SardA et al.: Spatio-temporal structure of a population of Aristeus antennatus 



603 



the deeper region between 1,000 and 2,200 m con- 

 sisted of a higher proportion of juveniles and males 

 (Cartes and Sarda, 1992; Sarda and Cartes, 1993b; 

 Cartes, in press). This contrasts with the high propor- 

 tion of females collected at depths of less than 1,000 m. 

 The present results support the hypothesis that 

 two different populations of A. antennatus persist in 

 the western Mediterranean. One population is 

 unexploited and stable throughout the year at depths 

 below 1,000 m and is characterized by lower densi- 

 ties and higher percentages of males and juveniles. 



The other population is commercially exploited and 

 variable over the year at shallower depths between 

 400 and 1,000 m. It is characterized by high abun- 

 dance and by seasonal variations in total number, 

 sex ratio, and depth distribution. 



The A. antennatus stock, though subjected to con- 

 siderable fishing pressure, has remained at near 

 optimum equilibrium levels (Demestre and Lleonart, 

 1993). Sarda (1993) and Sarda and Cartes (1993a) 

 attributed this equilibrium to the presence of 

 unexploited biomass at depths below 1,000 m, which 

 annually renews the exploited portion 

 of the stock. 



Though sampling was localized and 

 did not cover extensive areas of each 

 habitat, the samples from the US and 

 MS nonetheless reflected specific 

 spatio-temporal patterns in population 

 structure: females completely domi- 

 nated the population all year, forming 

 aggregations on the MS in spring and 

 summer. Earlier studies on fishing 

 patterns during the year (Figure 5A, 

 revised after Tobar and Sarda, 1987) 

 have referred to this population migra- 

 tion pattern. Changes in the coefficient 

 of variation of catch rate illustrated in 

 Figure 5B reflect a scattering of the 

 shoals in spring and summer (from 

 April to August) and a more highly 

 aggregated stock structure in autumn 

 and winter. Figure 6 summarizes our 

 conclusions regarding the distribution 

 and movement of A. antennatus. 



The increase in number of females 

 on the MS in spring and summer co- 

 incides with the period of gonadal rip- 

 ening and fertilization (Relini Orsi and 

 Relini, 1979; Arrobas and Ribeiro- 

 Cascalho, 1987; Sarda and Demestre, 

 1987; Demestre and Fortuno, 1992) 

 and shortening of the male rostrum 

 (Sarda and Demestre, 1989). These 

 authors also reported that shoals dis- 

 perse after spawning, which occurs 

 mainly from June to September. This 

 might suggest a specific mating area in 

 the MS. After September, shrimps 

 spread out over the slope and subma- 

 rine canyons, leading to a decrease in 

 density and increase in the proportion 

 of males on the slope. Processes linked 

 to the transfer of energy through the 

 slope and submarine canyon systems 

 (Reyss, 1971, 1973; or Rowe, 1971; 



