Northern shrimp live in the icy waters of the northern hemisphere. In the Atlantic they are to be found from New England along the Canadian coast, off Greenland, Iceland and Svalbard as far as Norway and the deep sea regions of the North Sea. And in the Pacific, in the Okhotsk Sea, the Bering Strait and in the waters off Alaska. The sub-arctic shrimp Pandalus borealis is known by various names: pink or Northern shrimp, Northern prawn, deepwater or deep-sea prawn, great Northern prawn or crevette nordique. Regional names like Maine shrimp are also common. And scientists are not even sure whether the animals in the Atlantic and Pacific belong to the same species. Some consider the Pacific Northern shrimp only as a subspecies (Pandalus borealis eous) of the Atlantic form (Pandalus borealis), but others accord it real species status with its own name Pandalus eous. The similarity between the 20 or so species of the genus Pandalus is so great that fishermen rarely bother to sort their catches accurately, record the different species separately or make precise appropriate reports. And so catch statistics by species reveal considerable inaccuracies which makes the interpretation of the data more difficult, and some evaluations even impossible.
In addition to the main species Pandalus borealis, the FAO database FIGIS lists six other species (P. goniurus, hypsinotus, jordani, kessleri, montagui, platyceros) whose total catch was fairly constant at around 400,000 t in the period from 2000 to 2010. Since then, however, the catch has declined to 327,000 t (2013). According to this statistic P. borealis accounts for 80 to 85% of the catches. And with that, P. borealis is seen to be by far the most frequent and therefore the most economically important species of the genus Pandalus. This can be said with some certainty despite the fact that the statistical data, as already mentioned, are not very reliable due to the insufficient species allocation. In the Atlantic, P. borealis is the largest of the three recognized Pandalus species whose distribution areas overlap geographically as well as in terms of water depth.
Heavy environmental demands on the habitat
The name "Pandalus borealis" already suggests that these animals prefer waters with low water temperatures ("boreal" = of the North or northern regions, adjacent to or including the Arctic). Their preferred temperature range is between minus 1 and plus 8°C. The upper tolerance limit of adult animals is around 14°C. Early larval stages tolerate temporarily higher temperatures up to 18°C, although mortality increases significantly then. However, P. borealis does not only react sensitively to high temperatures but also to low or strongly fluctuating water salinity. As a stenohaline (able to tolerate only a narrow range of salinity) species this shrimp prefers purely marine conditions with salinities between 32 and 35 per thousand. The interaction of water temperature and depth, salinity and substrate quality affect the distribution of P. borealis, with temperature being a particularly important factor. Some researchers believe that temperature has an impact on the sex ratio but these inferences are controversial. Usually the animals are to be found over soft muddy bottoms at depths of 80-500 metres, but in some areas they can be caught at depths of 1,300 m. Pandalus generally stays close to the seabed during the daytime. They are very active swimmers and with nightfall they rise to layers of water near the surface in search of food.
Northern shrimp is an important element in the marine food chain that connects several trophic levels. As opportunistic omnivores and scavengers they eat both invertebrates that live on the seabed, for example bristleworms, small molluscs and echinoderms, as well as plankton and other organisms in the open sea, for example copepods, mysid shrimps and arrow worms. At the same time, however, Northern shrimp is also an important source of food for seabirds, marine mammals and numerous species of fish such as cod, whiting, saithe, halibut or redfish.
With an average length of 10 to 12 cm P. borealis is one of the relatively small shrimp species. Females reach a maximum length of 16 cm, sometimes even 18 cm; males remain slightly smaller. They are often hardly more than three or four years old when they are caught in the fishermen’s nets. The maximum age is said to be eight years. All Pandalus species look quite similar. They are most easily identified by their pale-red colour, with only the abdomen or pleon having a slightly stronger colouring. The long forehead projection (rostrum) which protrudes beyond the front edge of the carapace has 6 to 8 teeth on the belly side and 12 to 16 on the dorsal side which extend approximately to the centre of the carapace. The eyes are wider than the eyestalk behind which on both sides of the carapace there is a small, forward facing barb. The second pair of walking legs is clearly dissimilar, the left leg usually being longer and slimmer than the right.
