FISHKKY BULI.KTIN: VOL 86, NO 1 



(e.g., when swimming towards the wall of the 

 pen) was to dive vertically in a tight circle until 

 swimming in the opposite direction in an inverted 

 position, and then spinning around the body axis 

 to brmg the dorsal side up, rather than turning by 

 banking to the right or to the left. 



An indication of the seasonal abundance of 

 M. thiirstoni in the surface waters of the study 

 area was obtained by the mean number of rays 

 caught daily from March to July 1983 (Fig. 6; 

 Table 3). Mean daily catch should be taken as a 

 rough indication of the relative abundance of M. 

 thurstom rather than as a precise index because 

 the fishing effort was difficult to quantify. Mean 

 effort, however, was roughly constant from March 

 through July because the mean monthly number 

 of working boats (about 20) and the number and 

 size of the nets set then was constant. Further- 

 more, the fishermen would harpoon a ray every 

 time they had the opportunity to do so. Peak of 

 abundance was in June, a result which appears to 

 be consistent with the fishermen's past experi- 

 ence, despite the 1983 abnormally high water 

 temperatures (Cane 1983). In July the number of 

 M. thurstom caught had dropped drastically, and 

 most of the catch consisted of M. japanica . No 

 information was obtainable for the August-Sep- 

 tember period. Eighteen specimens were cap- 

 tured during six fall and winter field trips (24 

 January-8 February 1981; 25 November 1981; 

 16-21 December 1981; 20-23 December 1982; 

 19-26 January 1984; 28 October-1 November 

 1984), but that figure was not comparable with 

 other data because part of the fishing cooperative 

 migrated south to Los Frailes during the cooler 

 months. It is common knowledge, however, 

 among the local fishermen, that M. thurstom in 

 the colder season is present, but in fewer numbers 

 than during the summer. 



The study area constitutes a feeding, mating 

 and nursery ground for M. thurstoni. The eu- 

 phausiid Nyctiphanes simplex , the main diet item 

 and the only food of the adults when in the area, 

 is the most abundant and widespread euphausiid 

 in the Gulf of California, and has been observed in 

 dense swarms (Brinton and Townsend 1980). Al- 

 though it is found in the study area year-round, 

 its juvenile and adult stages are most abundant 

 between February and August, peaking in June 

 on the west side of the Gulf of California (Brinton 

 and Townsend 1980). The seasonal abundance of 

 M. thurstoni in the southern Gulf thus seems to be 

 closely related to the seasonal abundance of its 

 main prey. It is impossible to describe the general 



Table 3— Mean number of daily captures of Mobula thurstom 

 a - total monthly number of captures; b monthily number of days 

 of monitonng; X = mean number of daily captures; SE = standard 

 error of tfie mean; so = standard deviation. 



movement and life history pattern of M. thurstom 

 in the Gulf of California from the fragmentary 

 information available. The scanty data, however, 

 suggest the following: 1 ) adult male and nonpreg- 

 nant adult female M. thurstoni enter the area in 

 spring to feed and to mate, 2) pregnant females 

 segregate from the rest of the population in 

 spring (as is also suggested by the slight predom- 

 inance of males in spring and early summer), 

 3) gestation period is one year and females give 

 birth to one young every two or more years, 4) the 

 young are born in the study area or near it in 

 midsummer and remain there throughout their 

 early life, and 5) in late summer, when the num- 

 bers of adult and juvenile Nyctiphanes simplex 

 decline due to intense heating of the water (Brin- 

 ton and Townsend 1980), adult M. thurstom leave 

 the area, whereas the young switch their diet 

 from euphausiids to mysids. Further investiga- 

 tions are needed for additional corroboration of 

 these hypotheses. 



Mobula japanica 



(MuUer and Henle 1841) 



Local name: cubana de lomo bianco 



A total of 78 specimens, 34 males (DW range 

 1,316-2,386 mm) and 44 females (1,470-2,302 

 mm), were caught at three stations (Punta Arena 

 de la Ventana, Cueva de Leon, and Ensenada de 

 los Muertos) and adjacent waters, between 16 De- 

 cember 1981 and 13 July 1983. Overall and sea- 

 sonal size-frequency distributions for M. japanica 

 are shown in Figure 7. With the exception of 

 April, when females were larger (T -value 

 = 4.697, df = 3, P < 0.02), there are no significant 

 size differences between the sexes (T -value 

 - 0.535, df= 76, P > 0.5). Most of the rays in the 

 sample were large; only three were < 1,900 mm 

 DW. 



Twenty-seven specimens (size range: 1,316- 

 2,285 mm DW; 18.6-115 kg) were weighed. The 

 WT/DW relationship is described by the equation: 



56 



