Understanding white sharks is critical to the safety of people using the oceans, and also to the conservation of this important and awe inspiring ocean predator. We still know far too little about white sharks (hence my support for significant investments in research), but this is a summary of what I’ve been able to find out from speaking to scientists and shark interaction enthusiasts, and by reading the published papers cited below.
Update Jan 2014: Some very interesting information – recent research reported in the media shows there is no proven correlation between the presence of sharks and the number of attacks in any location. Leading Australian shark researcher Barry Bruce said:
“One thing that is becoming clearer is that people and sharks (including white sharks) share the same space more frequently than we previously realised. This is not because numbers of sharks have suddenly increased; it is because we are getting better information on shark movements.
“The more we look the more we see. For example we know of some areas in eastern Australia where white sharks and people commonly swim at the same beach at the same time – yet there has never been an attack at these beaches – in fact in most cases you would not know they were there.”
Where do white sharks live in Australia?
White sharks are highly migratory. Tagging studies show white sharks migrate right throughout Australian waters but are most common in cooler southern waters. Tagged sharks have been found to travel from the south coast of Australia to as far as South Africa, and then return.
Recent studies suggest white sharks prefer waters cooler than 22 degrees Celsius, but they can also be found in warmer waters.
It is rare for white sharks to remain in an area for long periods of time. Usually they will leave once the local food source has moved on, or within a few days or a couple of weeks. In Australia I can only find evidence of long residency times (in months) having been recorded at aggregation sites off the big Australia Sea Lion colonies in South Australia.
Rumors of resident white sharks abound at some places around the coast, but have never been proven. There are examples of individuals or groups of other migratory species that occasionally take up permanent or temporary residency, so it is not impossible that white sharks could do the same, but remains unproven – and all scientific evidence seems to suggest highly unlikely.
When are they around on the West Coast?
In WA, white sharks are most commonly seen off Perth during the snapper spawning season (October to December) and whale migration (September to December) but may be present off Perth or along the West Coast at any time and particularly through the winter. It is not known if they come to Perth waters because of the snapper spawning and whales, or if they are here coincidentally on normal migration patterns, or perhaps a bit of both.
There are also increasing reports of white sharks in Geographe Bay right in spring and sometimes right into summer, particularly around Cape Naturaliste to Dunsborough, but also further east. It is unclear if this is new, or if people have started noticing more with increased awareness and patrols, but most likely a bit of both. Pure speculation on my behalf suggests that this could be a place where migratory routes come close to shore, given it is used by a number of other species including seabirds and whales. It is one of the only places in Australia that you can see blue whales from the shore as they migrate past on the way to feeding areas in the Perth Canyon and beyond. It is also known as a resting place for humpbacks and right whales on their migrations.
White sharks are slow breeding, late to mature and have few pups. Males mature at 3.5-3.7m length at 7 to 9 years old. Females will be 4.5 to 5m at 12 to 17 years and only reproduce every two to three years.
White sharks are thought to live up to 60 years and grow to 6m. Pups are born over a meter long, so no wonder the females need to be so big to breed. Some breeding areas have been found but the full range of breeding areas for white sharks remains a mystery along with their migration patterns.
Recent genetic studies have shown there are two distinct breeding populations in Australia, one on the East Coast, and the other being a Western and South Australian population. The populations intermingle between breeding seasons, travelling widely including to other countries, but return home to breed.
There is very little known about the social habits of white sharks, or changes in behavior with age. Reports from people (Squid’s conversations) who have spent time diving with white sharks believe they have observed established social orders and social influences on shark behavior amongst groups of white sharks, such as the presence of larger female sharks settling younger more agitated males, and also distinctive personalities (or at least behavioral differences) between individual sharks and age classes of sharks.
White sharks are protected by Federal Environment Legislation in Australia because they are vulnerable to overfishing. There are also international trade bans on products from white sharks.
Because there are so few white sharks it is difficult to estimate the population. Records from game fishing before protection in 1996 alongside commercial bycatch records suggested a rapid decline in numbers. Since protection, continued declines in bycatch across Australia in commercial fisheries and stable catches in beach netting programs in Eastern Australia suggest that numbers are continuing to decline or are stablising.
