Biology and fishery of the arched box crab Calappa convexa de Saussure (Crustacea, Brachyura, Calappidae) in the southeastern Gulf of California, Mexico

The arched box crab (Calappa convexa) is commonly caught in gillnets as by- catch of the spiny lobster (Panulirus spp.) fishery in southern Sinaloa, Me xico, a locally exploited fishing resource. Crabs were sampled monthly from May 1998 to April 1999 and a total of 274 specimens (132 mates and 142 fem ales) were captured. The distribution of the resource was homogeneous (pres ent in 72% of the samples) throughout the sampling area. The overall sex ra tio (female:male) was 1:0.92 (chi (2) > 0.05). The capture per unit effort (CPUE) varied from 0.81 g/100m(2)/h to 9.10 g/100m(2)/h. Maximum catch was obtained in June white minimum catches occurred in December and March. The CPUE values differed significantly (ANOVA P < 0.05) among the months of sam pling. Crabs were caught on sand, rock and boulders, with 43% of the CPUE c orresponding to rocky substrate. Mean size, based on carapace length (CL), was 72.5 +/- 5.8 mm for males and 74.5 +/- 7.2 mm for females. Individual s ize frequency distribution was unimodal and normal in males and unimodal an d not normal in females, with the largest number occurring in spring (107) and winter (73). Reproduction occurred throughout the year, with peaks from April to August. Females reached sexual maturity at an estimated size of 7 5 mm (CL-50) and carried an average of 638,885 eggs. The mean size differed significantly between females (75.7 mm CL, SD = 7.3 mm) and males (72.8 mm CL, SD = 5.8 mm) (P < 0.05). The modal classes displayed a slight seasonal displacement (spring to winter) towards larger sizes. Analysis of the cara pace width:length relationship shows variation during male and female devel opment, specifically, in larger sizes (> 75 mm CL), females were narrower w hile the opposite occurred in males. Left and right chelae reached a combin ed fresh weight of 100 g, in males of about 75.0 mm CL, while in females th is weight was reached at 79.0 mm CL.