Sought-after crustaceans from the deep waters of the Mediterranean
This article was featured in Eurofish Magazine 3 2022.
The deep-water rose shrimp (Parapenaeus longirostris), also known as pink shrimp or crevette rose, is one of the most economically important crustacean species for the fishing industry, particularly in the Mediterranean region. Because this species mainly occurs in relatively deep waters, comparatively little is known about the biology and way of life of these shrimp. This means there is an increased risk of overfishing, since market interest continues to be significant.
Shrimp and prawns have become an established and highly valued part of everyday diets in many countries around the world. Demand is growing and is largely being met by increasing aquaculture production. While landing quantities for crustacean fishing, which in addition to shrimp also includes lobster, crayfish, crab and swimming crab, is fairly stable at 6 million tonnes per year worldwide, the global aquaculture industry is currently producing around 10 million tonnes per year, including approximately 7 million tonnes of shrimp alone. However, the fishing industry very clearly differs from the aquaculture industry in the wider range of species caught. Shrimp production in aquaculture is quite clearly dominated by white shrimp (Litopenaeus vannamei), which is the most important shrimp at 5 million tonnes per year. The fishing industry, on the other hand, catches a much wider variety of shrimp species, ranging from the northern prawn (Pandalus borealis) to the Argentine red shrimp (Pleoticus muelleri). The deep-water rose shrimp (Parapenaeus longirostris) can also be classified as belonging to this varied assortment of species. Landings of these crustaceans in just the Mediterranean of approximately 20,000 tonnes per year may not be large, but they are still of economic significance. The actual catches may be significantly higher, because this species is not accurately recorded everywhere across its area of distribution. Depending on size (count) and season, these wild shrimp can fetch prices between 4.05 and 7.66 USD/kg on the Spanish market. In the Strait of Sicily, the passage in the Mediterranean between Sicily and the North African coast, Parapenaeus longirostris is the main target species for trawl net fishing, with an estimated annual market value of about 80 million euro. The economic, environmental and social interest in the sustainable management of this resource, which is shared by Italian, Tunisian and Maltese bottom trawlers, is correspondingly large.
Current stock estimates demonstrate a high fishing mortality rate as well as decreasing shrimp size in catches, which indicates possible overfishing. However, important requirements for developing a strategic plan for the sustainable management of this resource are still lacking. Despite its economic significance, our knowledge of the stock structures, the geographical distribution of stocks as well as the spawning and nursery areas in the Mediterranean is rather sparse. Fisheries scientists are working intensively on filling these gaps in our knowledge. The pressure to achieve reliable results as quickly as possible continues to increase, because the warming trend in Mediterranean waters is being reinforced by climate change and there is a risk that the deep-water rose shrimp, which is particularly sensitive to temperature, could also be affected. Because the Strait of Sicily is seen as an important biodiversity hotspot in the Mediterranean, this marine region was classified as a priority area for conservation as a precautionary measure as early as 2011, and in 2014 it was classified as an ecologically or biologically significant area (EBSA).
The main distribution area is most likely the Mediterranean
Deep-water rose shrimp are widely distributed across the North Atlantic. They occur in the East Atlantic from the Iberian peninsula, i.e. Spain and Portugal, to Angola and Namibia. In the West Atlantic, they extend from the USA (Massachusetts) to French Guyana. Although the name deep-water shrimp indicates a life lived at great depths, this species of shrimp prefers to live in areas nearer the coast, particularly where the shelf zones are not too wide and give way to the continental slope after only a few nautical miles. This pattern can also be seen in the Mediterranean, which is viewed as the centre of the distribution area for this species of crustaceans. These shrimp can be found almost everywhere from the coasts of Asia Minor to Spain, although the frequency of their occurrence varies significantly from region to region. In the Sea of Marmara, in the northern and central Adriatic, in the Gulf of Lion, the Alboran Sea, the Aegean, offshore from the Gaza Strip and off the French coast, this species is rather rare and is therefore only fished in limited quantities. In the Strait of Sicily and the marine areas around Greece, on the other hand, Parapenaeus longirostris is the most commonly occurring shrimp species almost everywhere. The deep-water rose shrimp is a benthopelagic (bottom-dwelling) shrimp species that prefers to live on muddy sand sea floors in the bathyal zone (bathyal, originating from the Greek bathys – deep, refers to the entire light-deprived area of the sea between 200 and 4,000 m depth). There they occur at depths between 20 and 700 m, but most commonly between 70 and 400 m. The species has a size-dependent bathymetric distribution, which basically means that the immature animals only live in shallower waters and as the animals get older they move to increasingly deeper areas. This is why mainly smaller specimens are caught on the continental shelf (50–200 m depth), while larger ones are found mainly on the upper continental slope between 500 and 700 m, with some even found at depths of up to 800 m.
