Thus, male sharks are prepared to deliver large quantities of sperm to the female when chances of reproductive success are highest. In some sharks, the females can store sperm for years, another tactic for ensuring fertilization in the uncertain life of a migratory animal. The interplay of all of these factors enhances the success of fertilization and survival of the young. However, these strategies work against sharks when they are harvested by man. The high maternal investment in each embryo means a very low rate of reproduction.
Sharks propagate by two's and four's, dozens at most.
These small numbers are enough to keep up with natural mortality, but not large enough to keep pace with human predation. Little is known about the resilience of sharks under these circumstances. In some animal populations, fecundity of individuals increases when the populations size decreases. The evidence of collapsed fisheries for threshers, porbeagles, school sharks, dogfish, and other species indicates that sharks lack the resilience to adapt to intensive harvesting. Because of these reproductive strategies and the limitations and unknowns surrounding them, we must be conservative in harvesting these fragile shark populations.