Blue Sharks: Bioindicators of Pollution and Health in the Atlantic Ocean

Pollution is becoming an ever-prevalent problem for marine ecosystems. Many of these ecosystems are being affected by contaminants produced from human activities. Pollutants can cause harmful effects at lower levels of biological organization, such as the organism level or at the gene and cellular level. In this study, these effects were examined in blue sharks (Prionace glauca).

Blue Shark (P. glauca) 

Blue sharks are one of the more common species of shark consumed by humans. They are also apex predators, meaning they are at the top of the food web. These components make them a prevalent species to study as it allows for examination of bioaccumulation, as well as the effect it could potentially have on humans. This particular study aimed to measure if Atlantic blue sharks have high levels of POPs and metals in their tissues that could effect human health as well. The contaminant body burden levels correlated with biochemical responses of stress in the organism were measured as well.

Twenty blue sharks (12 female and 8 males) were captured at a depth of approximately 20 meters in the Atlantic Ocean, southwest of Portugal. All individuals caught were considered to be juvenile sharks. After capture, liver, muscle, and brain tissue were collected for analysis. After preparation of these tissues, statistical analysis was done to determine the amount of contamination present.

Marine apex predators, such as blue sharks, tend to accumulate pollutants and contaminants in their bodies at higher levels that the majority of other animals in their food chain. When examining the tissues from this sample of blue sharks it was found that the liver tissue contained higher levels of persistent organic pollutants (POPs) than the muscle tissue did. This was to be expected as POPs typically accumulate in tissues that have higher lipid content, such as the liver. POP analysis also revealed that non dioxin-like polychlorinated biphenyls (NDL-PCBs) were the most abundant contaminant present in blue sharks. NDL-PCBs have been shown to produce toxic effects, and can affect fish behavioral responses. Some of the blue sharks sampled even had levels of NDL-PCBs present higher than what is legally allowed for human consumption. In addition to NDL-PCBs, brominated flame retardants (BFRs) were also found to be at high levels in these blue sharks captured from the Atlantic Ocean. BFRs have also been shown to have dangerous consequences to both marine organisms and humans, as they can impair organs such as the brain or thyroid.

Figure 1: Amounts of POPs found in liver and muscle tissue

Even more significant, as stated earlier, is the fact that all individuals were juveniles. Bioaccumulation has been proven to increase with size and age, so it is estimated that these already high levels of bioaccumulation of dangerous contaminants would have only continued to increase if the juveniles would have grown to full adult size.

Also examined in this study was the bioaccumulation of metals. In contrast to POPs, metals had higher levels of bioaccumulation in muscle tissue than liver tissue. These specimens showed higher levels of Mn, Cd, Fe, and Cu. However, in a previous study done approximately two decades ago that contained bigger organisms, showed lower levels of these contaminant burdens. This result raises red flags as it could indicate the possibility of a historical increase of contamination of these Atlantic Ocean waters. Mercury levels were also found to be above safe consumption levels. This is again troublesome as these blue sharks were juvenile and mercury levels are known to only increase as the size of the organism increases.

Because marine animals are constantly surrounded by xenobiotics, physiological alterations in response to stress could potentially be induced. When a Canonical Correspondence Analysis (CCA) plot was conducted of the liver tissue, it was discovered that nearly all contaminants had a positive association with DNA damage, which seems to indicate that when these blue sharks are exposed to POPs and metals, they are not able to counter the effects of the radical oxygen species (ROS) produced and prevent DNA damage.

Figure 2: CCA of liver tissue

It was concluded the lipid peroxidase (LPO), DNA damage, and inhibition of the enzyme glutathione peroxidase (GPx) were the main effects due to exposure to POPs and metals, which indicates they may be the best options to use for future biomonitoring. These contaminants can lead to ecological damage as well in terms of the blue sharks’ swimming, feeding, and reproduction habits. This conclusion can be seen and summarized in the figure below.

Figure 3: Blue sharks as bioindicators

This study also shows the prevalence of bioaccumulation and the potential human health risk it could have as blue sharks are a common species of sharks that are consumed by humans. In future studies, it would be beneficial to examine the individuals' stomach contents as well to see how feeding habits may influence bioaccumulation. It would also be important to examine larger sampling areas in the Atlantic Ocean as well as other oceans to see how contamination and pollution affect populations there as well.

Alves, L., Margarida, N., Marchand, P., Bizec, B., Mendes, S., Correia, J, Lemos, M., & Novais, S. (2016). Blue sharks (Prionace glauca) as bioindicators of pollution and health in the Atlantic Ocean: Contamination levels and biochemical stress response. Science of Total Environment, 563-564, 282-292.