living animals in the world.

They have a growth rate of 5 to 26mm per year. Whilst corals continue growing from their tips, the bottom parts, which are older, die and are colonized by organisms that cause their calcareous skeletons to erode. This results in the formation of mountains, with the accumulation of debris at their base, then dead coral, and finally a summit of living coral. The fauna associated with deep-sea coral is spectacular in terms of the high density of large organisms (megafauna), which forms a strong contrast to the surrounding sedimentary environment (inhabited by very diverse but small animals). It is thought that a minimum of around 2 000 species are associated with coral formations.

Many commercially important fish have been observed on cold-water coral reefs, including a dozen species that are very important to the fishing industry (for example cusk, cod, orange roughy, anglerfish, redfish and black pollock), as well as around ten species that are less heavily fished, but also marketed (including blue ling, grenadier and Alphonsino).

Coral reefs vary in shape and size. Reefs in Norway, mostly composed of the species Lophelia pertusa, are several kilometers long, around 500 meters wide and up to 35 meters high. Such coral reefs take over 9 000 years to form.

Sponge beds

Sponges have been found in all of the world’s oceans, in both deep and shallow waters. In the North-East Atlantic, fishermen working at depths of hundreds of meters often report the existence of sponge beds. “Glass sponge” reefs, once major structures in the deep sea, were thought to be extinct until recently. Yet a series of giant reefs has recently been discovered off the West coast of Canada, off of British Columbia. Some were already seriously damaged by deep-sea trawling. These glass sponge reefs are 9 000 years old and extend over an area of 700km2. In some places, they reach 19 meters in height! Individual sponges can be over 100 years old and weigh up to 80kg.

Deep-sea sponges, although little explored, have fascinating biochemical and genetic properties. Such properties make these animals rare or even unique, and mean that sponge concentrations are living laboratories.

Sponge reefs serve as a refuge for fish and invertebrates. They are used as a place to feed or reproduce, as a place for juveniles to develop, or as a sleeping place by diurnal species. Scientists have found almost twice as many species near sponge beds than on the surrounding seabed. They have also found areas damaged by deep-sea trawling. Like cold-water corals, deep-sea sponges are slow-growing, fragile and very vulnerable to damage by bottom fishing gears. Recovery can take centuries, if it ever happens.

Deep-sea fish

Deep-sea fish are typically small and not very muscular. They have watery flesh and soft bodies, properties which mean they are of no commercial interest even for fishmeal fisheries (where catch is turned into animal food).

Only a “handful” of species (those that often form aggregations and live on seamounts or continental margins) are marketable. These include orange roughy, redfishes, black scabbardfish, blue ling and grenadiers… They are characterized by their firm flesh, making them well-adapted to swimming in strong currents and above all… to human tastebuds!

An abundance concealing extreme vulnerability

The local biomass density of aggregating fish species is often astonishing.

When orange roughy stocks were discovered, a “goldrush” phenomenon occurred, with huge trawlers rushing to catch 50, 60 or even 70 tons in a single, less-than-five-minute haul! Fishermen had to time precisely how long the trawl spent in the school, for fear of losing their equipment under the weight of the catch.

It is easy to see how such localized abundance could give the impression of an inexhaustible goldmine. It is counter-intuitive to associate immense biomass with extreme vulnerability, yet this is exactly the situation for these aggregating species. The truth behind these “miraculous hauls” is much more complicated and so much more fragile.

Aggregating deep-sea species are characterized by low sustainable yields, high vulnerability to overfishing and slow recovery rates. However, their high market value has encouraged the development of new fisheries targeting them. The classic scheme for these fisheries is as follows:

  • a phase of rapid growth of the fishery, with high catch levels
  • followed by a contraction phase of the stock distribution, then finally an often drastic fall in catch levels when stocks are overexploited.

Deep-sea fishes: the equivalent to fossil fuels

The ecological characteristics of these fishes make them vulnerable to overexploitation and slow to recover from it. Catch levels can be high at first, but this is followed by dramatic and rapid decline (on average, the life-cycle of a deep-sea fishery lasts less than ten years). This is why we most often speak of deep-sea stocks being “extracted” by industrial fisheries (as if they were non-renewable fossil fuels), rather than exploited in moderation, with low catch volumes ensuring long-term sustainability.

Exceptions to the rule

Blue ling and black scabbardfish have moderate lifespans (20 to 30 years) which contrasts with species of high longevity (which often near or exceed a century). Fishing professionals argue that these stocks can be exploited sustainably, without taking into account the fact that fishing has already drastically reduced the virgin biomass of these fish populations and that they can no longer be fished sustainably without prior stock recovery. Finally, and above all, trawl fisheries are by definition  non-selective.  They cannot, therefore, be guaranteed to have a moderate impact on habitats, or on a targeted species whose characteristics are favorable to sustainable exploitation. Consequently, these fishing practices systematically and unavoidably lead to the capture of extremely vulnerable species, notably deep-sea sharks, some of which are critically endangered.

The very nature of deep-sea trawl mixed fisheries forbids them to comply with the holistic approach which is now expected from fisheries management (and so called “the ecosystem approach” which is mandatory).


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