NOTARBARTOLO-DI-SCIARA: NATURAL HISTORY OF MOHri.A 



nate denticles. Large adult size, often 

 exceeding 3 m in width. Dorsal side 

 greenish brown. Ventral side anteriorly 

 white, posteriorly gray). 

 lb. Branchial filter plates separate 2 



2a. Spine on base of tail M. japanica 



(Tail very long with a line of white tuber- 

 cles on both sides. Spiracle a short 

 transversal slit, dorsal to plane of pec- 

 toral fins. Teeth not in contact with each 

 other, at least twice as high as the crown 

 is wide; crown subtriangular in apical 

 view. Medium-large adult size, approach- 

 ing 2.5 m in width. Dark blue to black on 

 dorsal side, apex of dorsal fin white; ven- 

 tral side white. Skin rough to the touch). 



2b. No spine on base of tail 3 



(Spiracle small, subcircular, ventral to 

 plane of pectoral fins. Skin smooth to the 

 touch). 



3a. Base of tail dorsally depressed 



M. thurstoni 



(Double curvature of anterior margin of 

 pectoral fin. Surface of tooth crown 

 rugose. Medium adult size, exceeding 1.8 

 m of width. Dark blue to black on dorsal 

 side, apex of dorsal fin white; ventral side 

 white, with a dark greenish patch near 

 the posterior margin of each pectoral fin, 

 and with a pattern of dark and shiny sil- 

 very pigmentation on distal half of pec- 

 torals). 



3b. Base of tail laterally compressed 



M. munkiana 



(Anterior margin of pectoral fin straight 

 to weakly convex. Surface of tooth crown 

 smooth. Small adult size, barely exceed- 

 ing 1 m of width. Dorsum mauve gray; 

 ventral side white, tips of pectoral fins 

 gray). 



Mobula thurstoni (Lloyd 1908) 

 Local name: cubana de lomo azul 



Eighty one males (disc width range 630-1,770 

 mm), 69 females (210-1,801 mm), and three spec- 

 imens of undetermined sex (941-1,494 mm), were 

 caught at four stations (Punta Arena de la Ven- 

 tana. Cueva de Leon, Ensenada de los Muertos, 

 and Isla El Pardito) and their adjacent waters, 

 between 7 February 1981 and 30 October 1984. 

 Overall and seasonal size-frequency distributions 



for M. thurstoni are given in Figure 2. Mean size 

 varied with season, smaller rays being dominant 

 in winter catches, medium sizes prevailing in the 

 summer. The difference between mean disc width 

 (DW) in winter and overall mean DW is highly 

 significant (T-value = 5.189, df=169, P 

 << 0.001). There were no significant differences 

 between male and female DWs (T-value 

 = 0.3767, df = 145, P > 0.5), with the exception of 

 the November to February period, when females 

 were larger ( T-value = 2.331, df = 12, P < 0.05). 

 A total of 105 specimens (210-1,770 mm DW) 

 were weighed. The WT/DW relationship is best 

 described by the equation: 



WT - 4.817 X 10-8 ^Y)^^)2.^8 

 r = 0.99 



WT is given in kg, DW in mm. The largest speci- 

 men in the sample was a female; DW was not 

 measured because the fishermen had already 

 started filleting the pectoral fins. Calculated DW, 

 regressed from disc length, cranial width, and 

 upper toothband length, was 1,801 mm (multiple 

 correlation coefficient = 0.99). The second largest 

 specimen was also a female, 1,799 mm DW. The 

 largest male had a DW of 1,770 mm and weighed 

 53 kg. The smallest freshly caught specimen was 

 876 mm DW and weighed 6.4 kg. The smallest 

 postnatal specimen was a male, the carcass of 

 which was found drying on the beach in Ensenada 

 de los Muertos. Its calculated DW, regressed from 

 toothbands length, was 864 mm (multiple correla- 

 tion coefficient = 0.99). 



Overall ratio of males to females caught was 

 1.18 {N = 148). Catch sex ratios varied with sea- 

 son. Females appeared to be dominant in winter 

 (ratio of males to females 0.27; A'^ = 14). The re- 

 verse was true in March, in favor of males. A 

 significant difference from a 1:1 ratio (chi square 

 testP > 0.05) was not noted. Geographical segre- 

 gation, either of sex or size, was not apparent for 

 M. thurstoni during the warmer months when a 

 wide array of size classes and both sexes were 

 found in the same fishing area. Males and fe- 

 males were occasionally harpooned from the same 

 group basking at the surface. This fact argues 

 against behavioral sex segregation. Winter data, 

 however, were suggestive of size segregation at 

 that time of year. It was common knowledge 

 among local fishermen that during the winter 

 months all M. thurstoni caught are small. The 

 bimodal size-frequency distribution for early 



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