Recent genetic studies provide the only available estimate of the current population of white sharks in Australia, with scientists predicting an estimated Australian population of breeding sharks (females over 4.5 to 5m, males over 3.5-3.7m) at about 1500. The same studies estimate that the WA and SA population is approximately 700 breeding sharks. The study notes that the estimates come from a limited sample size and more research is required.
The tendency of white sharks to breed in smaller subgroups (East Coast and South/West Coast populations) within the greater Australian population makes white sharks more vulnerable than previously thought because they are two smaller populations rather than one larger one.
Despite protection, white sharks are still killed in significant numbers each year in Australian waters. Particularly in commercial fishing and aquaculture operations. Estimates of bycatch mortality vary widely from between 27 and 92 individuals per year depending on the method used to estimate mortality.
For this reason, and because white sharks are slow to grow and reproduce, reports of increased numbers off the West Coast are unlikely to be accounted for by a significant population increase. Further, if the shark population had significantly increased, it would likely show up in the bycatch figures from Western Australian fisheries, but it doesn’t. Below are the by-catch figures for WA fisheries for white sharks for the past 6 years sourced from the Department of Fisheries.
Table: Reported white shark by-catch in WA fisheries
|White shark catch||9||14||22||12||9||14|
However, anecdotally there have been increased sightings in WA. These are most likely due to a combination of three factors (although there are a range of other factors and theories that may contribute):
- Ocean currents in WA are changing. These changing currents are redistributing a range of marine life species as noticed by researchers, divers and fishermen alike. Part of this change is that waters offshore from the west coast are warming rapidly, but also cooler currents nearer the coast have strengthened. This situation is likely to be driving white sharks away from deeper water and towards WA’s West Coast.
- It is also likely that higher awareness due to the attacks combined with increased media focus and coverage of shark sightings on social media means that every sighting of a shark is now noticed far more than before, creating an impression of more sharks.
- Added to factor 2 is a likelihood of increased misreporting of size and species of sharks spotted by the community. A recent example was a report of a 6m white shark off Esperance that after speaking to locals the Squid suspects may have been a much smaller unidentified shark that was phoned in by a tourist concerned about the safety of her family at a nearby beach.
Update Jan 2014: Ananalysis by UWA scientists of Surf Life Saving WA data shows that the number of shark sightings per hour of helicopter time remains relatively constant over time. In 2012/13 285 sharks were spotted in 751 hrs, and in 2011/12 247 sharks were spotted in 620 hrs, approximately one shark every 2.5 hrs in both years. Noting these are all kinds of sharks, including small harmless sharks, this means you have to fly dedicated and trained spotters in a helicopter for 2.5hrs before you see a shark off Perth or the southwest. Much longer to see a white shark.
White sharks are one of the most potentially dangerous sharks. They have been implicated in most fatal attacks in Western Australia. There have been five deaths in Western Australia in the past two years attributed to white sharks by authorities.
However, the vast majority of white shark encounters by surfers, divers and boat users do not result in any kind of aggression from a shark. In most interactions sharks show very little interest in people, and in some they are curious. Also, white sharks are very rare – making an encounter, let alone an attack, unlikely for most ocean users.
We have no understanding of why sharks attack humans. There are many theories but none are proven or even well researched. We also do not understand what conditions make sharks dangerous, or put differently – why sometimes a shark encounter results in a bite and other times it does not. I have heard of some research suggesting that sharks feeding for a period of time in an area are less dangerous than sharks that have been migrating in the open ocean and have come in to shore for a short visit, but I haven’t seen anything conclusive on this.
References (Recommended reading – linked where I have a link)
Blower et al, 2012, Population genetics of Australian white sharks reveals fine-scale spatial structure, transoceanic dispersal events and low effective population sizes
West, 2011, Changing patterns of shark attacks in Australian waters
White Shark Recovery Plan, 2002. Commonwealth of Australia.