A pilot experiment of tagging the deep shrimp Aristeus antennatus (Risso, 1816)*

The blue and red shrimp, Aristeus antennatus (Risso, 1816), which is generally fished by trawling at depths between 300 and 700 m, is one of the most important demersal resources in the Mediterranean. A remarkable quantity of work has been devoted to describing its fishery and biology (Orsi Relini and Relini, 1979; Sardà et al., 1987; Cartes and Sardà, 1989; Matarrese et al., 1992; Mura et al., 1992; Spedicato et al., 1995). However, some basic aspects remain obscure, such as the age of exploited shrimps and the movement of this species during its lifespan, i.e. as larvae, juveniles and adults. The distribution of larvae (Heldt, 1955) is practically unknown. Juveniles appear with different relative densities on various fishing grounds in the Mediterranean: they seem most abundant at lower latitudes (Sardà and Demestre 1987; Mura et al., 1997; D’Onghia et al., 1997) while the largest, and probably the oldest, shrimps have been found in northern areas (Orsi Relini and Relini, 1979, 1998). Several fishing patterns suggest that shrimp may be capable of both vertical and horizontal displacement: 1) there is a unitary yield change with a daily periodicity, with a maximum in daylight conditions and minimum at night, suggesting a bentho-pelagic behaviour (Relini G., 1981; Bianchini et al., 1998); 2) in some canyons fishing is productive in restricted areas, where several trawlers work for long periods (Orsi Relini et al., 1986). The fact that the resource appears to be “resilient” means that shrimps come day after day to that particular fishing ground; 3) seasonal patterns of distribution of the trawling fleet and/or of the resource have been described (Sardà et al., 1997); 4) In areas where the study of red shrimps SCI. MAR., 64 (3): 357-361 SCIENTIA MARINA 2000


INTRODUCTION
The blue and red shrimp, Aristeus antennatus (Risso, 1816), which is generally fished by trawling at depths between 300 and 700 m, is one of the most important demersal resources in the Mediterranean. A remarkable quantity of work has been devoted to describing its fishery and biology (Orsi Relini and Relini, 1979;Sardà et al., 1987;Cartes and Sardà, 1989;Matarrese et al., 1992;Mura et al., 1992;Spedicato et al., 1995). However, some basic aspects remain obscure, such as the age of exploited shrimps and the movement of this species during its lifespan, i.e. as larvae, juveniles and adults. The distribution of larvae (Heldt, 1955) is practically unknown. Juveniles appear with different relative densities on various fishing grounds in the Mediter-ranean: they seem most abundant at lower latitudes (Sardà and Demestre 1987;Mura et al., 1997;D'Onghia et al., 1997) while the largest, and probably the oldest, shrimps have been found in northern areas (Orsi Relini andRelini, 1979, 1998). Several fishing patterns suggest that shrimp may be capable of both vertical and horizontal displacement: 1) there is a unitary yield change with a daily periodicity, with a maximum in daylight conditions and minimum at night, suggesting a bentho-pelagic behaviour (Relini G., 1981;Bianchini et al., 1998); 2) in some canyons fishing is productive in restricted areas, where several trawlers work for long periods (Orsi Relini et al., 1986). The fact that the resource appears to be "resilient" means that shrimps come day after day to that particular fishing ground; 3) seasonal patterns of distribution of the trawling fleet and/or of the resource have been described (Sardà et al., 1997); 4) In areas where the study of red shrimps has lasted for several years sudden events of appearance and/or disappearance have been recorded (Relini and Orsi Relini, 1987;Orsi Relini and Relini, 1988). Tagging techniques have been extensively used in the study of several important stocks of coastal peneids, especially in Australia (Dall et al., 1990). These experimental approaches have revealed unsuspected migrations totaling 1000 Km or more (Montgomery, 1981). During such tagging experiments basic information about the growth and maturation of migrating shrimps has been collected. Similar studies have never been extended to species living in deep waters. As the application of tagging techniques to the study of red shrimps could be a means of obtaining direct information on displacement and growth, we have developed a simple method to tag and release A. antennatus. The first recovery of a tagged specimen in the Ionian Sea has prompted us to publish information about this experiment to underline the possibility of catching live specimens which can survive after the release back to deep waters; our aim is also to promote the extension of such tagging studies.

