PRATT REPRODUCTION IN BLUE SHARKS 



breeding season for the blue shark on the conti- 

 nental shelf off southern New England. The 

 length-frequency histogram of these inseminated 

 females approximates a curve of normal distribu- 

 tion with a peak at 175 cm (Figure 25). The 

 phenomenon of carrying spermatozoa seems to 

 separate an age-class from the combined length 

 frequency of the female population. If this is an 

 age-class, then Stevens' (1975) age curve for the 

 blue shark indicates that the inseminated females 

 are 5 yr olds and the uninseminated fish are 

 primarily fours and fives. Since most of my sam- 

 ples were taken in June and July, it is possible 

 that the unfertilized 5 yr olds would be insemi- 

 nated later in the season. Since all nongi-avid 

 females on the continental shelf bear tooth cuts 

 and many have vaginal scars, it would appear that 

 only females >4 yr have organ systems developed 

 enough to retain spermatozoa. The growth curve 

 ofovarianeggsin inseminated females (Figure 25) 

 shows the eggs as half-mature in 5 yr olds. Five- 

 yr-old inseminated females that I sampled as late 

 as October in continental shelf waters off southern 

 New England do not contain mature ova or em- 

 bryos. Due to immature egg size and the lack of 

 developing embryos, 1 conclude that this age class 

 is not ready to bear young during the summer of 

 insemination. If fertilization occurred during the 

 winter, the 9-mo gestation proposed by Suda 

 ( 1953) and Aasen ( 1966) would produce full-term 

 embryos in gravid females through the following 

 summer and into the fall. This is not the case. 

 Full-term females occur most frequently during 

 the spring and early summer (Figure 20). 



The age-6 female length-frequency mode (190 

 cm; Stevens 1975) is conspicuously absent from 

 the shelf waters in the summer months (Figure 

 27), while males of this size are numerous. Backus 

 (see footnote 7) caught two females offshore that 

 could be 6 or 7 yr old by Stevens' (1975) criteria 

 (197.4 and 209.5 cm); each carried 11.0 cm em- 

 bryos in September and October, respectively. I 

 examined one gravid female in July with two em- 

 bryos 1 1 and 13 mm. The uterine eggs were other- 

 wise undeveloped. Typically, gi^avid females in 

 this population are of lengths indicating 7 yr of age 

 and older. 



Based on these findings, the sexual cycle of the 

 female blue shark in the western North Atlantic 

 would start as 4- and 5-yr-old fish arrive on the 

 feeding/mating grounds of the continental shelf in 

 late May and early June. Here they interact with 

 males receiving dermal punctures and lacerations 



(tooth cuts). The 5-yr-old females and some 4 yr 

 olds, copulate with the males of 180 cm and larger 

 judging from the size of the tooth interspace 

 reflected in bite marks and male sexual maturity. 

 This process is known to continue as late as 

 November and may continue year round in Baha- 

 mian waters (Stephen Connett*). The followmg 

 spring, the 6-yr-old females remain offshore and 

 fertilize their eggs (May, June). Embryos reach 

 full term in 9-12 mo. Puppingisfrom April to July. 

 At this time the female is 7 yr old. This is the 

 probable trend for most female blue sharks. There 

 are many exceptional bits of data such as reliable 

 reports of small (165 cm) gravid females (Suda 

 1953; Tucker and Newnham 1957) and embryo 

 sizes that depart from the trend, especially in the 

 eastern North Atlantic ( Figure 20). These are to be 

 expected in a wide ranging, abundant species with 

 a long breeding season. A small number of females 

 in the inshore population have very advanced or- 

 gans and egg development for their length. It is 

 possible that these precocious individuals bear 

 young a year earlier than their siblings or shift 

 completely out of phase by bearing young at ran- 

 dom seasons. 



Stray gravid females occur regularly in south- 

 ern New England shelf waters. Their diminuitive 

 numbers are an insignificant part of the spawning 

 population. Too little is known of the early life 

 history and feeding habits of the blue shark to 

 determine whether the young would fare better in 

 the rich waters of the continental shelf or offshore 

 along the margins of the Gulf Stream. 



Sex Ratio 



Suda ( 1953) reported the blue shark sex ratio at 

 birth to be 1:1. Data from a population of 2,174 

 males and females sampled at Bay Shore, Long 

 Island, from 1965 to 1972 is presented in Figure 27. 

 In this sample immature females consistantly 

 outnumber the males until a fork length of 150 cm 

 is reached because unlike females, the males only 

 move inshore when the sex organs start to mature. 

 The sex ratio then becomes equal in the subadult 

 sizes when a large number of mating wounds and 

 inseminated oviducal glands are prevalent. In the 

 adult size group ( 180-250 cm) the sex ratio shifts 

 rapidly to a preponderance of males. The inflexion 

 point at 180 cm coincides with the size at which 



'Stephen Connett. instructor, summer oceanography program 

 St. George's School, Newport. R.I., pers. commun. April 1977. 



467 



