Mollet et a\: Reproductive biology of Isurus oxynnchus 



315 



tive gi-ow-out studies indicated that the nurse shark 

 Ginglymostoma cirratinn has a 2-year reproductive 

 cycle with a 3.5-4.5 months gestation and a -20 

 months resting period.-'^ A 3-yr reproductive cycle 

 was proposed for the dusky shark and the tiger 

 shark with 22-mo. and 15-16 mo. gestation periods, 

 respectively (Musicke et al., 1993; Crow*^). Francis 

 and Stevens (2000) reported that gestation in the 

 porbeagle lasts 9 months, which would allow for only 

 a very short resting period if they have pups every 

 year. Francis (1996) suggested that the great white 

 shark might have little or no resting period because 

 mating was observed immediately after parturition. 



In the 3-year cycle of the shortfin mako, mating 

 could occur either after parturition, which would 

 require sperm storage, or after the resting period 

 and before ovulation. Two recently fertilized shortfin 

 makes from South Africa caught in March and June 

 had fresh mating scars, suggesting mating takes 

 place in late summer-fall before ovulation. It may be 

 selectively advantageous for mating to occur in sum- 

 mer-fall prior to ovulation when the females appear 

 to be in a different geographical area, rather than 

 after parturition in spring of the previous year near 

 the pupping grounds, because both sexes may be 

 feeding actively at that time. 



There was an uneven distribution of reproductive 

 stages in our shortfin mako data. Pre-ovulating and 

 early to mid-term pregnant females were poorly rep- 

 resented. Casey and Kohler (1992) suggested that 

 adult females remain far offshore in more tropical 

 waters. We propose that this suggestion applies par- 

 ticularly to pre-ovulating females ready to be mated 

 and early to midterm females. Because the shortfin 

 mako prefers 18°C water (Casey and Kohler, 1992), 

 these females are likely to be found in deeper water 

 and are less likely to be caught. 



Regional differences 



The studies of Garrick (1967) and Heist et al. (1996) 

 have suggested that there is only one circumglobal 

 species of shortfin mako, Isurus oxyrinchus. Despite 

 this conclusion, we found that there were significant 

 differences in the median TL-at-maturity of females 

 and in their mass-length relationship between the 

 Northern and Southern Hemispheres. Small regional 

 differences in size at maturity have been reported 

 for a number of other elasmobranch species, includ- 

 ing widely distributed species such as the sandbar 

 shark (Last and Stevens. 1994), scalloped hammer- 



2" Carrier, J. C, and H. L. Pratt. 1998. Albion College, 470.5 Kel- 

 logg Ctr., Albion, MI 49224. Unpubl. data. 



head shark, Sphyrna lewini (Branstetter, 1987; Chen 

 etal., 1990), soupfin shark (Peres and Vooren, 1991), 

 and dusky shark, Carcharhinus obscurus (Natanson 

 and Kohler, 1996). Our estimates of the duration of 

 gestation and reproductive cycle are unlikely to be 

 greatly affected by these differences. 



Determining reproductive parameters for the large, 

 wide-ranging and oceanic shortfin mako proved dif- 

 ficult. A dedicated sampling program conducted 

 throughout the year within a particular region may 

 still not provide the necessary data. We believe 

 our approach of combining scarce data from widely 

 spaced localities was justified. 



Acknowledgments 



The data collected by Abner Kingman as an observer 

 on a swordfish longliner proved to be very useful. 

 We are most grateful to Malcolm P. Francis and Ken 

 J. Goldman for many suggestions and for data on 

 porbeagles and salmon sharks. We thank Fabio E. 

 S. Costa and Alberto F. Amorim for litter data from 

 Brazil, David W. Welch for help with the statistical 

 analysis, Nancy E. Kohler for providing HSI data, 

 Dave B. Holts for the California drift gillnet fishery 

 data, and Gregor M. Cailliet for reviewing the man- 

 uscript. A large number of people provided data 

 and useful suggestions, and we acknowledge Shelton 

 P. Applegate, Steven Branstetter, Barry D. Bruce, 

 Jose I. Castro, Tris Colket; Leonard J. V. Compagno; 

 Gerald L. Crow; David A. Ebert, Ian K. Fergusson, R. 

 Grant Gilmore and Oliver Putz, William C. Hamlett, 

 Glen Hodson, Leanne Laughlin, Robert N. Lea, Jean 

 R. C. de Marignac, Sanford A. and Barbara Moss, 

 John A. Musick, Gavin J. P. Naylor, Giuseppe Notar- 

 bartolo di Sciara, Tsuguo Otake, Daniel Pauly, Julian 

 G. Pepperell. John E. Randall, Peter Saul, Jeffrey 

 A. Seigel, Colin A. Simpfendorfer, Antonio D. Testi, 

 and Senzo Uchida. GC wishes to thank his colleagues 

 at the Natal Sharks Board for assistance in provid- 

 ing and examining specimens. HFM wishes to thank 

 Cherilyn Chin, Juan E. EzcuiTa, Scott R. Greenwald, 

 John B. O'Sullivan, and Gilbert Van Dykhuizen at 

 the Monterey Bay Aquarium for help and support. 

 Anne L. Mollet proofread the manuscript. Comments 

 by four anonymous and two NMFS reviewers helped 

 us to focus and strengthen our findings. 



Literature cited 



Applegate, S. P. 



1966. A possible record-sized bonito shark. Isurus oxyrin- 

 chus Rafinesque, from Southern Cahfornia. Calif. Fish 

 Game 52:204-207. 