The hidden lives of these crustaceans in relatively deep-water areas is the main reason that our knowledge of this species of shrimp is still very patchy. This is shown by the differing information available regarding the question of what the temperature preference of this subtropical (or tropical?) crustacean species is. Some authors state 8°C–15°C, others believe the preferred water temperature is more like 14 to 16°C. Some sources even refer to 24°C, however this is probably quite unlikely at the animals’ preferred depths of around 700 metres. Important details regarding the biology and way of life of these shrimp are unknown, because studies in their natural habitat would be very expensive and difficult. Much of what we know or suspect consists of assumptions and conclusions drawn from other penaeid shrimp species that are closely related to deep-water rose shrimp, or from specimens that rise to the surface of the sea at specific times of the day and therefore end up in nets at shallower depths. Similarly to some other deep-water species, rose shrimp move vertically between depths during the course of the day, i.e. they migrate to the layers of water close to the surface to feed there. In doing so, the crustaceans follow the swarms of marine plankton organisms, which can be concentrated so densely that they even reflect the sonar echos of fishing vessels (deep scattering layer). While the typical pattern for these diurnal migrations is to rise at night and return to the depths at dawn, there seem to be significant deviations from this rule for the rose shrimp in some regions. For example, off the southern Portuguese coast, daytime fishing has been observed to be significantly more productive (measured by catch per unit effort, CPUE) than night-time fishing trips, which indicates reversed vertical migration behaviour.
Feeding opportunist with a broadly diversified range of prey
The descriptive facts regarding this species of shrimp are of course uncontroversial and certain. The deep-water rose shrimp is a decapodic (ten-footed) crustacean that reaches a maximum total length of 16 to 18 cm (the females tend to be somewhat larger than the males). This makes the species approximately one third larger than the northern prawn Pandalus borealis, for which a maximum length of 12 cm is given. However, rose shrimp as large as this are really only caught by offshore trawlers in commercial deep-sea fishing. In regular fishing, which takes place near the surface, the landing quantities for the males are normally between 8 and 14 cm and for the females between 12 and 16 cm length. Since the size of crustaceans is rarely given as the total length of their body, but rather as the carapace length, these dimensions correspond to sizes between 16 and 42 mm. The pinkish-orange coloured body armour and red rostrum, which extends to the front from the head and body armour (carapace) as a pointed nose are typical of the species and provide the reason for its name. There is a long groove on the carapace beginning around the height of the eye stalks and extending back over the entire length of the carapace. At the final tail segment (telson), where the anus of the crustacean exits the carapace, three small, sharp and hard teeth are located.
The facts regarding the maximum age that shrimp can reach are disputed. For crustaceans that moult regularly, reliable age determination is notoriously difficult. Many authors date the normal life cycle at two years, but do not rule out the possibility that particularly large specimens can reach three years of age. New studies give reason to suspect that individual animals could perhaps even reach an age of four years.
It also seems certain that shrimp do not live individually and in isolation from each other as is frequently the case with deep-water species, but instead remain together sociably in larger groups. Deep-water rose shrimp feed on both benthic organisms such as mussels, bristle worms (Polychaete) and Foraminifera (these are single-celled, usually amoeboid protists with shells) that live on or in the sediment on the sea floor, as well as on zooplankton, primarily small fish and fish larvae, cephalopods and small crustaceans. Accordingly, for food intake a distinction can be made between the “digging phase” in the depths, where the shrimp root out small food particles from the mud on the sea floor, and a hunting phase in the upper layers of water, where the shrimp actively hunt their prey species. The limited stomach content analyses that have been done for this species of shrimp show that the shrimp probably prefer feeding on certain food organisms. Around three quarters of the stomachs studied contained Foraminifera, followed by Polychaete at almost 60% as well as molluscs at 53%. Other organisms such as radiolaria, sponges and small echinoderms were only represented with average frequencies below 10 percent, which indicates their lesser importance. The wide spectrum of prey animals consumed shows that Parapenaeus longirostris is a typical food opportunist that is not specialised in a particular prey, but will eat almost anything that seems exploitable and can be overpowered.
