Sibling Rivalries Of The Grey Nurse

By Chris Doyle on August 3, 2012 in Other

Anyone with a sibling will be familiar with the fights that occur over the last potato chip in a bid for some extra nourishment.

While this often causes a fuss for parents, spare a thought for the grey nurse shark, where competition between siblings over nutrition is a matter of life and death.

Female grey nurse sharks do not have a placental connection to their developing young like we humans do. All the nourishment for the developing pups (as baby sharks are called) comes from the eggs from which they hatch and from the lining of the mother’s uterus. But this may not always supply the nutrition necessary for growth, so grey nurse shark pups have found an additional source of nutrients to help them grow – their siblings.

A pregnant grey nurse shark can have as many as 40 shark pups inside her two uteri (yes, that is the plural for uterus) but she will only ever give birth to two pups at most. Once the fastest growing pups develop their teeth and jaws, at around 10cm in length, they begin devouring their less-developed siblings for a little extra nutrition. Eventually, only two pups remain – one in each uterus. After a pregnancy lasting up to 12 months, the pups are born at around one metre in length.

Although it has a fearsome appearance, the grey nurse shark is a relatively placid animal and is not considered a danger to humans. It is the most widely kept shark in public aquariums, owing to its tolerance for captivity. They grow to over three-and-a-half metres in length and come together in aggregations when feeding and to mate. They are slow swimmers and are usually observed hovering motionless just above the seabed, or in rocky caves and overhangs.

Grey nurse sharks have one of the lowest reproductive rates of any species of shark in the world and this has made them very susceptible to fishing activities. The NSW government declared the grey nurse shark a protected species in 1984, making it the first species of shark in the world to be given formal protection status. This was in response to reports of a decline in the numbers of grey nurse sharks along the east coast, thought to be the result of spearfishing, accidental catches in bather-protection nets and commercial fishing.

Despite being protected, threats to the grey nurse shark still continue. Ongoing accidental catches by fishermen and bather-protection nets are still placing the grey nurse shark population under pressure. Many grey nurse sharks carry injuries caused by fishing gear, and it is not uncommon to see sharks swimming around with lines or hooks still attached. Even more worrying is that some sharks may have internal injuries from fishing hooks, which can cause serious disease and potentially death.

The Eastern Beaches is home to a nursery area for grey nurse sharks. Magic Point, south of Maroubra, is mostly populated by female and juvenile sharks, so it is thought that grey nurse sharks come to this site to breed. The area is considered to be crucial to the survival of the grey nurse shark population on the east coast. Not surprisingly, Magic Point is one of the best places to see grey nurse sharks and the spot has become an extremely popular dive spot, with numerous dive boat operators using the area. A code of conduct exists for diving with grey nurse sharks and this code of conduct should be observed at all times. Hopefully, with an increased appreciation of this species, we will see numbers increasing right along the east coast in the near future.