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Sharks and the City

Human beings are notorious for altering the natural environment to fit our needs. As architectural and engineering methods have developed over the decades, our buildings have grown larger and taller, and as the global population has exploded, we have built increasingly wide, sprawling cites. In response, animals have been forced to adapt to a new habitat: the urban environment. Many have done this with great success, with especially bold and explorative species, like foxes and seagulls, and adaptable animals, like crows, doing incredibly well within the city. We know a lot about how urbanisation affects wildlife in the terrestrial environment, but what about marine creatures? Does our activity on land affect those living in the ocean? Are animals like sharks affected by urbanisation?


Nurse sharks in a non-urbanised habitat (Image Credit: Yann Hubert / Shutterstock)

How Does Activity on Land Affect Marine Species?

The marine environment - especially in nearshore habitats like reefs, mangroves and seagrass beds - is affected by many human activities on land. The process of urbanising a coastal region has a direct impact on marine animals, like sharks, as construction can physically alter the natural geography. Development might mean altering the shape of the coastline, closing off some waterways, or changing the direction and quantities of water flow. Furthermore, construction methods, like drilling and pile-driving, can also lead to noise pollution, which can disrupt marine life. There can also be chemical pollution and litter washing into the oceans and injuring marine life (Knip et al, 2010 & Rangel et al, 2021).


There can also be indirect effects of urban development. For instance, runoff from agricultural land, which is unnaturally rich in nutrients, can overstimulate aquatic plant growth, encouraging invasion by alien species or throwing off the natural balance of the ecosystem (a process known as “eutrophication”) (Knip et al, 2010 & Rangel et al, 2021).


The process of eutrophication (Image source: US EPA / WikimediaCommons)

Sharks Accumulate Toxic Chemicals in Very High Concentrations

As predators, high up on the food chain, sharks absorb minerals and compounds from their food. This means the sharks absorb all the chemicals that smaller fish are exposed to and concentrate these within their bodies. Pollutants like heavy metals, toxic chemicals and pharmaceuticals which run into oceans from the land, can affect sharks especially because they build up to harmful levels within their bodies. If we measured the levels of, say mercury in a fish, it would be lower than in the shark that eats those fishes. This process is known as “biomagnification” (Moorhead, 2019 & Rangel et al, 2021).



City Life versus Country Life

Scientists are now trying to understand exactly how coastal urbanisation impacts on neighbouring sharks... In one study, researchers in Miami took samples of blood from nurse sharks they caught (and safely released) near the city. The scientists analysed the levels of cholesterol, fatty acids, ketones and triglycerides in the blood plasma. These compounds, known as “biomarkers”, can tell us how well sharks are storing and utilising their energy sources, and can help us to determine where the sharks find their food (Moorhead, 2019).


Locations where sharks were tested for blood tested (Rangel et al, 2021)

The scientists discovered that there was a significant difference in the concentration of these biomarkers in the plasma of sharks which lived off the coast of the metropolitan area versus individuals that lived within the nearby Biscayne National Park. They found that the sharks with a better body condition, had higher concentrations of triglycerides in their blood, and that the urban sharks had lower levels of fatty acids and higher levels of triglycerides in their blood (Moorhead, 2019).


Levels of blood biomarkers associated with body condition in urban / non-urban nurse sharks (Moorhead, 2019)

To advance on these findings another study was conducted in the same area. Scientists again looked at the blood biomarkers of nurse sharks, including analysing of the concentrations of the fatty acids omegas -3 and -6, but they also considered bacteria levels as well. They found that the urban sharks had significantly higher levels of bacterial markers, where the sharks from the Biscayne National Park had much higher levels of fatty acids in their blood (Rangel et al, 2021).

