Small invaders conquer our waters
This article was featured in EUROFISH Magazine 1 / 2021.
Almost half a dozen goby species originating from brackish water areas of the Black, Azov and Caspian Seas are spreading almost explosively in Central and Eastern Europe. The round goby (Neogobius melanostomus) has been particularly successful, even making it to North America’s Great Lakes. Such neozoa, which are difﬁcult to contain, are a risk to regional ecosystems.
Growing demand and attractive prices are accelerating overuse
This article was featured in EUROFISH Magazine 4 / 2020.
Sea cucumbers, also called holothurians, are the most species-rich group of echinoderms (which also include sea urchins and starfish). On the seabed these worm-like creatures are of similar ecological importance as earthworms in the garden. Sea cucumbers take up sediments, digest the organic matter they contain and excrete the sand in a purified state… so act like “marine vacuum cleaners”.
Most of the time sea cucumbers seem to lie motionless on the seabed and with their worm-like, cylinder-shaped bodies some of them actually resemble the vegetable whose name they bear. Other forms are often compared to fat caterpillars, thick prickly sausages, or even the little heaps that dogs leave behind on pavements. Indeed these strange animals exist in a huge variety of shapes. They live in the benthic habitats of all seas from shallow tidal zones to the dark bottom of the deep sea. Among the more than 1,700 known species there are tiny representatives such as the Rhabdomolgus species, which measure only a few millimetres, but also giants such as Synapta maculata, which can grow up to two and a half metres in size. However, the majority of species measure only 10 to 30 centimetres in length and live for about 5 to 10 years. Sea cucumbers play an important role at the bottom of the oceans. They feed on the seabed, digesting sedimented plankton (detritus) as well as organisms that live in the seabed, or the remains of dead organisms that have sunk to the bottom of the sea, and then excrete the sand again – now freed from organic pollution. This “sanitary service” that the sea cucumbers perform is of great importance for the health of the seabed: the cylindrical animals clean the ocean floor, removing dead organic matter, thereby preventing excessive oxygen depletion in the depths. At the same time, on the light-flooded surface they release nutrients that enable renewed the growth of microalgae with which the marine food chains begin. Sea cucumbers have a special function in tropical coral reefs: they “recycle” calcium which many marine animals need to build their shells or skeletons. A high calcium carbonate content also increases the alkalinity of the water and acts as a buffer against local acidification. In some areas of marine life, especially in the deep sea, sea cucumbers are the dominant life form. In fact they sometimes account for almost 90 per cent of the biomass present there. Common to all sea cucumbers is the soft but muscular tubular body with its thick leather-like skin. The mouth opening at the front end of the trunk, which is often surrounded by tentacles, bears a ring of calcareous plates that surrounds the oesophagus and serves as an attachment for the often very complex tentacles and longitudinal muscles that extend backwards in the body. In contrast to starfish and other echinoderms, sea cucumbers do not have an exoskeleton but have spine-like hard formations, so-called sclerites, on their skin. These are attached flexibly to the body, can be moved with special muscles and are used for digging in the sediment and for protection against predators. The five-beam radial symmetry characteristic of all echinoderms (many starfish have five arms, the chewing apparatus of sea urchins has five teeth) can only be recognized externally in sea cucumbers by the five rows of ambulacral feet. These are small movable feet with tiny suckers, hundreds of which are arranged on the body in long rows. With their help sea cucumbers can move forward, but only very slowly. In adaptation to life on the ground, some holothurians have developed flattened bodies where a sole (for crawling) and a back can be seen. However, this bilateral symmetry is a secondary development and does not replace the five-beam radial symmetry. Some sea cucumbers can also swim and in doing so detach themselves briefly from the seabed.
This article was featured in EUROFISH Magazine 3 / 2020.
Some people only know octopods from seafood salads at the Greek restaurant, others from seafaring stories that describe how the animals use their strong fleshy suckers to pull ships down into the depths. But they don’t frighten us anymore today: in fact, they rather stimulate our appetites, for they are now a popular marine delicacy. Demand for them is so great that some stocks are regionally overused. That is why great hopes now rest on aquaculture, although breeding technology is not yet fully developed.
Cephalopods (from the Greek ‘kephale’ for head and ‘pod’ for foot) belong to the group of molluscs (mollusca) like snails and mussels. The evolutionary development of this species group which began 700 million years ago has led to a wide variety of shapes, sizes and colours. Today, the cephalopod class comprises nearly 1,000 species, including not only squid and cuttlefish but also approximately 170 octopus species, some of which differ quite considerably in appearance and lifestyle. For example, in the case of the octopus species Octopus horridus the head is relatively small and the arms conspicuously elongated, whereas the musky octopus O. moschata has a large head and short arms connected by an umbrella- like membrane. In contrast to many related species of molluscs (such as snails or mussels) the octopus’ body is not protected by any outer shell or carapace – with the exception of female pearl or paper nautiluses (Argonauta). Most octopods live near the bottom (benthic) of shallow waters down to about 200 metres off the coasts of warm, temperate seas where they hide in underwater rock caves, thickets of seagrass beds, or in tropical coral reefs. However, some species have penetrated into the depths of the ocean where they swim freely in the abyssopelagic zone. The body tissue of deep-sea octopods contains a lot of incompressible fluid, which is why the animals are hardly affected by the enormous pressure at great depths.
