Exposure of SBT to oil

The first and most obvious way that tuna would be exposed to oil would be through direct contact. SBT are migratory fish, usually as they pass through the Great Australian Bight they will surface. However, it is unlikely that SBT would surface if an oil slick was on the surface and thus unlikely they would undergo direct contact.


SBT are more likely to be exposed to the toxic effects of oil via direct ingestion. SBT feed on sardines that pass through the GAB. Sardines are likely to come into direct contact with oil; thus, tuna can ingest oil that is on the sardines.


Natural dispersion (breaking down oil into small oil droplets soluble in water) can result in the presence of oil droplets within the water column. Rough waters, like that in the GAB, exaggerate natural dispersion, increasing the concentration of oil droplets. Migrating SBT can ingest the oil droplets that are present in the water. The use of chemical dispersants will increase the likelihood of tuna being exposed to oil in this way.


It is important to remember that everything in the marine ecosystem (and any ecosystem) is connected. Anything that’s affected at the bottom of the food chain can affect the levels above. For example, zooplankton is sensitive to oil exposure and in response experience developmental abnormalities as well as lower rates of feeding and reproduction. Sardines feed on plankton and tuna feed on sardines. There are two main problems here. Firstly, a reduced population of zooplankton (due to oil caused lethality) results in reduced sardine numbers and consequently reduced biomass of SBT. The second problem is the effect of biomagnification. Biomagnification is the increase of oil concentration over two or more food-chain levels. For example, one organism (e.g. sardines) can ingest and retain oil, and then an organism on a higher feeding level (e.g. SBT) may eat the first organism. If biomagnification were occurring, the organism at the higher level (SBT) would receive an increased exposure to the oil by eating contaminated food. Biomagnification is a problem for human consumption.


Use of chemical dispersants

Application of dispersants – is the cure worse than the ailment?


In case of an oil spill at sea, Equinor has to decide on the most effective response to minimise damage. The dominant strategy remains the removal and containment of spilled oil by mechanical technology. Such containment may be difficult in the Great Australian Bight, considering its rough waters. The other option that Equinor have outlined is the use of chemical dispersants.


Dispersants enhance the natural break-up of floating oil into small drops. In this way, coating of coastal areas and oiling of sea birds and mammals can be reduced. To the human eye, chemical dispersants make oil spills disappear.


However, the treatment could be worse than the ailment. Dispersants essentially break up the oil and reallocate it from the surface to the water column. The effect of an oil spill on surface dwelling marine organisms (e.g. birds and sea-lions) is reduced as a consequence. However, the increased concentration of oil in the water increases the toxic effects on organisms in the water column and on the bottom of the sea floor.


It is important to note that toxic effects caused to organisms in the water column and on the bottom of the sea floor, are likely to work their way back up to the surface dwelling organisms anyway. For example, a bird may no longer be at risk of being coated in oil; however, the fish it feeds on are dying off or contaminated with oil. Thus, that bird may die from starvation or due to the toxic effects of oil.


Additionally, usually less than 100% of the treated oil will disperse. This means that the effect from undispersed oil will still occur.


Understandably, there are conflicting views concerning the potential risks and benefits for human health and the environment generated by the use of dispersants during oil spills. Currently, there is not enough data to make an informed decision on the most environmentally friendly response. Importantly, each case varies so what worked in one scenario may not necessarily work in the Great Australian Bight.


What does appear to be clear is that even cleaning up oil spills has toxic effects. In the words of South Australian senator Tim Storer – the risks of drilling in the Bight simply don’t justify the rewards.


Oil drilling in the Great Australian Bight

Port Lincoln proudly proclaims itself “the Seafood Capital of Australia”; but, Norwegian energy giant Equinor threatens this status with plans to drill for oil in the Great Australian Bight. Such plans will have dramatic repercussions for the thriving aquaculture industry of Port Lincoln which relies heavily on the Bight.


Equinor plans to start operations in October next year with a drill located 370km from the nearest coastline at water depths of 2339 metres. Critics of the drill fear that an oil spill in this productive and pristine region will damage coastal communities and devastate wildlife.


What happens when oil is spilled?


Initially after an oil spill oil will spread out across the water, moving with the wind and the current. The amount of spreading depends on the action of winds, waves, water currents, oil type and temperature. At the same time the oil undergoes a number of chemical and physical changes due to weathering, evaporation, oxidation, biodegradation and emulsification.


Effect of an oil spill on the SBT population and industry


Oil spills can seriously impact the marine environment in a range of ways. Examples include:

  • Physical and chemical alteration of natural habitats e.g. resulting from oil incorporation into sediments
  • Physical smothering effects on flora and fauna
  • Lethal or sub-lethal toxic effects on flora and fauna
  • Changes in biological communities resulting from oil effects on key organisms e.g. death of zooplankton.


An oil spill in the Great Australian Bight (GAB) will inevitably have lethal and/or sub-lethal effects on the prized Southern Bluefin Tuna (SBT). Sub-lethal effects may include fin and tail rot, altered reproduction, decreased growth rates and lowered immune function. Overall, exposure to oil will reduce the chance of survival.


SBT are at a critical level that the industry sustainably manages with strict quota regulations. The recovery of the species is dependent on the strong conservation efforts at the international, national and state level. Considering the status of the species, a mass mortality event caused by an oil spill could see the extinction or reduction below recovery level.


The ability of a species to recover from exposure to toxic chemicals like that in oil depends on a number of factors. These include:

  • Sensitivity/tolerance to chemicals
  • Potential for (im)migration from non-impacted areas
  • The reproductive dynamics i.e. generation time
  • Number of offspring


In general, ubiquitous species with high reproduction rates and short generation times have a significantly higher potential for recovering than isolated species with a slow reproduction.


SBT are expected to have a low recovery potential. Firstly, SBT are considered to be a single stock since they have a single spawning ground. Consequently, an oil spill in the GAB is expected to affect the entire population of SBT. Secondly, SBT have slow reproduction rates and a long generation time. Therefore, it may take many years for the population of SBT to recover, if at all. Lastly and most importantly, the numbers of SBT are most likely not great enough to withstand the effects of oil spill.


Exposure of seafood to oil can make it unfit for human consumption. There are two ways that this can happen. First is through the presence of chemicals known as polycyclic aromatic hydrocarbons (PAHs), some of which are cancer-causing. PAHs are one of the components that make up oil.



The second way that an oil spill will make SBT unfit for human consumption is through tainting. Tainting is defined as the presence of an “off taste” that can be due to oil; however, it can also be due to natural causes. Tainting isn’t necessarily harmful; however, it can significantly decrease the marketability of the affected fish.


Even if SBT remains unaffected by the oil spill, just the perception of tainting and contamination will have significant negative effects on SBT marketability.


Port Lincoln upholds a title for clean seafood, caught in the pristine waters of the Great Australian Bight. After an oil spill, it will be possibly contaminated seafood, caught in waters thick with oil.


Australian Southern Bluefin Tuna Industry Association (ASBTIA) chief executive Brian Jeffriess says an oil spill will mean the end of the tuna industry. This could be due to loss of the species or contamination or perceived contamination of fish. Furthermore, “drilling in the Bight is not just a direct threat to tuna, but to all of the city’s aquaculture,” he said.