FISHERY BULLETIN: VOL. 84, NO. 4 



Embryos collected in early May on the Outer Banks 

 and in the lower York River average 259 mm (range: 

 221-276 mm; n = 7), and those collected from Cape 

 Lookout, NC, in mid- April (Otwell fn. 4) average 264 

 mm (range: 222-281 mm; n = 11) (Fig. 2). By late 

 June and early July the embryos are full term (x 

 = 413 mm; n = 4). Parturition occurs at this time 

 and the first free-swimming young appear in pound 

 net catches. Embryo weight gain in spring is note- 

 worthy; three-quarter term embryos in April and 

 May average 310 g (range: 192-392 g; n = 16), while 

 the weight of full-term individuals in late June in- 

 creases fourfold averaging 1,291 g (range: 1,134- 

 1,409 g; n = 3). Schwartz (1967) reported that term 

 individuals average 305 mm DW, however, embryos 

 we considered full term are considerably larger (ca. 

 400 mm) and the smallest free-swimming ray we col- 

 lected was 323 mm. Perhaps, the embryos Schwartz 

 (1967) considered full term were taken in early June 

 and were not yet ready for parturition. 



Female rays ovulate following parturition. We 

 found encapsulated uterine eggs in specimens taken 



on 28 June and 21 July. In early August the embryos 

 are 20-30 mm wide and have lost the shell capsule. 

 By late August they average 125 mm (Figs. 2, 3). 

 When adult rays leave the Chesapeake Bay in late 

 September and early October, the embryos are 

 relatively large, up to 220 mm. 



Reproductive cycles of large elasmobranchs are 

 often difficult to describe because during certain 

 stages of pregnancy, individuals may be inaccessible 

 as a result of schooling and migratory behavior 

 (Holden 1974). Since cownose rays leave Chesapeake 

 Bay by November and do not return until May, we 

 could not determine precisely the length of gesta- 

 tion. Nevertheless, an 11-12 mo gestion period 

 seems most probable. Within this context, the rapid 

 embryonic growth observed in summer would slow 

 during winter. A slowdown or cessation of intra- 

 uterine growth would be expected if gravid females 

 experience high energy demands during an exten- 

 sive migration to distant wintering grounds, possibly 

 northern South America as suggested by Schwartz 

 (1965). Thus, the embryos from late summer and fall 



Figure 3.— Series of Rhinoptera bonasus embryos ranging from 18 to 140 mm disc width collected in late summer and fall. 



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