Urban Sharks, Is This Possible?

Sharks live closer to people than we think [1]. Compared to the open ocean, coastal areas generally have more food for sharks, which makes these areas attractive to them. Young sharks often use shallow waters, which are protected from predators, as nursery grounds during their first few years of life, before moving to deeper water as adults. Shallow areas near shore also tend to be warmer, which helps speed up the growth of young sharks (Figure 1). However, the sharks that use coastal waters are also exposed to numerous human threats, especially near big cities. These threats include fishing, boat traffic, and pollution (Figure 2) [2]. For example, light pollution from cities can alter the behavior of sharks and their prey. Contaminants like metals and oil that leak into the ocean can directly affect shark health and reproduction [3].

Studying the Nutrition of Urban Sharks

To better understand the influence of urbanization on shark nutrition, we studied 52 blacktip sharks in the waters off South Florida, close to the city of Miami, and in more pristine waters farther away from Miami. We measured shark girth (how wide they are around the waist) and we took small blood samples to check what they were eating. To do this, we carefully and temporarily trapped sharks using a shark-friendly fishing method called a drumline (Figure 3). We then brought the sharks alongside our boat, collected blood samples through a vein in the tail, recorded body-size measurements, and then released the sharks, which swam off. We tested shark blood to examine the types and proportions of substances called fatty acids, which told us about the sharks’ nutrition (Figure 3). Individual sharks sampled close to Miami were classified as “urban sharks” and individuals sampled farther from Miami were classified as “non-urban sharks.”

What Are Fatty Acids and Why Study Them?

You have probably seen the words “omega-3” written on products being sold in supermarkets, pharmacies, and grocery stores. Omega-3 and–6 fatty acids are a type of healthy fat that animals (including humans) need but cannot produce, and thus must get from their foods [4]. These healthy fats are essential for survival and health, since they help to form some of our cells. They are also essential for inflammatory and immunological processes—two ways that our bodies cope with diseases [5]. Omega-3 fatty acids differ from saturated fats, which are a type of fat that tends to be solid at room temperature and, therefore, unhealthy when in excess in an animal’s body. Many processed and deep-fried foots have high saturated fat content, which can cause cardiovascular disease in humans [4, 5].

In the marine environment, omega-3 fatty acids are produced by microscopic algae called phytoplankton. These omega-3 fatty acids are then passed along the food chain to animals, for example, when a small fish eats the phytoplankton, then a bigger fish eats the smaller fish, and then a shark eats the bigger fish (Figure 3) [4]. If sharks cannot feed on prey with high levels of omega-3 fatty acids, then they may have to eat foods with higher proportions of saturated fats, which is unhealthy.

Urban Shark Are Chubby!

We discovered that urban sharks had larger girths than non-urban sharks and also had higher levels of unhealthy saturated fats in their blood. We believe this means that urban sharks are eating more food, and food of poorer quality in terms of saturated fats. This may be equivalent to urban sharks feeding on lots of “fast food” in the city!

But why do urban sharks eat more food than non-urban ones? First, we suspect that increased nutrients coming from sewage are helping phytoplankton and smaller animals to proliferate, and these organisms serve as food for larger animals, including sharks [6]. Second, the waters around Miami have many fishing marinas, where anglers often cut the meat off their daily catches and discard the remaining carcasses in the water, where those leftovers can be consumed by urban sharks [1, 7]. Lastly, it is also possible that urban sharks do not have to swim as much, so they do not use up as much energy doing physical exercise.

We also expected to find that urban sharks were feeding on prey that was low in important nutrients, like the essential omega-3 and omega-6 fatty acids. In a previous study we did with nurse sharks—a species that does not move around a lot—we found a reduction in omega-6 fatty acids [8]. However, we did not detect a reduction in diet quality in urban blacktip sharks, which tend to move around a lot. This made us think that the lifestyles and behaviors of each urban shark species might influence the quality of the food they eat. Thus, we suspect that impacts of urbanization are stronger in sharks that do not move around a lot, because, compared to other species, they probably have increased exposure to the pollutants emanating from big cities.

In summary, our results and those from previous studies suggest that sharks living near cities may eat more, and the quality of their food may not be as good as that found in more pristine areas. We are not sure what this means for shark health, so we suggest that future studies should continue to investigate the nutrition and health of urban sharks—an inadequate dietary intake of essential nutrients could compromise shark health and development.

Glossary

Contaminants: ↑ Chemicals that are discharged from industries, including things like mercury, pesticides, and hydrocarbons (petroleum).

Drumline: ↑ Shark-friendly fishing method, where the shark can constantly swim after being hooked.

Omega-3 and–6 Fatty Acids: ↑ A group of essential fatty acids that is important for the survival and health of all animals.

Saturated Fats: ↑ A type of fats derived from both animal fats and plant oils that tend to be solid at room temperature and are unhealthy when in excess in an animal’s body.

Cardiovascular Disease: ↑ Are some conditions that affect the heart and blood vessels, like when there are high levels of blood lipids (fats).

Phytoplankton: ↑ Microscopic algae that like plants use nutrients, carbon dioxide, and sunlight to grow while releasing oxygen.

Food Chain: ↑ Feeding relationships between organisms, i.e., prey and predators. The chain consists of producers, consumers, and decomposers.

Acknowledgments

We thank the members of the University of Miami Shark Research and Conservation Program, especially to Abigail Tinari for research support. We also thanks Kelly Quinn and Alexandre Huber courtesy of using their illustrations and Dr. André Julião for his amazing review. This work was supported by the Disney Conservation Fund, the Batchelor Foundation Inc., Save Our Seas Foundation and Fundação de Amparo à Pesquisa do Estado de São Paulo- FAPESP (Grants #2014/16320-7 and #2017/25273-0, Ph.D.’s scholarship to BR).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.