Most sharks have an unusual combination of biological characteristics: slow growth and delayed maturation; long reproductive cycles; low fecundity; and long life spans. These factors determine the low reproductive potential of many shark species.
Slow growth and delayed maturation: Some species of sharks, including some of the commercially important species, are extremely slow growing. The picked dogfish (Squalus acanthias) has been estimated by Jones and Geen (1977) to reach maturity at about 25 years. The sandbar shark (Carcharhinus plumbeus), the most economically important species along the southeastern coast of the United States, has been estimated to reach maturity from 15-16 years (Sminkey and Musick 1995) to about 30 years (Casey and Natanson 1992).
Long reproductive cycles: Sharks produce young that hatch or are born fully developed, and that are relatively large at hatching or birth. The energy requirements of producing large, fully developed young result in great energy demands on the female, and in reproductive cycles and gestation periods that are unusually long for fishes. Both the reproductive cycle and the gestation period usually last one or two years in most species of sharks, reflecting the time it takes a female to store enough energy to produce large eggs and to nurture her large young through development (Castro 1996). The reproductive cycle is how often the shark reproduces, and it is usually one or two years long. The gestation period is the time of embryonic development from fertilization to birth, and is frequently one or two years long. The reproductive cycle and the gestation period may run concurrently or consecutively. For example, in the picked dogfish, the reproductive cycle and gestation run concurrently and both last two years. A female carries both developing oocytes in the ovary and developing embryos in the uteri concurrently for two years. Shortly after parturition, it mates and ovulates again, and the process begins anew. In this case, both ovulation and parturition are biennial. In most of the large, commercially important, carcharhinid sharks, the reproductive cycle and the gestation period run consecutively. These sharks have biennial reproductive cycles (Clark and von Schmidt 1965) with one-year gestation cycles. They accumulate the energy reserves necessary to produce large eggs for about a year, then, mate, ovulate, gestate for one year, and give birth. For example, after giving birth in the spring, a blacktip shark (Carcharhinus limbatus) enters a "resting" stage where it stores energy and nourishes its large oocytes for one year. After mating and ovulation, it begins a year gestation period, giving birth in the spring of the second year after its previous parturition (Castro 1996). Thus, these sharks also reproduce biennially. Some of the hammerhead sharks (Sphyrna) and the sharpnose sharks (Rhizoprionodon) reproduce annually (Castro 1989, Castro and Wourms 1993). Even longer cycles of three and four years have been proposed for other species without adducing any evidence.
Low fecundity: The small size of their broods, or "litters", is another factor contributing to the low reproductive potential of sharks. The number of young or "pups" per brood usually ranges from two to a dozen, although some species may produce dozens of young per brood. Most of the commercially important carcharhinid sharks usually produce less than a dozen young per brood. For example, the sandbar shark averages 8 young per brood, while the blacktip averages 4 per brood (Castro 1996). An exception, among the targeted species, is the blue shark for which broods of over 30 young have often been reported.
Long life spans: Although, many species of sharks are known to be long-lived (Pratt and Casey 1990), the reproductive life span of sharks is unknown. Because the long time before maturation and the long reproductive cycles, it appears that a given female may only produce a few broods in its lifetime (Sminkey and Musick 1995).
Many of the commercially important species use shallow coastal waters, known as "nurseries", to give birth to their young, and where the young spend their first months or years (Castro 1993b). The mating grounds are often close to the nurseries, and thus adults of both sexes congregate close to shore in large numbers. These areas are highly attractive to fishermen, because of their nearness to shore and the high concentration of sharks. Most of the commercially important species (e.g. the genera Carcharhinus, Sphyrna, Rhizoprionodon, Negaprion) have shallow water nurseries (Castro 1987, 1993b). These sharks are very vulnerable to modern fishing operations, and are easily overfished.
There is no evidence of any compensatory mechanisms by female sharks that will increase brood size or decrease the length of the ovarian and gestation cycles in response to overfishing. It is highly unlikely that those mechanisms can be evolved rapidly enough to compensate for the increase in mortality. Even if such mechanisms could be evolved, brood size would be limited by the maximum number of young that can be carried by a female, and ovulatory and gestation cycles are limited by complex metabolic processes. The long ovarian cycles and long gestation periods probably reflect he minimal times required by the species to acquire and transfer the necessary energy to large ova and young.