Seamounts

Seamounts are underwater mountains that are formed by volcanic activity and rise at least 1000 meters above the sea floor. Some are 3000 or even 4000 meters high. Their peaks often rise up into the upper layers of the mesopelagic zone. Seamounts can be regarded as islands or volcanoes that do not reach up to the sea surface. It was long believed that these were rare occurrences. Today it is known that seamounts are present in all oceans. The total number is estimated in the thousands.

Research has shown that some seamounts are home to communities of unique, endemic species. These include lower animals like sponges and sea cucumbers, relatives of the starfish, but also vertebrates such as fish, which can occur in large schools around seamounts with high species diversity. This makes the seamounts especially interesting for fisheries.

Wild-caught fish of a certain size (not typical pelagic fish, such as herring and mackerel) normally have gill arches cut shortly after having been hauled aboard the boat. This is especially important if the method of fishing is such that the fish are still alive when brought aboard, as the gill-arch cut quickly kills the fish. Another common method of killing the fish is gutting and de-heading immediately when hauled aboard. Chilling or freezing of wild-caught fish is very important to secure fish quality.

Pelagic fish, such as herring and mackerel, are normally pumped or hauled into the hold, and are not individually killed. These die due to the lack of oxygen in the boat’s hold. It is very important to cool down and freeze pelagic fish very quickly as they normally have not been gutted. The intestines contain nutrients, and these must not be allowed to spread in the haul, as they provide fertile conditions for rapid bacterial growth, which will impede the quality of the caught fish. This process must be conducted with care.

PELAGIC FISHES | An Introduction to the Biology of Pelagic Fishes

Marine fishes live in every oceanic ecosystem, ranging from shallow waters to the vast, deep, open ocean, which is characterized by having a distinct thermal, toxic, and photic stratification. Open ocean (pelagic) fishes do not have access to any refuge and they must swim to escape predators, capture prey, or to search for more favorable environments. In general, pelagic fishes are continuous swimmers that routinely undergo seasonal migrations and some are capable of extensive vertical excursions from the surface to the deep layers of the ocean. Numerous pelagic species co-inhabit the same areas of the open ocean, but have different depth distributions and vertical movement patterns. This article reviews some general adaptations of pelagic fishes and describes the physiological specializations that allow some species to undergo large vertical-movement patterns.

Fisheries in the deep sea

Commercial fishing has only been carried out in deep waters over the past few decades. Although longline fishing has been practiced since the 18th century, industrial fishing far out in the ocean first became practicable in the 1950s with the availability of seaworthy refrigeration ships. Deep-sea fishing received a boost in the early 1970s with the introduction of the 200-nautical-mile zone, or Exclusive Economic Zone, which made it impossible for foreign ships to fish close to the coasts of another country. The high seas, including the deep sea, were an alternative fishing area. The Soviet Union and Japan in particular were soon specializing in the deep-sea regions. In the beginning the catch amounts were enormous – especially around structures such as seamounts and banks.
To the extent that fish stocks were gradually shrinking in the coastal areas, deep-sea fishing became increasingly interesting for other countries as well. According to a survey by the FAO, there were 27 countries conducting deep-sea fishing in the year 2008, with Spain, South Korea, New Zealand and Russia at the forefront. Around 70 per cent of the ships employ trawl nets, and these are often demersal-trawl nets. Today these can be deployed to a depth of 2000 Meters. It soon became obvious that deep-sea fishing is problematic in two respects. For one, valuable habitats such as cold-water corals or the ecosystems at seamounts are destroyed when nets come in contact with the bottom. Secondly, fish species are quickly decimated, particularly the K-strategists. For example, newly discovered stocks of orange roughie were reduced to 15 to 30 per cent of their original size within just 5 to 10 years. In many areas the species was commercially depleted. This “boom and bust” kind of fishery is typical in the pursuit of deep-sea fish species. The reason for this is that species like the orange roughy not only produce a small number of offspring, their reproductive performance is also very erratic and episodic. Several years can pass with low production of offspring before a strong season occurs again. It is still not known what controls or triggers these fluctuations. Investigations at the Great Meteor Seamount west of Madeira have indicated an influence of changes in the eawinds affecting eddy currents above the seamount.

It is a certainty that the deep-sea species cannot compensate for heavy fishing activity. Deep-sea fishing is also both ecologically and economically questionable. For one thing, it is very destructive, and for another the catch levels are relatively low because most deep-sea fish stocks are comparatively small due to their K-strategy. Thus, taken as a whole, the deep-sea fisheries represent only a small proportion of the worldwide catch amounts. Basically, they can only be maintained because of the high subsidies, since the costs for fuel are high for the great distances ships often have to cruise out.