Sharks. Love them or hate them, they’re an important part of our planetary ecosystem. As you probably know, many shark species are in trouble, due to overfishing, habitat destruction, climate change, and other factors. As scientists work to restore shark populations, they stress the importance of understanding how shark biology work. Let’s take a look.
Population Growth
When looking at any species as a whole, population growth depends on a number of factors, including reproduction, life span, and number of offspring each member of the species can be expected to produce. These factors can also help scientists determine how rapidly a certain population can recover after a catastrophic event, and they can help fisheries officials make regulations that will positively impact populations as a whole.
Comparing Fish, Sharks, and Mammals
Species mature, reproduce, and live out their natural lifespans at variable rates. When it comes to bony fish, even those capable of growing to large sizes, these animals tend to grow fairly quickly, reaching sexual maturity at a relatively young age. They have the capability of reproducing often, and each fish is capable of producing a high number of offspring. Swordfish – some of the biggest bony fishes in the sea – reach maturity by the age of five, and once mature, are able to spawn many times during just a single year. During their lifetimes, female swordfish produce millions of eggs, many of which grow to maturity themselves.
Mammals grow at variable rates, usually determined by their size. The smaller an animal, the faster it tends to reproduce. Mice, rabbits, cats, dogs, and other small-sized mammals reproduce quickly, and it is easy for their populations to explode when not kept in check. Large mammals, such as giraffes, bears, and domesticated animals including horses and cattle grow to maturity at slower rates, as do marine mammals like whales and dolphins. In addition, these larger mammals are capable of bearing fewer offspring, and their gestation periods are much longer than those of smaller mammals. For example, manatees carry their young for 13 months before delivery, and walrus pregnancies last an average of 15 to 16 months. Orca pregnancies last about 17 months, and sperm whales carry their young for as long as 19 months – more than twice the amount of time the average human pregnancy takes. In case you’re wondering, elephants hold the record for the longest pregnancies among mammals, with gestation taking approximately two years.
Despite the fact that sharks are fish, their growth rates and reproduction rates are more similar to those experienced by mammals, and like large mammals, they take a long time to mature, and they have relatively few offspring. For example, dusky shark females mature at age 21. Once pregnant, their gestation period lasts between 22 and 24 months, after which they give birth to a litter of three to fourteen pups that might or might not make it through the first year of life, as they swim off on their own once born. Adult females take approximately one year off between pregnancies, and may take longer to mate again, depending on the availability of a suitable mate. Due to its slow reproductive rate, the Dusky Shark is extremely vulnerable. In addition, its vulnerability is increased due to its popularity with commercial and recreational fishermen; in the United States, the population has dropped an estimated 15 to 20 percent from levels recorded in the 1970’s.
In each of these examples, animal populations are affected by life-history characteristics that impact the overall population’s capacity for growth. A shark producing ten pups every two years for a length of twenty years would add approximately 100 individual sharks to the overall population, while a swordfish could potentially add millions of new members to its species, with thousands of these surviving the pressure caused by disease, starvation, predation, and other environmental factors. This comparison illustrates the importance of managing shark populations like large mammal populations, rather than treating them like other fish. With careful management, shark populations may finally rebound, contributing to a healthier ecosystem all species will ultimately benefit from.