Following last Wednesday's fatal shark attack at Muriwai Beach, we thought our readers might have some questions about sharks they'd like answered.
NIWA fisheries scientist and fish biologist Dr Malcolm Francis is New Zealand's foremost shark researcher. Francis has been a scientist, diver and underwater photographer for more than 30 years and is a representative to the IUCN Shark Specialist Group.
In 2008 he was awarded the prestigious New Zealand Marine Sciences Society Award for his "continued outstanding contribution to marine science in New Zealand".
Here Francis has answered the first batch of your questions. The next round of answers will be up tomorrow at 1pm:
As a keen scuba diver/spear fisher, what is the best action to take when coming across a shark in NZ while diving? (Robert Glasgow)
Sharks are rarely seen underwater by divers but they can be attracted to dead or struggling fish. If you are spear fishing, remove dead fish from the water immediately and get out yourself. Sharks will key in on the sounds and smells of dead fish, and the latter linger long after the fish is removed from the water.
If you find yourself in the water with a shark and can't readily get out, face the shark and keep your eye on it - sharks often use surprise in their attacks on prey and if they know they've been seen they may not think it's worth an attack (because their prey are often more agile and can escape easily) or may realise you aren't their normal prey.
Stay calm, don't splash or swim fast, and try and get something between you and the shark - a rock, a speargun, camera, or anything else you can use to fend it off. If you are attacked, try and hit the shark on the sensitive snout or gills.
Is the reason why sharks come to places they do not usually go due to decreasing number of fish in the sea? Is the overfishing from the fishing industry part of the cause for this? (Kazuhide Shaun Okuda)
Sharks of various species occur throughout NZ waters. The larger species are mobile and continually cruise around looking for food, and hope to surprise their prey. So there aren't many places where they don't usually go.
During summer, sharks often come close inshore because their prey (often schooling fish) may aggregate in shallow coastal waters. There is no evidence that sharks have difficulty finding prey because of low fish abundance.
With everything we know about Great Whites and their migration, behaviour and feeding etc, how likely is it really to have three Great Whites, travelling together and evenly feeding on the same prey? (Nicole Fleetwood)
White shark tracking indicates that the sharks travel separately rather than in groups, although the accuracy of the fixes from some tag types is low, so the position estimates are not good enough to be sure how close the sharks are together.
However, great white sharks are extremely good at finding and scavenging on dead animals such as whales, and it is not unusual to see several sharks feeding on one whale carcass. They also aggregate around seal colonies.
So if the food source has been around for more than a day or so, it is quite possible for more than one great white to be feeding on the same prey at the same time.
Is it true that some "families" (groups of siblings) within the bull, great white and tiger sharks are known to intentionally attack humans (or maybe their ability to distinguish between seals and humans was inferior to other shark families) or can they simply not distinguish between us and seals? (Georgina Cuttance)
Some shark attacks on humans appear to be intentional, but I'm not aware of any evidence that some groups of sharks are more likely to do so than others.
Is it possible that the globally increasing number of shark attacks can be linked to the rapid decrease in global fish stock in the oceans? (Hans Botha)
Probably not. Sharks do not appear to be suffering from starvation, and in fact shark populations have also declined under the influence of fishing in some countries, so the balance between the numbers of sharks and the numbers of their prey may not have been upset too much.
Shark attacks may be increasing for a range of reasons, including better reporting of attacks worldwide, and especially the much greater number of people entering the water for recreation.
Please explain why, although nearly every scientific documentary I've seen about sharks insists:
- that they are misunderstood and are suffering from fictional accounts of shark attacks by movies, books and folklore;
- that sharks don't knowingly attack people;
- that they either mistake them for normal prey (eg seals) or have been conditioned by human interference (eg feeding or fishing) to attack;
- that they release humans as soon as they realise they have the wrong species;
we still hear and see shark attacks on people that cannot be explained by any of the above? The Muriwai case appearing to be one of these. (Simon Prince)
Many shark attacks on humans appear to be a case of mistaken identity. They often occur in dirty water or in surf zones where visibility is poor, and the victim is usually not eaten (though in some cases that may be because rescuers don't give the shark a chance to do so).
In the north-east Pacific sea otters are often attacked and killed by great whites, but are usually left uneaten; this is thought to be because they lack a high-energy layer of blubber that most other marine mammals possess.
So white sharks at least seem to be able to discriminate among prey types after they are bitten (and we assume they have made a mistake in attacking an animal that is not desirable as food). It has recently been suggested that sharks possess taste buds in their teeth, which would give them the sensory ability to analyse their prey during a bite.
Nevertheless, some shark attacks on humans do appear to be intentional and feeding is likely to be the motivation in a proportion of these attacks.