ICES mar. Sei. Symp., 199: 183-188. 1995

Geographical distribution of Aristaeomorpha foliacea (Crustacea: Aristaeidae) in the Sicilian Channel (Mediterranean Sea)

Sergio R agonese

Ragonese, S. 1995. Geographical distribution of Aristaeomorpha foliacea (Crustacea: Aristaeidae) in the Sicilian Channel (M editerranean Sea). - ICES mar. Sei. Symp., 199: 183-188.

The distribution of the red giant shrimp (Aristaeomorpha foliacea) in the Sicilian Channel (Central M editerranean Sea) is described and geographical differences sug­gested by the available data set are qualitatively analysed. The unit stock assumption does not seem an unrealistic hypothesis, considering the state of knowledge.

Sergio Ragonese: lstituto di Tecnologia della Pescaedel Pescato, Via Vaccara 61, 91026 Mazara, Italy [tel: (+39) 923 948 723, fax: (+39) 923 906634],

Introduction

The deepwater shrimps Aristeus antennatus (Risso, 1816) and Aristaeomorpha foliacea (Risso, 1827) consti­tute an important resource for the Mediterranean Sea fisheries. In the Sicilian Channel (sensu lato), both the red giant {A. foliacea) and the blue-red (A. antennatus) shrimp are actively sought by local and foreign trawlers and are often caught together. Exploratory surveys indi­cate that the red giant shrimp outnumbers the blue-red by 13:1 in the Sicilian Channel, just the opposite of the relationship of these two species elsewhere in the west­ern Mediterranean (Sardà, 1986).

Table 1. Location, depth (m) and source of the geographical zones in Figure 1.

UM A zonesDepth

range (m) Source

A1 West 450-700 Maurin and Di Meglio, 1961B1 Cap Blanc 500-1000 Ghidalia and Bourgois, 1961B2 Pantelleria 550-750 Ghidalia and Bourgois, 1972C Pantelleria 400-750 Bombace and Sardà, 1972D West Egardi 500-650 Arena and Li Greci, 1973E l West Egardi 540-630 Arena, 1985E2 South Malta 540-630 Arena, 1985E3 Capo Rossello 540-630 Arena, 1985FI Northwest

(NW) 410-668 Ragonese, 1989F2 Far South (FS) 401-817 Ragonese, 1989F3 Southeast (SE) 393-796 Ragonese, 1989G Capo Passe ro > 4 0 0 Local fishermenH “Triangle” > 3 5 0 Local fishermen

In spite of its importance, little information is avail­able on the red giant shrimp trawled in the Sicilian Channel. This article provides the first distributional map of the local population(s) of red giant shrimp, plus a preliminary evaluation of the “unit stock” hypothesis.

M ethods

The area of the central M editerranean Sea between the Sicilian and North African coasts (the Sicilian Channel sensu lato) is considered a “Unitary Management A rea” (UM A; Fig. 1) for demersal resources (Ragonese, 1989). The 1000 m isobaths encompass the northwest (Capo S. Vito, Sicily-Cap Ferrat, Algeria) and the southeast (Capo M urro di Porco, Sicily-Cape Misratah, Libya) limits (Fig. 1).

Information was gathered from the local fishermen, from the available literature on “UM A ” zones, and from two years of seasonally stratified random trawl surveys (eight trawls; I and IV: spring 1985 and 1986; II and VI: summer 1985 and 1986; III and VII: autumn 1985 and 1986; and IV and VIII: winter 1986 and 1987) carried out by the I.T .P .P. of Mazara on the Italian side of the “U M A ” . The commercial trawler, “S. A nna” , with commercial trawl gear lined with a codend w:th mesh size of 18 mm on a side was used (see Levi (1990) for details).

Average yields (kg/h), sex-ratio (F/F + M), spawning probability (percentage of “ready to spawn” females) and length-frequency distribution (females only; obli­que carapace; 1 mm below) for trawl seasons and for

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34

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Figure 1. Geographical distribution of zones of Aristaeomorpha foliacea in the proposed “Unitary Management A rea” (see Table 1).

three specific zones were derived and their shapes com­pared qualitatively. Overall, the valid hauls (1 hour long) were 30, 69 and 64 in the “northwest” (NW; FI in Fig. 1), “far south” (FS; F2 in Fig. 1) and “southeast” (SE; F3 in Fig. 1) zones, respectively. The numbers of hauls for each survey were 5-3-3-4-3-4-4-4, 8-10-8-9-10- 8-7-9 and 7-7-7-8-7-9-10-9 for the “northwest” , “far south” and “ southeast” zones, respectively.

Results and discussion

The red giant shrimp probably migrates vertically (it is almost exclusively trawled during daylight hours), and

seems not to be limited to one specific type of soft bottom. For example, it is often captured along with the burrowing Nephrops norvegicus. The red giant also behaves like an opportunistic predator that can survive independently of the benthic facies (e.g., the reduction in numbers of the anthozoan Isidella elongata caused by trawling did not have any appreciable consequence). However, the red giant shrimp supposedly prefers the tem perature of the so-called “eastern water” (13.5°C), an intermediate stratum of warm and salty water which originates in the eastern M editerranean Sea and flows towards the Strait of Gibraltar.

