FISH FACTS by Dr Ben Diggles
Shark Biology
THERE has been a lot of lines of print written lately about sharks. This hasn’t been because they sometimes eat people (even though it’s always newsworthy when they do). Instead, it’s because there is evidence suggesting there are fewer of them around than there used to be.
The underlying reasons why are fundamentally based on their unique biology. Sharks and rays are elasmobranchs which means, unlike bony fish, they have skeletons made from cartilage instead of bone. It is also certain that they do not possess the receptors that would allow them to feel pain (while in fish the jury is still well and truly out on the pain issue). But the key biological feature of elasmobranchs that is bringing them attention nowadays is their relatively limited reproductive capacity.
Most sharks are ovoviviparous, meaning that their eggs develop and hatch in the uterus, and the foetuses continue to develop until birth. Many species are also oophagous, which means that the sharks that hatch first often eat the remaining eggs or other smaller foetuses. A few examples may assist with visualising why this is a potential problem.
Mako sharks are open ocean speedsters which have been the subject of much recent debate. The shortfin mako (Isurus oxyrinchus) are reasonably long lived, with the age at maturity for males being around 7–9 years old (at about 2m long), and 19–21 years for females (at about 2.7 to 3m long). The gestation period for females is 15–18 months, with a three year reproductive cycle, and they give birth to between four and 30 pups (most litters number between 10 and 18) which are about 60–70 cm long. The maximum age of female shortfin makos is again thought to be only around 30 years, meaning that mature female makos may get only three or four chances to reproduce in their lifetime.
It’s clear that the larger sharks have limited reproductive capacity compared to most bony fishes. But what about the smaller sharks? Galapagos sharks (Carcharhinus galapagensis) are found around oceanic atolls and grow to a little over three metres long. Both male and females mature at between 2.1 to 2.5m long, at ages estimated between 6-8 years old. The females probably breed only every second or third year, producing litters ranging in size from four to 16, with young born at 60 to 81cm long.
So you can see the reproductive story remains fairly similar for sharks large and small. They mature late in life, and the females have protracted gestation periods which result in few young. Clearly these characteristics are very unlike bony fish and together they make sharks quite vulnerable to fishing pressure, which is why species such as the great white and grey nurse are now protected over much of their range, and why draconian fisheries management controls are now in place for other sharks in many parts of Australia.
Recreational fishers mostly release sharks nowadays, and rightly so. If you accidentally catch one, or are targeting them for tag & release, they should be handled with care using the usual best practice methods. Try to keep them in the water if you can, firstly to avoid being bitten, and secondly to avoid lifting the shark by the tail. While their sandpaper like skin is very tough and they cope with air exposure reasonably well, their relatively weak cartilage skeleton is prone to traction injuries whenever they are removed from the water.