Early sexual maturity and high fertility
Due to the short benthopelagic life cycle of only two to three years, this species of shrimp becomes sexually mature at a relatively young age. In the Mediterranean, both sexes are already reproducing in their first year, once they have reached a carapace length of about 2.5 centimetres. The female ovaries change in colour with increasing maturity from white to dark green. Mating and fertilisation are preceded by a brief mating ritual in which both sexes synchronise with each other by means of olfactory and tactile signals. At the climax, an indirect semen transfer most likely occurs, in which the female independently takes the male’s spermatophores into her sexual opening. The fertilised eggs are simply expelled into open water, where they drift on the currents. Brood caring, as can be seen with other crustaceans that carry eggs under their body, is not usual for Penaeid shrimp. After hatching, the plankton larvae first go through the phases of development typical for all crustaceans (nauplius, zoea and mysis) before beginning their post-larval life near the sea floor on muddy sand sediment on the continental shelf. As far as is known, the larval phase lasts for about two months. The young animals initially remain near the sea floor at depths of 100 to 200 m. When mapping nursery areas in the Strait of Sicily, two nurseries with particularly high concentrations of young animals were discovered on the northern side of the strait. With a carapace length between 15 and 18 millimetres, these shrimp reach fishable status (“recruitment”).
Analyses of the stages of maturity have shown that deep-water rose shrimp do spawn asynchronously, but taken as a whole the stock is spawning continuously almost all year round. This means that not all animals in the population become mature at the same time, but there are always individual animals who are ready to reproduce. This results in a kind of continuum of spawning across the year. In some regions, reproduction has one to two peaks of activity, for example in the Strait of Sicily in January as well as in August and September. Off Tunisia, the high point of spawning activity is reached in June and July. The most important criterion for individual maturity is the maturity index (MI), which is calculated from the ovary weight in relation to the carapace length (CL). As for all free-spawning shrimp, fertility is comparatively high for this Penaeid species. Depending on the body size, approximately 20,000 eggs for first-time spawners and significantly more than 100,000 eggs for very old animals have been counted.
Precautionary approach to protecting stocks
The economic and socioeconomic importance of the deep-water rose shrimp is great. It is exploited by the fishing industry almost everywhere throughout its area of distribution, both as the object of targeted shrimp fishing or as a commercially valuable bycatch species. It is caught mainly with bottom trawl nets, with the catch sizes and landing quantities varying significantly depending on the catch area, catch depth and time of year. Deep-water fishing on the outer continental shelf and the upper continental slopes of the southern Mediterranean supplies a variety of fish, mollusc and crustacean species, including, among others, hakes, musky octopus and calamari, monkfish, red mullet, gilthead seabreams, John Dory and rays. Apart from the deep-water rose shrimp, crustaceans are also represented by the Norway lobster (Nephrops norvegicus), giant red shrimp (Aristaeomorpha foliacea) and blue and red shrimp (Aristeus antennatus) As regards biomass, the deep-water shrimp is the most important species of crustacean in many fishing areas of the Mediterranean, accounting regionally for approximately one quarter to one third of all crustacean landings according to the FAO (FishStat). The catches are brought to market both fresh and deep-frozen (e.g. under the brand name “Aegean Harvest” for goods frozen on board). Deep freezing is particularly common on deep-sea trawlers, where fishing trips last 20 to 30 days on average. For coastal fishing, where catches are landed regionally, deep-water shrimp are rarely sorted according to size, although larger sizes command better prices. The demand is so strong, however, that this work is not necessary, because customers will happily accept a smaller product.
Currently there are no formal management objectives for fishing P. longirostris either in the Strait of Sicily or in other regions of the Meditteranean. The management of this resource is based primarily on controlling catch capacities (number of fishing licences), fishing effort (days at sea, number of trawl nets) and some technical specifications (mesh opening in cod-end, area closures and size restrictions). According to the European Common Fisheries Policy, a reduction in fleet capacity has been targeted since 2000. For Italian trawlers that fish for deep-water shrimp in the Strait of Sicily, there is also a trawl net ban of 45 days per year between January and March. In Tunisia, the authorities can limit the number of boats in one region and impose fishing bans of up to three months. The legally prescribed minimum mesh opening for shrimp trawlers in Tunisian waters is 20 mm.
The EU has gone a step further in this regard. EC Regulation 1967 of 21 December 2006 specified a carapace length of 20 mm as the minimum landing size for deep-water rose shrimp for Italian and Maltese trawl net fishing. Square mesh openings in the cod-end of bottom trawl nets must measure at least 40 mm for EU fishing vessels. For diamond-shaped mesh openings, a mesh width of 50 mm is prescribed. To protect stocks, according to Article 8 of Regulation (EU) 2015/812 of the European Parliament and of the Council of 20 May 2015, the minimum marketing size was raised to 22 mm CL in some fishing regions outside the Mediterranean. Moreover, fishing technicians are trying to improve the size and species selectivity of trawl net fishing in the Mediterranean. Previous attempts to prevent unwanted bycatch with square mesh sheets positioned laterally in front of the cod-end have not been very effective, however.