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An introduction to Mediterranean deep-sea biology* - page 19 / 32





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(Sardà and Martín, 1986; Demestre and Lleonart, 1993; Bianchini and Ragonese, 1994; Carbonell et al., 1999). This species is a characteristic compo- nent of the demersal muddy bottom community on the middle slope at depths between 400 and 800 m (Cartes and Sardà, 1993), where Cartes and Sardà (1992) and Maynou and Cartes (2000) have defined it as a nektobenthic species of moderate to high swimming mobility. However, this species is also fished frequently between 400 and 800 m and in other Mediterranean areas (Bianchini and Regonese, 1994; Carbonell et al., 1999; Papaconstantinou and Kapiris, 2001; Cau et al., 2002). The distribution of this species is nonetheless considerably broader, reaching depths of at least 3300 m (Sardà, 2003), indicating that the species is eurybathic with a dis- tribution that is considerably broader than that of other decapod crustacean species.

The spatio-temporal behavioural pattern of A. antennatus is well-known, with the species forming seasonal aggregations on the middle slope at depths between 400 and 900 m. These aggregations form between late winter and early summer (Tobar and Sardà, 1987; Demestre and Martín, 1993; Sardà et al., 1994). Towards the end of summer the shrimp shoals tend to break up and move inside submarine canyons, with the shrimp being fished at shallower depths along the margins of the canyons (Sardà, 1993; Sardà et al., 1994; Sardà et al., 1997).

Studies carried out on the catchability of shoals of this species (Sardà and Maynou, 1998) have sug- gested that the shoals take on an elongate shape par- allel to the coast between early spring and summer. This is when the shrimp stock bears the brunt of the fishing effort (Tudela et al., 2003; Sardà et al., 2003a), because shoal formation is at its peak on the part of the slope most readily accessible to trawlers, and females attain maximum size, that is, the bio- mass concentration is also at its peak. In addition, the marketability of this species is also highest at this time (Sardà et al., 2003b).

In the last decade a great number of studies have focused on the estimation of the level of exploitation of A. antennatus populations. Furthermore, different management options have been studied, taking into account biological and fishing aspects, to offer the most appropriate and sustainable fishery (Demestre, 1990; Demestre, 1993; Demestre and Martín, 1993; Carbonell and Alvarez, 1995; Carbonell et al., 1999). Despite fluctuations in landings over the annual series, a clear seasonal periodicity is detect- ed, with increased landings in late winter, spring and

early summer. Also, inter-annual fluctuations of around 8 years have been detected (Tobar and Sardà, 1987; Carbonell and Acevedo, 2003). The general trend of all the results shows a situation of the stocks not being overexploited yet, but with a tendency to go far from the optimum exploitation.

The fact that A. antennatus appears to be under- exploited or near the optimum levels on the Spanish coast, in contrast with the majority of demersal resources, is due to at least two characteristics of this species. Firstly, the whole stock is not available for fishing since only a certain proportion of the stock is accessible to commercial fishing (Demestre and Martín, 1993; Sardà et al., 2003b), and the species has an extremely wide distribution in bathyal waters (Sardà et al., 1997). Secondly, the turnover rate is high. Most demersal resources, par- ticularly fish, have a low turnover rate that makes them “easy victims” to overfishing (Demestre and Lleonart, 1993).

On the lower slope, between 1000 and 3300 m depth, the density is lower, there is no fishing activ- ity (virgin grounds), the sex proportions are not sig- nificantly different from 1:1 (Sardà and Cartes, 1993; Sardà et al., 2003b; Sardà et al., 2004), and no information is available on seasonal population movements at these depths. The study of the life his- tory of Aristeus antennatus presents an interesting scenario: the fishing grounds are mainly occupied by female, large-sized individuals showing a high density, while the virgin grounds are occupied by smaller-sized individuals showing a low density. However, Aristeus antennatus does not seem to be overexploited in its fishing grounds (Demestre and Lleonart, 1993; Tursi et al., 1996; Martínez-Baños, 1997; García-Rodríguez and Esteban, 1999). Thus, considering the present state of knowledge of A. antennatus, the questions addressed are about the relationship and interaction between exploited and virgin populations. The grounds below 1000 m depth are beyond the reach of fishing boats and we can assume that the shrimp populations are virgin at these depths. These virgin populations have lower densities and a higher proportion of males than the exploited populations (Sardà et al., 2003a,b).

As a matter of fact, considering the multi-species nature of the Mediterranean fisheries, deep-water shrimps are caught between 400 and 800 m together with many other species considered by-catch, some of which, such as Norway lobster (Nephrops norvegicus), hake (Merluccius merluccius), anger- fish (Lophius spp.) or conger (Conger conger) have


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