The red giant shrimp is distributed throughout the

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Aristaeomorpha foliacea in the Sicilian Channel 187

188 S. Ragonese ICES mar. Sei. Symp., 199 (1995)

UM A over a wide range of depths (300-1000 m), but shows a marked preference for (or a higher vulnerability to trawling on) muddy bottoms at depths between 500 and 750 m (Table 1), especially where narrow de­pressions or canyons notch the seabed. The lower bathy­metric limit for this species is probably deeper than 1000 m, but trawling at these depths is technically diffi­cult and unprofitable because the small number of shrimp captured at those depths is not adequately com­pensated by the increase in individual size.

The bathymetric map suggests two main distributional zones for this species in the Strait of Sicily (west and east sides, Fig. 1). Between them there is a narrow passage with sufficient depth (200-400 m) to allow adult shrimp to move from one side to the other. In addition, trawl catches of this species suggest that the east zone could be further divided into a “far south” (FS) and a “ southeast” (SE) zone, partially separated by two deep depressions (each deeper than 1000 m) known as Pantelleria and Malta (Fig. 1).

Differences between the west and east shrimp (those in the west are larger) have been noted by other authors, but it has been difficult to support an hypothesis of “more than one stock” because of the limited time series of comparable data.

The yields and sex-ratio plots (Fig. 2a, b) show no specific pattern and a high level of “interaction” . The only important feature is the sharp and recurrent spring peak relative to the northwest (NW) area. In contrast, the three zones have similar summer peaks in repro­ductive pattern (Fig. 2c).

As far as the length-frequency distributions are con­cerned (Fig. 3), the data show general seasonal varia­bility between and within zones, a relative scarcity of adult females (CL > 40 mm) in the SE, a greater similar­ity in the adult female component of the NW and FS, and a virtual non-occurrence of spring recruitment in the NW.

According to local fishermen, who fish with a trawl codend mesh size of about 18 mm on a side, large catches of juveniles occur in wintertime, at least over the wes­ternmost seabed on the west side, but the available data do not permit verification.

The red giant shrimp of the Sicilian Channel has a wide geographic and bathymetric distribution, but it is impossible to state from the current data whether these shrimps comprise a single stock or two stocks (west and

east UM A) with limited interchange. The unit stock hypothesis is sufficiently realistic to permit preliminary stock assessment and management of this important resource, but the problem of stock identity should not be ignored in the process of developing a management strategy.

A ckn ow led gem ents

The authors thank Dr D. E. Aiken for his helpful assist­ance in revising the text, and colleagues D r D. Patti and D r F. Bertolino for improving the quality of the graphs.

R eferences

Arena, P. 1985. Studio sulla possibilité di razionalizzare e rendere più produttiva la pesca a strascico nel Canale di Sicilia e nel M editerraneo Centro-M eridionale. ESPI, Palermo, Italy (mimeo). 214 pp.

Arena, P ., and Li Greci, F. 1973. Indagini sulle condizioni faunistiche e sui rendimenti di pesca dei fondali batiali della Sicilia occidentale e della bordura settentrionale dei banchi della soglia Siculo-Tunisina. Quad. Lab. Teen. Pesca, anno 4°, 1(5): 157-201.

Bombace, G ., and Sardà, R. 1972. La pesca a strascico sui fondali da -5 0 0 a —700 metri nel settore a Sud-Est di Pantel­leria. Ministero Marina Mercantile, memoria no. 33. 77 pp.

Ghidalia, W ., and Bourgois, F. 1961. Influence of tem perature and light on the distribution of shrimps in medium and great depths. Stud. gen. Fish. Coun. M edit., 16. 49 pp.

Levi, D. 1990. Ministero Marina Mercantile - Consiglio Nazio- nale delle Ricerche. Sessione valutazione delle risorse demersali. Relazione sull’attività svolta dall’U .O . 1st. Tec- nologia della Pesca e del Pescato - Mazara. N .T .R .-I.T .P .P ., 15. 170 pp.

Levi, D ., and Vacchi, M. 1988. Macroscopic scale for simple and rapid determination of sexual maturity in Aristaeom or­pha foliacea (Risso, 1826) (Decapoda: Penaeidae). J. Crust. Biol., 8: 532-538.

Maurin, C ., and Di Meglio, S. 1961. Campagne du navire océanographique “Président-Théodore-Tissier” dans le sec­teur Atlantique Ibero Marocain et sur les côtes sud de la M éditerranée occidentale. Rapp. Comm. int. M er M édit., 16(2): 269-273.

Ragonese, S. 1989. First stock assessment of Aristaeomorpha foliacea (Crustacea: Aristaeidae) in the Sicilian Channel. Nova Thalassia, 10 (suppl. 1): 457-465.

Sardà, F. 1986. D ata of Aristeus antennatus (Risso, 1816), distribution and abundance in the W estern Mediterranean Sea. Some biological aspects. Rapp. Comm. int. Mer Médit., 30(3): 118-20.