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Australia is an island continent famous for its oceans - the Great Barrier Reef, endless sweeping beaches, seafood and surf culture. Yet for all their diversity and beauty, Australia's seas are relatively low in nutrients. A lack of river outflow and large upwelling currents means that Australia's surface waters are relatively nutrient poor. Therefore, phytoplankton, the primary producers of the ocean ecosystem, exist in low densities. In turn, this feeds a low amount of fish, squid and crustaceans, which feed the top predators such as sharks, tuna, dolphins, seabirds and seals. While seal populations in North America, South America, Africa and the Southern Ocean number in their millions, Australian seals exist in smaller numbers (<250,000).
Three species of seal are found in the coastal waters of Australia: Australian fur seals, New Zealand fur seals, and Australian sea lions.
Arctocephalus pusillus doriferus
Zealand fur seal
Photo: Brian Hunt
Population size: 120,000
Population size: 100,000
Population size: 14,000
seals are the largest
of the 10 types of fur seal. Females can weigh up 60 - 100kg, while males are much larger and can weigh up to
300kg - as big as a bear! Aussie fur seals are a golden brown colour and have a call that sounds a bit like
a cow mooing (adults), or a lamb bleating (pups). They have a relatively small range that is concentrated in Bass Strait.|
Occurring alongside Australian fur seals are New Zealand fur seals. To tell the difference, notice the dark grey colour of the fur, the white whiskers, pointier face and smaller size. The adult call is a high pitched trumpeting sound. New Zealand fur seals are also found all along Australia's southern coast as well as in New Zealand (hence their name!). In Bass Strait, Australian and New Zealand fur seals are sometimes found on the same islands.
|Australian sea lions are a similar size to Australian fur seals but have a lighter yellow-brown coat. They have a more rounded snout, making them look a bit like a dog. They occur to the west of Bass Strait and are more sparsely distributed than the other 2 species. The population is declining and classified as vulnerable, making them of high conservation importance. What's the difference between a sea lion and a fur seal? Not much: see the Fast FAQs!|
Several other seals are somtimes seen in Australian waters but they are only visitors from the subantarctic. Henry the elephant seal likes to moult near Geelong, Victoria, and leopard seals are also seen occasionally.
Seals are easily hunted
they form dense colonies on land for breeding and resting. Plus, having flippers for feet means they can't run very fast. With the exception of a small amount of subsistence hunting by indigenous Australians, seals were safe on islands for thousands of years. However, the sailing revolution of the 1700s
brought seals within reach of European commercial hunters who sold seal fur and blubber for use as clothing and oil.
'Seal Shooting in Bass's Straits', 4 May 1881.
Source: La Trobe Picture Collection, State Library of Victoria
Australian and New Zealand fur seals have become more numerous in recent years. This has lead to a perception, particularly amongst some fishers, that fur seals are reaching "plague proportions" and need to be culled before they "destroy fish stocks". Let's look at the facts behind this issue:
-Both species were hunted to the brink of extinction during the commercial sealing era of 1798 - 1830 (see History section above).
-It's difficult to know exactly what the population sizes were before then, but they have been estimated to be at least double what they are now.
-It is true that the population has increased recently: The Australian fur seal population has doubled since the 1980s.
-So, Australia's fur seals are still recovering from the extreme amount of hunting they experienced almost 200 years ago.
-The rate of recovery has, in fact, been much slower than that of other hunted fur seal populations. This is because of the low level of nutrients, and therefore food, in Australian waters. This is probably why female Australian fur seals have a very low birth rate (about 50% per year) and only about half of those pups survive to adulthood. This has kept population growth down to around 2% per year, which is much lower than the rates of 10 - 15% seen in some other fur seal populations.
It is true that seals eat fish, but that doesn't necessarily reduce the amount of fish available for humans to catch. Most research shows that top predators are, in fact, essential to ecosystem balance. South African researchers estimated that a reduction in the local Cape fur seal population could result in population increases of certain large predatory fish. With more predatory fish, smaller sized fish suffered from greater amounts of predation. Removing seals could actually reduce the overall biomass of fish in the ocean.
This is an example of top predators being "keystones" to ecosystem balance. Another example is that of the dingo - excluded from south-eastern Australia by a huge fence for the protection of livestock. Free from dingo predation, kangaroos can become over-abundant and degrade vegetation. Unlike seals, kangaroos are believed to be more numerous now than before European settlement. Similar conflicts between predators and primary producers exist throughout the world.
So do we want more seals in Australia? Most people enjoy seeing seals in the wild. Yet some people blame their declining seafood catches on the increasing seal population. Researchers say that no such link exists and that seals balance the marine ecosystem. When you hear an opinion, consider the expertise and motivation of the people making it.
A leading cause of death in seal pups is caused by getting entangled in rubbish like old bits of fishing net, rope and packing bands. Pups will play with floating objects in the same way a young dog might play with a piece of rope - by biting it and thrashing their head. When seals do this with a fishing net, sometimes the net gets caught around their neck. The direction of the fur helps work the net down around the neck where it can become embedded.
Fast growing pups will eventually become too big for the netting which may end up strangling them or cutting into the skin. Monofilament fishing line is especially nasty and has been found cutting into the flesh as deep as 15mm. Death by entanglement is slow and painful.
Seal researchers and park rangers free entangled seals when possible but usually they are too difficult to catch.
Hungry seals sometimes follow fishing boats hoping to partake of the catch that is brought in. Unfortunately a seal can scare off a school of fish that the fishers would otherwise have caught, and interfere with nets.
Being a large predator at the top of the food chain, seals eat plenty of large fish - Australian fur seals are estimated to consume 5 times more seafood than humans catch in their range. This leads to resentment from some who feel they are in competition with seals for their livelihood. Although seals are protected, some people still kill them illegally such as in August 2006, when up to 100 seals were shot. Most shootings occur at sea and go unobserved.
There is no evidence that the growth of Australia's fur seal populations is related to declining catch rates. As in many other cases around the world, declining catches have coincided with increased rates of fishing. Research suggests that seals and fisheries can co-exist because they generally use different areas and methods to get their prey.
Another hazard for fur seals around fishing boats involves being caught in the fishing net and drowning. An estimated 500 seals are drowned each year in Australia's South East Fishery. Hundreds more die from entanglement in lost fishing nets.