Saturday, 23 January 2010

A Jagged Truth

Learn how Global Warming could potentially save a sharks population in Australia from becoming extinct.  Caitlín McCann takes us for a ride in the fascinating world of one the ocean's most  misunderstood apex predators. 

Man, Climate Change and the Labrador of the Sea

Sharks have been demonised and mythologised by many cultures for centuries. The erroneous fear that has been past on through generations (in particular over the past 30 years) by literature, television and even cinema, labeling sharks as man-eaters and vicious killers has only, in recent decades, been combated by a global network of researchers whose aim is to stop some of the most remarkable apex predators in our ocean from going extinct. Ironically, the current foe of wild populations globally, namely climate change, has created an advantage for an otherwise quasi-extinct shark population.

The aforementioned population is that of the Grey Nurse Shark (Carcharias taurus) found in temperate and sub-tropical coastal waters and currently dwindling off the east coast of Australia. This relatively placid and slow moving shark was once one of the most feared encounters for divers in the Australian waters (but in recent years has earned itself the endearing name of Labrador of the Sea). Owing to its name (‘grey’ is often the number one attribute given in reports of shark attacks) and ferocious appearance the C. taurus had fallen prey to governmental precautionary measures known as shark control programs, such as coastal meshing and baited drumlines in addition to pressures from commercial and recreational fishing.

In 1992 the Queensland’s Nature Conservation act classified the two remaining C. taurus populations in Australia as endangered. The globally recognized IUCN Red List has listed the shark population off the west coast of Australia as vulnerable and the east coast population as critically endangered after the Environment Protection and Biodiversity Conservation Act 1999 listed the species as two separate populations. However measures in place to control other species still have horrific impacts on C. taurus populations in Australia.

Late Bloomers

A conventional assumption about marine species is that they have a high productivity rate and thus their populations are not as severely impacted by large scale mortality events owing to anthropogenic impacts such as commercial fishing and destruction of habitat. However, this is often not the case as species such as the C. taurus have one of the slowest productivity & lowest fecundity rates (produce two pups every two years) of all the elasmobranchs. It takes approximately 6-7 years for males and 9-10 years for female sharks to become sexually mature. This makes the species very vulnerable to overexploitation and extinction.

The eastern Australian population of C. taurus has been currently estimated at 500 individuals. It stands that many of the individuals that die via by-catch of commercial fisheries and entrapment in beach meshing are female sharks that tend to live closer to the coast where they give birth. Of the approximate 500 individuals (figure most likely outdated 2007), scientists believe the effective population, or the number of individuals breeding, could be as little as 50. This means that every casualty could potentially have dreadful effects for the population as a whole.

100,000 years in Isolation

Although anthropogenic impacts have had an enormous effect on reducing the eastern Australian population of C. taurus, Adam Stow and colleagues from the Macquarie University in Sydney have published findings that the ‘critically endangered east Australian C. taurus are characterized by less genetic variation than any other populations’, in Western Australia or South Africa. These finding suggest that dwindling eastern population might also be affected by ‘events that pre-date mankind, a founder effect in the population’. This essentially means a small number of sharks potentially established the eastern colony and became isolated from a larger, genetically more diverse breeding system. This causes susceptibility to inbreeding, genetic drift and an overall loss of genetic diversity. This can lead to inheritable diseases and disorders becoming more common in the populations making for a very ‘unhealthy’ population.

Using various genetic analyses Stow and the team were able to identify the first account of ‘genetic variation and geographical partitioning in C.taurus’. The eastern populations have only 1 mitochondrial haplotype solidifying the theory that the population had a few ‘founding members’.

With this lack in genetic variation in the eastern population and the mortality rates of C. taurus rising, one of the few remedies seen by researchers is a merger of the eastern and western populations, increasing numbers and diversity, and making accidental casualties less significant to the survival of the population. Until recently this was seen as absurd. 

Anthropogenic Monster gives “2nd chance”

Associate Professor Corey Bradshaw of the University of Adelaide in Australia commented that even though “this is probably one of those one in a hundred examples where climate change may actually be somewhat beneficial for this particular species”, the potential to save one species is not a balanced return for the cost of global warming around the rest of the world.

After receiving a grant to pursue studies on the extinction risk, threat assessment and priority management actions for the East Coast population of C. taurus in Australia, Prof. Bradshaw commented to ABC News in September 2008 that a major reason for such a succinct separation in the two Australian populations came down to the species sensitivity to colder waters. The two populations have been separated for over 100,000 years; however the increasing temperatures owing to global warming have already seen an increase in water temperatures in the Bass Strait, the body of water that divides the shark populations.

Now with global water temperatures rising, the possibility of the eastern and western grey nurse shark populations meeting is less bizarre. This will not ‘save’ the populations, however, it does give more time to make positive steps towards preserving them by rethinking shark control programs and the use of beach meshing, recreational spear fishing and commercial fishing near the sharks’ habitat.

Future still unknown…

Although rising temperatures in Southern Australian waters will undoubtedly heighten the chances of the two populations eventually meeting, no one knows yet just how the groups will interact. Even if the two populations meet and begin interbreeding, the eastern population is already vastly inbred and human activities (until such an event occurs) are still having drastic effects on the 500 or so individuals that are thought to remain on the east coast from Queensland to New South Wales.

The continuing presence of commercial fishing, recreational spear and game fishing and various shark control activities in Australia will continue to cause the decline of C. taurus numbers even in light of the apparent ‘second chance’ the populations have received by this biggest of man-made monsters, global warming. Various recovery plans, legislative efforts and charity organizations are continuing in the name of endangered marine species across the globe, however, only time will confirm the fate of the so called: Labradors of the Sea.

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