Sex change as a successful reproduction strategy
Pandalus is a "protandrous hermaphrodite": the animals reproduce first as males and later change into "secondary females". The sex change occurs mostly at an age of between 2 and 4 years. However, under certain conditions individual specimens can skip the male phase and develop directly into "primary females". As far as we know today, water temperatures play a decisive role in this process – an ingenious reproduction strategy that improves the species’ reproductive record. When living conditions are particularly favourable more male animals change into females. The populations then produce more eggs, with the result that the number of young increases.
As in all crustaceans the release of sperm packets (spermatophores), the discharge of eggs, and their fertilization are separated in time in P. borealis. These processes, which can last up to 36 hours, mostly take place from July to October depending on the region and temperature. The colder the water, the sooner does reproduction begin and the longer it takes for the eggs to develop. As a rule the females carry the eggs under their abdomen for at least 5 to 6 months, but in cold northern waters even up to 11 months. In such cases, the larvae do not hatch until the late spring of the following year, making it difficult to re-fertilize the females in the same year. For this reason they can sometimes only reproduce every two years. As the embryos mature the females migrate gradually from deep water areas to the shallow shores where the larvae will find more food after hatching and have more favorable conditions for their development. Depending on their size, age and nutritional status females produce between 300 and 4500 eggs, on average about 2,000. The larvae usually hatch at night. With the aim of distributing the new generation over as wide an area as possible the females swim around restlessly at this time and support the hatching process through rigorous movements of their abdomen.
The larvae of P. borealis live a purely planktonic life in the first few months and during that time hardly resemble the adult animals. They hatch as so-called zoea which, although they have compound eyes, a carapace and walking legs, do not possess swimming legs (pleopods). After several molts and other larval stages the zoea changes into a megalopa larva and develops functional swimming legs for the first time. From hatching to sexual maturity shrimp goes through about eight to eleven larval stages during which their typical crustacean shape emerges more and more. The larval phase can take between 45 and 120 days depending on the temperature. During this period, the animals drift with the currents and waves, and are spread over a wide area whilst at the same time their number is strongly decimated by plankton-eating fish and other predators. Although the tiny crustaceans have little with which they can oppose the currents they try to stay in the shallow water areas for as long as possible. Usually they only move to deeper areas after about a year as adult shrimp. This behaviour is also evident in the count of P. borealis in the catch. While offshore catches are usually in the range of 100-300/ kg (pure meat, cooked & peeled) inshore shrimp are usually much smaller at 250-400/ kg.
Targeted fishing only began a few decades ago
Fishing specifically for P. borealis began around 1898 in Norway after Johan Hjort who is regarded as the practical inventor of the Northern shrimp fishery modified the design of the deep-sea demersal trawls used at that time and discovered huge swarms of shrimp in Norwegian fjords. Because other fisherman showed no interest he went out with a boat himself and on his return simply dumped his catch on the quayside. This "argument" convinced the obstinate fishermen and in a relatively short time there emerged a profitable fishery whose products could be exported. In 1936 during a visit to the US Johan Hjort foresaw comparable occurrence of shrimp off the coast of New England where the prevailing conditions were similar to those in the Norwegian fjords. He was able to prove this hypothesis during a trip aboard the research vessel Atlantis and with that the foundation stone was laid for the shrimp industry on the Atlantic coast of North America. Technical progress in ship building and fishing gear made it gradually possible to go further and further north where there are especially large numbers of P. borealis. More and more nations joined in this profitable business with the small but tasty shrimp.
Norway began fishing for Northern shrimp in the Barents Sea and off Spitsbergen in 1970, and in 1974 Russian ships followed. Canada, the EU, Greenland, Iceland and the Faroes are also entitled to fish shrimp in this area. In the mid-1980s more than 125,000 t Pandalus were caught there but today stocks are significantly weaker: the quota recommendations of ICES are only half that amount. In order to prevent overfishing the fishery is now closely regulated and monitored. Norwegian and Russian fishing vessels need special shrimp fishing licenses for the area, and the number of vessels and fishing days for fleets from other countries are strictly limited. The minimum mesh size of the nets is 35 mm (stretched). And because Northern shrimp are often found in the by-catch when fishing for other species the gear for many of these species has to be fitted with escape windows and sorting grids that allow shrimp and young fish escape.
