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Fishery Bulletin 100(1) 



or planktonic blooms, which in turn would require a higher 

 oxygen demand in the area, subsequently provoking drops 

 in oxygen and causing mortahties of high-oxygen-demand- 

 ing invertebrates. In any event, these results suggest that 

 fluctuations in oxygen levels might be a key element in 

 regulating portunid populations at Ensenada de La Vela 

 and merit further research efforts. 



Berried females were remarkably scarce during our 

 study. Only two, both belonging to C danae, were caught 

 throughout the first period at the estuarine site, and none 

 were caught during day and night samplings in the surf 

 zone nor offshore. Nonetheless, scarcity of ovigerous fe- 

 males of swimming crabs in these coasts is not exceptional. 

 During a 2-year survey of crustaceans along 700 km of Fal- 

 con's shoreline, Carmona-Suarez and Conde (19961 caught 

 very few berried females of several of the littoral portunid 

 species. They caught only one berried female of A. cribrari- 

 us and no ovigerous females of C. sapidus, C. larvatiis, C. or- 

 natus, or C. danae. However, in estuarine areas, substantial 

 numbers of berried females of C. bocourti. C. maracaihoen- 

 sis, and C. exasperatus were caught in the tidal zone. The 

 scarcity or sheer absence of egg-bearing females in some 

 species of swimming crabs might be the result of habitat 

 partitioning by sex. Differential distributions by sex have 

 been reported for C. sapidus (Williams, 1965; Perry, 1975; 

 Archambault et al, 1990), C. maracaiboensis (Norse, 1977), 

 and C. bocourti (Taissoun, 1969; Norse, 1978). However, for 

 the dominant species in the surf, A. cribrarius. ovigerous 

 females do not seem to be segregated into deeper waters. In 

 southern Brazil, ovigerous females of this species appeared 

 in shallow waters close to the coast (Pinheiro et al., 1996). 

 Similarly, many ovigerous females were collected in very 

 shallow water, at the sui-fs edge in North Carolina (Wil- 

 liams, 1984). Likewise, for C. bocourti and C. maracaiboen- 

 sis egg-bearing females have also been reported in marine 

 shallow waters (Norse, 1977). Furthermore, adult females 

 of most species inhabiting the surf zone at Ensenada de 

 La Vela were observed in this area year-round. Thus, alter- 

 native explanations should be considered, such as lack of 

 estuarine habitats or sustained harsh environmental con- 

 ditions that do not allow energy to be invested in reproduc- 

 tion. For instance, a highly seasonal reproductive pattern, 

 with periods without berried females, has been observed in 

 populations of the mangrove tree crab, Aratus pisonii, liv- 

 ing in hypersaline lagoons in this area (Conde, 1989); this 

 pattern contrasts with the pattern for populations inhabit- 

 ing other localities, where these crabs reproduce continu- 

 ously throughout the year (Conde and Diaz, 1989a; Diaz 

 and Conde, 1989). Also, undergi-own or stunted specimens 

 of various species of crustaceans have been reported in this 

 area (Conde and Diaz 1989b, 1992a, 1992b; Carmona, 1992; 

 Carmona-Suarez and Conde, 1996). Thus, it is possible that 

 this arid coast lacks the necessary resources for these crabs 

 to reproduce, except in a few estuarine spots. This hypothe- 

 sis is also supported by the fact that the body size of several 

 species of swimming crabs collected in our study was small- 

 er than that reported in other locations (Fischer, 1978; Wil- 

 liams, 1984). 



The only river near the Ensenada de La Vela is the Coro 

 River, which lies approximately 2 km westwards. Because 



of current direction (east-west), it cannot influence estua- 

 rine conditions to the sampled area. The small estuary in 

 the Ensenada de la Vela could be a possible local nutrient 

 supplier But its influence is restricted to a few days dur- 

 ing the end of each year, when the estuary opens to the 

 sea. The setup of an untreated sewage discharge in the 

 small estuarine basin at the beginning of 1997 could in 

 fact have a long-term impact, but it is possible that vari- 

 ous species of swimming crabs may not be affected nega- 

 tively, because of their capacity to live in polluted areas. 

 Such is the case with C. bocourti (Taissoun, 1972; Wil- 

 liams, 1974), and C, sapidus, the main species in the Lake 

 Maracaibo crab industry (Oesterling and Petrocci, 1995), 

 where contamination due to several sources (i.e. sewage 

 and oil) has reached high levels (Rodriguez, 2000). 



Because trawl studies have shown greater abundances 

 of blue crabs (C. sapidus) and, in general, other decapods 

 at night (Wilson et al., 1990), we ran a series of day and 

 night samplings at the marine front over a six-month pe- 

 riod and later also undertook diel offshore pot sampling on 

 several occasions. Although Fischer ( 1978) has stated that 

 A. cribrarius burrows into the bottom during the day and 

 emerges at night, we collected A. cribrarius in the same 

 range of abundances and sizes during both day and night 

 samplings. Similar results were achieved by DeLancey 

 ( 1989) in South Carolina, where no significant differences 

 were obtained from samples collected at day and night. 

 Wilson et al. ( 1990) ascribed the lack of differences in day 

 and night abundances of C. sapidus to the use of more 

 effective sampling devices than previously employed. In 

 our study, no major differences were obsei'ved in diversity, 

 abundance, body size, or sex ratios for most species, even 

 though two kinds of collecting gears were used; thus, it 

 is feasible to consider that if daily cycles exist in the spe- 

 cies, they do not have a significant impact on daily density 

 variations. In turn, these findings may have practical con- 

 sequences for the decisions regarding sampling schemes 

 to assess fisheries in this area. 



The exploitation of swimming crabs at Ensenada de La 

 Vela must be considered only at the artisanal level be- 

 cause of the low abundance of all species treated in our 

 work and their wide and random density fluctuations. 

 In fact, local fisheries are currently limited to the arti- 

 sanal capture of portunids by hanging nets or hand-driven 

 trawling nets. The most captured species by fishermen is 

 C. bocourti (senior author, personal obs.), but C. danae is 

 also a promising staple to be hai-vested because it appears 

 in all three major biotopes (marine inshore, offshore, and 

 estuarine). Arenaeus cribrarius, a species commercially 

 exploited in Brazil (Pinheiro and Franzoso, 1998) and re- 

 garded to have an excellent fiavor (Fischer, 1978), may al- 

 so be considered a target species because of its great abun- 

 dance, although its small size may make it less desirable 

 commerciallv. 



Acknowledgments 



We heartily thank Sebastian Trompiz, whose involvement 

 in most of the phases of the project was instrumental to 



