A significant change in the genetics of tiger sharks reveals a vulnerability to direct exploitation and shark control programs.
Jaws was the only word needed to give the iconic 1970's thriller about a great white with a preference for humans its eerie title. Though a strong and important player at the top of the foodchain, sharks face a range of enemies: overfishing, habitat loss, pollution and climate change and human fear resulting in the use of shark control programs in some locations.
The fear and fascination for sharks have made people collect shark jaws for decades. These collections of shark jaws from museums, national fishery institutes and personal collections, including modern samples from fishery institutes represent a great opportunity for scientists.
Using genomic data retrieved from historical tiger shark jaws, an international group of scientists including Professor Einar Eg from the Technical University of Denmark has found evidence of the disappearance of a local southeastern Australian population of tiger sharks. A disappearance associated with a documented local decline in abundance of tiger sharks, likely caused by the ongoing shark control program.
The international study Retrospective genomics highlights changes in genetic composition of tiger sharks (Galeocerdo cuvier) and potential loss of a south-eastern Australia population has just been published in the journal Scientific Reports
“Our study shows that tiger sharks can have local and genetically isolated populations at a restricted geographical scale – such as the south Eastern Australian coast – and that these local populations are vulnerable to direct exploitation and shark control programs,” says Einar Eg.
Top predator controls the ecosystem balance
The study shows that there are still tiger sharks in the area. However, these individuals belong to an, apparently, more widespread population found across the east/north coast of Australia.
“When we, through genetic analysis, better understand the distribution and migration of shark populations and their responses to human activities over historical time, we are better able to design proper management plans and actions at the appropriate geographical scale. Not only for the benefit of sharks, but for marine ecosystems as a whole,” says Einar Eg and explains:
"Genetic diversity within a species, is the fuel that drives future evolution and adaptation to the environment, e.g. climate change. Without historical genetic/genomic data, there is no way of assessing the loss of genetic diversity within a species"
Professor Einar Eg
“Sharks are top predators. They control the abundance of other species below them, and sick fish, in the food chain, ensuring species diversity. I.e. they are important for maintaining ecosystem balance. They are generally long lived and slow reproducers, so a healthy shark fauna signals a healthy ocean and ecosystem.”
Genetic diversity is the fuel that drives future evolution
Before the new study, it was believed that tiger sharks did not display local population structure. Thus, genetic differences among tiger shark populations were only found at a basin wide scale, such as between tiger sharks in the Pacific and Atlantic oceans.
Accordingly, tiger sharks were expected to display low vulnerability towards local depletion. Therefore management of the species at a large geographical scale was in focus.
“From our samples alone, it appears that the historical local population has been extirpated or significantly reduced. This means that management of the species also has to focus on regional processes and exploitation patterns in order to protect local populations and biodiversity of the species as a whole,” says Einar Eg and points to the crucial aspects of genetic research:
“Genetic diversity within a species, is the fuel that drives future evolution and adaptation to the environment, e.g. climate change. Without historical genetic/genomic data, there is no way of assessing the loss of genetic diversity within a species.”
Fear and facts – are sharks moving North?
With regards to the shark control programs having an impact on shark numbers, the obvious question arises How afraid should one actually be to go swimming in Australia or South Africa?
“In 2021, there were 73 cases of unprovoked shark bites worldwide, with a total of 11 fatalities. Most attacks were related to surfing and board sports. In Australia, there were three fatalities and 1 in SA. So, the chance of being attacked and killed by a shark is almost non-existing. One should definitely be more afraid of driving in your car writing txt messages,” says Einar Eg.
As climate change causes sea temperatures to rise, some researchers say that we may be looking into a future with large sharks entering Danish/European waters. However, Einar Eg stresses that though changed temperature conditions could allow for more large sharks occurring in Danish/European waters, many other factors determine the distribution of a species.
“The Mediterranean, for instance, is very suitable for large sharks, but we do not see large assemblages of white, tiger, mako sharks there. If they come, it is highly unlikely that this would result in any bather-shark conflicts. As an example, there were no reported shark bites in Europe for 2021,” says Einar Eg.
A future for sharks
On a global scale, the tiger shark is near threatened. According to Professor Einar Eg that covers a significant species depletion in some areas, while they’re doing ok in other regions of the world:
“We need to shift tiger shark management conceptually from an exclusive species view to also include the local population aspect. I.e. saving global populations has to go through protection and proper management of local populations,” says Einar Eg.
“Now, by having our temporal genetic data, we can study the genetic impact of anthropogenic pressure on marine species, enabling us to improve management in order to secure biodiversity.”
How can genetic research continue and help improve shark control and hunting in favour of sharks?
“Genetic research can help to elucidate the proper biological units (genetic populations), which should be the target for fisheries management, conservation and biodiversity protection,” says Einar Eg and concludes:
“Studies like ours can illustrate the likely consequences of local over-exploitation in relation to shark control and make us realize what we can lose by not paying attention to the distribution of genetic variation within a species.”