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Fishery Bulletin 92|2). 1994 



of food items was determined. Following Perez and 

 Aurioles-Gamboa 2 , we recorded some of the phyto 

 and zooplankton components in four major groups: 

 1) phytoplankton, 2) zooplankton, 3) particulate or- 

 ganic matter (POM), and 4) inorganic matter (small 

 grains of sand, clay or mud). The number of 

 diatoms, crustacean parts, foraminifers, and other 

 components, such as small agglomerations of POM, 

 were counted and their numbers converted to rela- 

 tive frequency. 



Proximate analyses was based on 200 g 8 and 

 200 g 9 of sun dried and milled pelagic red crabs 

 (about 4 kgs of fresh crabs). The techniques used were 

 those of the Association of Official Analytical Chem- 

 ists (A.O.A.O, 1984): moisture (7.007), ash (7.009), 

 crude fiber (7.006), crude protein (2.057), ether ex- 

 tract (7.060), carbohydrates (by difference from all 

 other determinations at 100%). This methodology had 

 been used previously to analyze pelagic red crabs 

 sampled from the benthos of the continental shelf 

 (Castro, 1993). Two-sample f-tests (Zar, 1984) were 

 conducted to identify differences in food composition, 

 proximate composition, and mean SCL between 

 stranded pelagic red crabs in Bahia Magdalena and 

 breeding pelagic red crabs collected on the continen- 

 tal shelf in March 1990. Achi-square test for deter- 

 mining a possible deviation of sex ratio was applied 

 for the stranded pelagic red crabs (Zar, 1984). 



Results 



The stranded crabs formed a long brilliant red line 

 of several kilometers on the interior coast of the 

 northern part of Bahia Magdalena (Fig. 1). In addi- 

 tion, there were two lines of dried crabs separated 

 by a few meters, higher on the beach and stranded 

 during the previous days. 



Behavior of pelagic red crabs 

 during stranding 



Two hours of observations on a surface swarm about 

 12 m long and 1 m wide were conducted after 1100 

 hours (11 May 1991) during the receding tide. The 

 swarm was propelled to and from the beach by the 

 waves and was unable to move offshore. When the 

 swarm was pushed toward the beach by the wave 

 action, some animals were thrown onto the sand and 

 exposed as the water receded. 



During sampling on the beach, the pelagic red 

 crabs moved their legs and actively used their chelae 

 as pincers. This behavior was typical of pelagic red 

 crabs caught in trawls from the continental shelf. 



Pelagic red crabs had been caught from the shelf in 

 all seasons, but only during mid-summer (when the 

 bottom-surface temperature difference is as great as 

 17°C) did the crabs show signs of damage as they 

 moved slowly and died rapidly on the deck (Aurioles- 

 Gamboa, unpubl. data). In contrast, crabs were very 

 active in the first minutes after stranding, and moved 

 less frequently later. The crabs were brilliant red, 

 which differentiated them from the lighter color of crabs 

 collected on the continental shelf. Based on their vigor- 

 ous activity, the pelagic red crabs stranded on 

 Magdalena Island (Fig. 1 ) appeared to be in good health. 



Stomach contents of stranded 

 pelagic red crabs 



The total number of items and relative frequency of 

 the four major groups found in the stomachs is shown 

 in Figure 2A. For comparison, the results of a typi- 

 cal sample taken on the continental shelf is provided 

 in Figure 2B (Perez, 1992). The minimum number of 

 items counted was 457 and the maximum about 2,266. 

 This range was greater than that found in crabs from 

 the continental shelf (841-1,495 items). However, 

 there was no significant difference in the mean num- 

 ber of items (x=l,027 versus 1,100) between stom- 

 achs from the stranded crabs and those from the shelf 

 (two sample *-test, P>0.05, df=9; r=-0.3082). 



Food composition (particulate organic matter, zoop- 

 lankton, and phytoplankton) was not different be- 

 tween crabs from the two regions. Inorganic matter 

 (grains of sand, clay, etc.) was more abundant in the 

 stomachs of the pelagic red crabs collected from the 

 shelf (x=365 versus 60; two sample ^-test, P<0.05, 

 df=9; ^=-0.0046). This difference, however, does not 

 account for a significant change in the feeding hab- 

 its of crabs from the two samples. 



Proximate composition of stranded red crabs 



In stranded pelagic red crabs, the sexes were not dif- 

 ferent in protein and crude fiber (Table 1), however 

 they differed significantly in lipids and ash contents 

 (two sample t-test, P<0.05, df=9; *=1.870 and 10.012 

 respectively). Females had higher lipid and lower ash 

 content than males, both by about 1.5%. 



There were significant differences when the chemi- 

 cal composition of stranded crabs (sexes combined), 

 were compared to crabs from the continental shelf 

 (Table 1 ). Crabs from both areas were similar in their 

 protein content, but differed in lipids and ash. The 

 lipid content in Bahia Magdalena crabs was almost 

 nine times higher than that in shelf crabs U-test, 

 P<0.05, df=16, r=35.664). Crabs from the continen- 

 tal shelf were higher in fiber and ash but lower in 

 carbohydrate content. 



