The first and most obvious way that Southern Bluefin Tuna (SBT) would be exposed to oil would be through direct contact. Usually as SBT pass through the Great Australian Bight they will surface, which puts them at risk of surfacing and suffocating in an oil spill. 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 (see this page for more information on the use of chemical dispersants)
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.