The situation in the area off the east coast of North America and West Greenland is much more dramatic. This area provides approximately 85% of global catches of P. borealis. Canada’s shrimp fishery began in 1979 with 2,000 t, but the catch was then steadily increased to 182,868 t in the year 2007. To preserve resources, the fisheries authorities limited the total allowable catch to 164,000 t in 2008 but this quantity has since then never been fully exploited. Mostly the catches were just over 140,000 t. In 2013, the Atlantic States Marine Fisheries Commission decided that the P. borealis stocks were too weak and closed the fishery off New England completely. The first fishing ban in 35 years! And there was worse to come: in 2014 the Canadian fishing quota was reduced by a further 22% over the previous year and in 2015 there was even a moratorium in some fishing areas, so that fishing was not possible there at all. In Greenland the TAC, (total allowable catch) decreased by almost a third from 2010 to 2014. Biologists from Greenland’s Nature Institute recommended in 2015 that the catch be reduced by a further 25,000 t. These developments have shaken up the market for Northern shrimp in recent months. There are less raw materials available and the prices for several grades have increased by up to 50 per cent. Sometimes demand cannot be met at all. An end to this development is not yet in sight.
This is an unusual situation for the shrimp industry in the northern countries. From 1990 (226.033 t) to 2004 (446.909 t) the P. borealis catch rose steadily, with volume increasing annually by 6%. Since then, however, it has decreased again just as steadily, with catches dropping by 4% every year for more than ten years. In 2014 only 267,500 t were landed worldwide and preliminary estimates for 2015 predict 235,000 t. And forecasts are bleak because it is to be expected that catches will continue to fall... in spite of the fact that the Northern shrimp fisheries in Canada and Greenland, which account for the bulk of global volume, are well managed, continuously monitored, strictly controlled and sustainably operated. The MSC certificates awarded to these fisheries are visible evidence of these efforts.
Temperature rise threatens reproduction
So if the stocks are evidently not overfished, why then are less and less Northern shrimp being caught? The proliferation of cod in the waters in question, particularly off Iceland and south of West Greenland, could be an important factor. Fisheries scientists point out that Northern shrimp belong to the preferred food of cod. As the cod stocks gain strength this can therefore decimate the Northern shrimp stocks. The second factor, which is discussed again and again in this context, is the ongoing climate change, which is already becoming apparent in the Arctic seas through rising water temperatures. With this, the southern parts of the original habitat of Northern shrimp is lost and the cold stenothermal species is pushed further and further north. That such habitat shifts are not mere speculation can be seen in the declining catches off Cape Cod and in parts of the Gulf of Maine, which are already affected by these developments. If the temperature increase of 2°C as calculated by climate models is right and its effects become more noticeable in the future the distribution patterns of P. borealis in the Northwest Atlantic, which provides the bulk of the catches, will likely undergo massive changes. The stocks would decline because if more animals are crowded together in a smaller area, food quickly becomes a limiting factor.
Versatile uses for Northern shrimp
High-quality food and industrial raw material
The temperature increase might even affect the shrimp’s reproduction, as studies carried out by a group of researchers from the Bedford Institute of Oceanography in Dartmouth (Canada) show. In the past, the larvae of P. borealis hatched almost simultaneously with the spring algal bloom in their habitat. This meant that the newborn shrimp mostly found a rich food supply, which then secured them high survival rates. Whilst the time of algal blooms is primarily dependent on light supply, which does not change with climate change, the egg-laying and hatching of shrimp depends on temperature. In the course of evolution the two processes, algal blooms and hatching of larvae, have "synchronized" so strongly that their maxima are barely more than a week apart in many marine regions. This finely balanced interplay is now being disturbed by global climate change and temperature rise. And that increases the likelihood that at least some of the larvae will enter the world during phases with poor food supply which will weaken, and perhaps even endanger, the shrimp stocks.
It is difficult to say exactly what is responsible for the current decline in catches. Fisheries researchers avoid making clear statements because so far they cannot even be sure whether the shrimp stocks have actually already decreased or whether the animals have simply migrated from the traditional fishing areas. Either way, the result is the same and provides little cause for optimism. It rather seems as if there are hard times ahead for the Northern shrimp industry.