Sharks near Miami have more accumulated fat than in better conserved areas, study shows

Analysis of blood samples showed that sharks living near urban areas have a lower-quality diet than those living in wilderness areas. Dietary imbalances can impair important physiological processes, such as cardiovascular tone, inflammatory response and reproduction.

By André Julião  |  Agência FAPESP – Not even marine animals appear to be immune to the American way of life. Researchers affiliated with institutions in Brazil and the United States have discovered that Nurse sharks (Ginglymostoma cirratum) living near urban Miami have more fat in their blood and bacteria in their stomach contents than individuals of the same species living in better conserved areas.

The study was supported by FAPESP, and an article reporting its results was published in the journal Science of the Total Environment last year.

“The species is apparently affected by urbanization as it lives near marinas with pollution, many boats and fish remnants. Nevertheless, this environment may also offer a larger amount of prey, as well as protection from predators, all of which probably contributes to an accumulation of fat,” said Bianca Rangel, first author of the article. The study was conducted at the University of São Paulo’s Institute of Biosciences (IB-USP) in Brazil during her doctorate. 

In another article published in the same journal, the researchers conclude that urbanization does not seem to have had such intense effects on a different species, the Blacktip shark (Carcharhinus limbatus).

This species has less sedentary habits, moving between areas near the city and wilderness areas such as Florida Bay and Everglades National Park, consuming more varied prey and living in more diverse environments.

“We expected to find that this species was nutritionally deficient, mainly in essential fatty acids, because it’s exposed to an environment more influenced by urbanization, but this wasn’t the case. We also failed to detect the expected higher levels of fatty acids that serve as indicators of the presence of bacteria,” said Renata Guimarães Moreira, a professor at IB-USP and principal investigator for both studies.

Curiously, the physical condition of these sharks was better in the urban area, possibly for the same reasons as for G. cirratum, i.e. the larger supply of high-calorie food including remnants of fishing by humans, and less competition from other predators. However, levels of bacterial fatty acids were higher in urban than non-urban specimens, as expected, probably because of sewage and hence more bacteria. 

For the researchers, future studies should monitor nutritional quality of both predators and prey, particularly because increased levels of saturated fatty acids and the imbalance between these and polyunsaturated fatty acids can be physiologically harmful, affecting cardiovascular tone, inflammatory response, reproduction, kidney function, and brain development.

Bahamas

The results described in the article were obtained by the researchers at IB-USP, who analyzed samples collected by Neil Hammerschlag and his group. Hammerschlag is a professor at the University of Miami who since 2011 has been monitoring sharks in Florida and the Bahamas, an archipelago some 500 km to the south.

The researchers avoided killing sharks for specimens, most of which came live from commercial fisheries. They were measured and weighed, blood and tissue were sampled, and they were marked with a number before being released so that data can be compared if they are captured again in future.

In another article also published last year in the same journal, Rangel, Hammerschlag et al. describe the results of an analysis of blood markers in Tiger sharks (Galeocerdo cuvier), also from Florida and the Bahamas.

Although these large migrants do not approach cities, they may suffer from the influence of human activities. Levels of bacterial fatty acids were higher in specimens from the Bahamas than Florida, for example.

“In this case, we only analyzed juveniles,” Rangel said. “Competition among adults is probably considerable in the Bahamas. We inferred that juveniles avoided disputes by feeding on bottom-dwelling fish, which are significantly influenced by organic matter and hence bacteria.”

Pollution by chemical fertilizer is also a factor, according to the researchers. Heavy use of fertilizer tends to lead to a buildup of these nutrients in rivers and the ocean, favoring proliferation of microorganisms.

The researchers were surprised to find that nutritional quality among G. cuvier in the Bahamas was poorer than in Florida, possibly because the sharks inhabited areas near tourist diving sites. Tourist diving guides often use dead fish as bait to attract sharks. The researchers believe sharks expend a significant amount of energy to obtain low-quality food, as the carcasses contain more bone than meat.

Few studies have involved this type of approach, Rangel and Moreira noted, stressing that the results are preliminary and they require more analysis in order to reach robust conclusions. Meanwhile, they see their findings as an important warning about the environmental hazards and the need to consider these in conservation policies.

“Any action taken to conserve these animals requires a database on their biology, physiology, genetics, behavior, and other aspects. Only if we understand the changes to metabolic and endocrine processes caused by urbanization can we formulate conservation and management plans that minimize the impact of human activities on these species,” Moreira said.

The article “Urban living influences the nutritional quality of a juvenile shark species” is at: www.sciencedirect.com/science/article/abs/pii/S0048969721010925. 

The article “Effects of urbanization on the nutritional ecology of a highly active coastal shark: Preliminary insights from trophic markers and body condition” is at: www.sciencedirect.com/science/article/abs/pii/S0048969722011743. 

The article “Metabolic and nutritional condition of juvenile tiger sharks exposed to regional differences in coastal urbanization” is at: www.sciencedirect.com/science/article/abs/pii/S0048969721016168.