MATERIAL AND METHODS
The tagging experiment was carried out in the Ionian Sea, along the Calabrian coast, offshore from Roccella Jonica (RC) (Fig. 1). This area was chosen because vertical displacements of A. antennatus occur in the Roccella canyon during night hours (Matarrese et al., 1995). Thus, A. antennatus was caught by trawling at only 100-150 m in depth. This small depth of catch reduced the traumatic effects of hauling on the survival of the specimens. The impact of trawling was also diminished by tows of short duration and by the fact that the tow was carried out at night when smaller thermic and light changes occur from water to air.
The sampling of A. antennatus specimens was conducted on 22 May 1998. A commercial vessel of 75 tons gross tonnage, with 360 Hp engine, was hired for two trawling operations. It was equipped with a nylon otter-trawl net with 40 mm stretched mesh (20 mm side) in the cod-end. The vessel speed, measured using GPS, was maintained at 2.5-2.8 knots. Two tows were carried out during the night in shelf waters close to the canyon: times, depth and geographical positions are reported in Table 1.
The catch was rapidly sorted and all the live specimens of A. antennatus were put in a tank containing cooled sea water (approximately 13°C). After half an hour in the tank, all swimming individuals were measured, sexed and tagged. The tag was a yellow streamer 95x4 mm, narrower in the middle (20x2 mm), placed through the pleon as shown in Figure 2. Each tagged specimen was replaced in the tank and left from half an hour to two hours to allow the vessel to reach the release site. This time lag was also useful to evaluate vitality after tagging. The tank was covered with a dark 358 M. RELINI et al.  screen to reduce stress due to the lights of the vessel. Each tagged specimen was released from a segment of PVC tube (40 cm length and 10 cm in diameter) full of water, closed and ballasted at the lower end (Fig. 3). The tube was closed at the bottom using plastic or paper sheets, in order to investigate the durability and hence the ease of release using different materials. The cylinder was gently released overboard with the ballasted end down, so that it descended to the bottom in a vertical position and then lay down on it, allowing the shrimp to swim out if the basal plug was still in place. The total number of specimens tagged and released during the two hauls is reported in Table 2.

RESULTS AND DISCUSSION
The present pilot experiment involved only 45 shrimps. In commercial trawl catches the majority of shrimps come on board damaged or dead. For tagging purposes the duration of the trawl was reduced, but one-hour trawling is probably still too long as the dead shrimps outnumbered the others. Live specimens, when put in a provisional aquarium on board, seemed to survive well in this artificial environment, at least for the time necessary for the tagging and release operations: only 3 out of 45 specimens died.
One month after tagging, the commercial vessel "Lucia madre" recaptured one specimen (104 tag number) on 26 June 1998 at 20:00, during a haul carried out at a depth of 506 m. During its month of free life, the tagged specimen, a female, covered at least 10 nautical miles, moving north-east and to greater depths (Fig. 1 It bore a spermatophore as at the moment of tagging and its carapace length was unchanged: therefore apparently no moult had occurred. This report can be considered important from the technical point of view. Its main significance is to show the feasibility of tagging a deep-living species of Decapoda Natantia. Undoubtedly these species are more delicate than strictly benthic forms of Brachyuran Decapods, to which large-scale tagging has already been applied successfully (Melville-Smith, 1987). This first recapture also gives interesting indications about the displacement of the shrimp both in vertical and horizontal space, giving confirmation of the above-mentioned bentho-pelagic behaviour inferred from daily fluctuations in fishing yields and movements into and out of canyons, deduced both by persistent yields in limited areas such as in the Ligurian Sea and by seasonal variation of the productive fishing grounds such as along the Catalan coast (Sardà et al., 1997).
A. antennatus has been monitored from a manned submarine (Campillo, 1994) and the shrimps were observed hovering on the sea bed at 1-2 m from the bottom. Their escape reaction involved moving up in the water column. These facts indicate strong swimming abilities that are probably favoured by a lighter body than in coastal species. In the latter the great distances recorded by tagging experiments have suggested speeds of up to 7 Km/day, which are unlikely to be achieved by walking and have therefore been assigned to swimming (Dall et al., 1990).
It is well known that, even when few individuals are recaptured, tagging can give very important information both in terms of general knowledge and for management purposes. Decapoda Reptantia which were thought to be linked to the bottom like the spiny lobster Palinurus elephas or strictly benthic fish such as the angler Lophius piscatorius, showed unsuspected displacements which sometimes crossed hypothetical stock borders (Relini and Torchia, 1998;Pereda and Landa, 1997).
At present it is impossible to predict the recovery rate from a large-scale experiment on red shrimps, but perhaps even on the basis of the limited number of recoveries typical of migratory species, some basic questions about their life history (moult rate, growth per moult, displacements) could be answered.