Little is known about the growth and age of elasmobranchs. Many of the conventional methods for aging animals, such as examining teeth, will not work with elasmobranchs.
Sharks grow slowly compared to bony fishes, possibly due to sharks' slow digestive time and feeding rates. There is considerable variation in age and growth rates between species and even between populations of the same species.
Growth rings are periodically deposited on the vertebrae of some sharks. Vertebrae can be stained and examined for these growth rings. Growth rings may stop developing in older sharks.
Examining the vertebrae of captive-born sharks after their death enables researchers to compare the number of growth rings with the shark's known age.
In some areas, tagged sharks are providing information about growth rates. Once a shark is caught, it is measured, tagged, and released. The shark is measured again when it is recaptured. Researchers correlate the measurements with the number of years since recapture and calculate a yearly growth rate.
Depending on the species, sharks and batoids have several predators, including other sharks, elephant seals, and killer whales.
Sharks are vulnerable to overfishing. Because sharks are slow-growing and a single female produces only a few hundred pups or less in a lifetime, depleted populations may take years or decades to recover.
Recreational and commercial shark harvesting has increased in the past several years due to an increased demand for sharks and shark products.
In some cultures, the meat of the milk shark (Rhizoprionodon acutus) is believed to promote lactation in human females.
Shark skin can be abrasive, so it is used as a sandpaper (called shagreen) in some countries. If the denticles are removed, leather products of the finest quality can be manufactured.
Teeth of the Greenland shark (Somniosus microcephalus) are used by natives to cut hair and the skin is placed on the outside of their boats for added strength.
Hawaiian villagers used shark teeth to create exotic tools ranging from cutting awls to weapons of war. These tiger shark teeth are still valued as jewelry.
Tiger shark vertebrae are crushed into the powder used by geisha girls. Tiger shark teeth are used for jewelry, with a complete jaw worth approximately $200 in places like Tahiti.
Medicinally, parts of sharks are used for a variety of reasons including hemorrhoid treatments, vitamin supplements, artificial skin to treat burn patients, acne medication, anti-clotting blood compounds, and human cornea transplants.
A substance called squalamine, found in the liver, stomach, and gall bladder of some dogfish, is believed to slow the growth of human brain tumors. Shark cartilage is also being tested for treatments against the growth of tumor cells.
No part of the shark, however, is as economically valuable as their fins. Particularly wasteful, shark finning is the practice where the fins are removed from the shark and the rest of the shark is simply discarded. Shark fins can be worth up to $25 per pound, compared to shark meat which is valued at only $0.50 to $7 per pound. The fins are used primarily to make shark fin soup, a delicacy in Asia. A bowl of shark fin soup can cost as much as $150 a bowl.
Each year, thousands of sharks are taken unintentionally in nets set out to catch other types of fishes.
It's estimated that at least 100 million sharks are killed by humans every year.