King Crabs successfully hold their own in the Barents Sea
This article was featured in EUROFISH Magazine 1 / 2020
Red king crab (Paralithodes camtschaticus) is one of the best crustaceans that the cold northern seas have to offer. The species was originally only found in the north Pacific but in the 1960s Russian scientists introduced it to the Northeast Atlantic. The conditions in the new habitat suit the crab’s biological requirements and so their number has in the meantime spread there extensively.
Made-to-measure for aquaculture
The African sharptooth catfish Clarias gariepinus is a hardy fish of modest needs that is not particular about its diet. Moreover it is an accessorial air-breathing fish that is sometimes to be found living in groups in the most confined spaces at great densities. Its protein- and omega-3-rich fillet tastes good, is healthy and can be processed in many different ways. All this makes C. gariepinus an ideal candidate for aquaculture and it is produced in considerable quantities worldwide.
This article was featured in EUROFISH Magazine 5/2019.
Farmed fish with a long history and an uncertain future
The common carp (Cyprinus carpio) is not only one of the best-known but also one of the most frequently produced freshwater food fishes. Nearly 4.2 million tonnes of this species were reared in carp ponds or in polyculture in 2014, plus a further 150,000 tonnes from fishing. Carp were already popular as food fish in the ancient world, and in Central Europe centuries-old carp fishing ponds are today part of the cultural landscape.
The original distribution area of common carp is in the warm temperate regions of South East Europe and Asia from the Black Sea, through Asia Minor and China, to Japan. The Romans introduced the species to Central Europe about 2,000 years ago and today it is to be found all over Europe, with the exception of Scandinavia. Within this extensive area, however, the species structure is widely controversial. Some taxonomists distinguish four subspecies whose core centres are thought to be found from the Danube River basin to the Ural Mountain range, in the Aral Sea, in the Amur River basin to southern China, and in the waters of North Vietnam. Other experts differentiate only two subspecies – C. c. carpio and C. c. haematopterus, while a third group sees rather a uniform species status. Morphological methods alone hardly enable any satisfactory distinction for, with regard to its body shape, the common carp is one of the most variable freshwater fish species.
Their fillets are used for fish burgers, fish fingers, as well as different surimi products. Alaska pollock can be utilized for different purposes and at relatively affordable cost. Their stocks constitute the largest and the most economically important ground fish resources in the North Pacific Ocean. They are both, well managed and fished on sustainable basis. Yet depletion of their stocks occurs primarily due to natural causes. On that score, a dramatic role is played by the climate change.
Although the walleye pollock (Theragra chalcogramma) known also as the Alaska pollock is a close relative of Atlantic cod, saithe and haddock, intensive fishing started only 50 years ago. Before that, interest in this species was surprisingly low which could possibly be explained by its relatively small body Despite a record length of 91 cm, the length of individuals caught in the net seldom exceeds 50 cm while the weight barely reaches 500 to 600 g. The brownish or greenish coloured, beautifully spotted fish live in large schools at a depth of between 30 and 500 m. In terms of biomass, the stocks of Alaska pollock belong to the largest and most important stocks of food fish due to three main reasons. First, they grow relatively quickly reaching 30 to 40 cm length already at the age of three or four years. From this point onwards, they can spawn virtually every year thus contributing to the survival of the species. It is universally assumed that Alaska pollock lives a maximum 14 to 15 years. Second, their fertility rate is really high. The exact number of eggs depends upon a number of factors including the age, the size and the nutritional status of females, however on average it ranges from 100,000 to 1.2 million. The third reason for the high importance of Alaska pollock is the wide spectrum of feed they use. Although carnivorous they can consume plankton-eating krill or young herrings available in copious amounts in the North Pacific. In the Bering Sea, Alaska pollock accounts for almost 60% of the total fish biomass, while in the Gulf of Alaska and Aleutian Islands they constitute about one fifth of the total fish biomass.
When people talk about halibut in Europe they usually mean Reinhardtius hippoglossoides: Greenland halibut or black halibut. The name halibut is derived from haly (holy) and butt (flat fish) due to its popularity on Catholic holy days.
The species’ geographic range extends along the Norwegian coast as far as the cold North Atlantic waters off Greenland, Iceland and Spitzbergen. It is thus to be found in the same habitat as the Atlantic halibut Hippoglossus hippoglossus, a relatively close relation that is considered to be the “real” halibut in a lot of other countries. Although the two species are similar in appearance and behaviour closer examination reveals a number of noteworthy differences.
Meagre (Argyrosomus regius), also called croaker, has been highly valued as a consumption fish since ancient times. The catch volume from the fishery cannot satisfy demand. Production of the species in aquaculture began in France and Italy at the end of the 1990s and in the meantime nearly 20,000 tonnes are supplied annually. Meagre is still considered a niche product but its potential is by no means exhausted yet.
It would be lying to describe meagre as a particularly striking or attractive fish. Its body shape with its two dorsal fins is perch-like, its sides and belly are pale to silvery white, its back and fins brownish or grey-black. The only spot of colour is to be found on the inside of its mouth cavity which in live fishes has a yellow or orange shimmer. What makes this fish so popular is its firm meat that renders it suitable for frying, baking or grilling but also for steaming, smoking or even marinating raw. It has a strong aroma and does not spoil easily because due to its moderate fat content it is less susceptible to oxidation.
Arctic charr (Salvelinus alpinus), also called mountain trout or salmon trout, is a member of the salmon family (Salmonidae). It is a coldwater species that occurs in Arctic, sub-Arctic and Alpine waters as well as in northern coastal waters. Like salmon, Arctic charr enters the upper reaches of rivers to breed. Some populations inhabit freshwater permanently, others migrate to the sea and only return to the rivers for reproduction.