Nutritional quality of the diets of urban and non-urban nurse sharks (Rangel et al, 2021)


Diet & Exercise

So what does all this mean? The scientists concluded that the sharks in the urban area were eating nutrient-poor foods, which were high in calories, but low in fatty acids. They suggested that this might be happening because the city sharks have more prey available to them. Firstly, because they are probably scavenging on human food sources (like fishermen's discarded catches). Secondly, because the unnatural light and the presence of man-made structures (like docks, jetties, and artificial reefs) causes fish to aggregate in high numbers around the urban area (Moorhead, 2019).


Nurse sharks in crystal clear waters (Image Credit: Jakob Owens / Unsplash)

The scientists also theorised that the levels of these biomarkers might have been affected by the sharks’ activity levels in the different habitats. In the urban environment, the high prey availability meant that the sharks were able to expend less energy to catch a meal. Whatsmore, due to the presence of so many anthropogenic structures and human debris, these sharks did not have to travel so far to find a safe shelter to rest in (Moorhead, 2019).


This might sound like good news… sharks have more food available to them in urban environments and have more safe places to relax… Right?




You Are What You Eat

Well… don’t forget that those fatty acids were significantly lower in the urban sharks. High concentrations of fatty acids in the blood can suggest that a shark is eating very high quality food items. Therefore, these studies actually show us this study seemed to imply that whilst the urban sharks had easy access to more food, they might not actually have been eating the healthiest diet (Moorhead, 2019).

Nurse sharks near a dock (Image Credit: Cam Green / Pexels)

Basically, the non urban sharks had a healthier diet… While the urban sharks were able to survive, it’s like a human being living solely on McDonald’s rather than eating 5-a-day.


The researchers determined that this was happening because the sharks' food webs had shifted and the diets of other animals in their community had deteriroated. The non-urban sharks had a broad diet; feeding on fish, squid, octopi, crabs and shellfish. Whastmore, In the national park, the “consumers” (animals which eat plant life) were eating brown algae, sponges, seagrasses and corals, which are rich in fatty acids. These fatty acids subsequently bioaccumulated in the sharks (Rangel et al, 2021).


In comparison, the urban sharks had a much more restricted diet; primarily predating on fish (known as “piscivory”). The fatty acid profiles of these sharks implied that their prey were eating very different foods, like phytoplankton. Whatsmore, the bacterial markers suggested that the sharks' prey had been exposed to significant amounts of sewage and other decomposing organic materials. This is indicative of eutrophication in the urban environment (Rangel et al, 2021).


Comparative food webs of urban and non-urban nurse sharks in Miami (Rangel et al, 2021)

Sharks Along urban Coastlines Are Less Healthy!

These findings are quite worrying because fatty acids are critical for physical development, immune system functioning and reproductive success. Omega - 6 is vital for immune and inflammatory responses, so sharks which are low in this fatty acid are likely to have a weakened immune system and be more susceptible to disease. It is also important for cardiac and brain development (Rangel et al, 2021).


As the sharks that were sampled in this study were juveniles, these deficiencies could mean they suffer reduced growth rates, or poor health during an important life stage. Similarly, such deficiencies have also been associated with “feminisation” in male fish. This means that the males lack the natural hormones necessary to develop to maturity. This could have serious population-level implications, as it could limit the ability of these sharks to breed in the future (Rangel et al, 2021).


Nurse shark (Image Credit: Stevelaycock21, CC BY-SA 4.0 / WikimediaCommons)

These studies show us that our activities on land do not only have an impact on terrestrial animals, but can have far-reaching implications, even in the oceans. If we continue to develop and urbanise our planet at the current rate, there is no telling how significant the damage on wild animals might be. We must consider the way we alter our habitat and strive to live more in harmony with the natural world around us.



References

Knip DM, Heupel MR & Simpfendorfer CA (2010). Sharks in nearshore environments: models, importance, and consequences. Marine Ecology Progress Series, 402, 1–11. Access online.


Moorhead SG (2019). Variation of Body Condition and Plasma Metabolites in a Population of South Florida Nurse Sharks, Ginglymostoma cirratum. Masters Degree Thesis, University of Miami, USA. Access online.


Rangel BS, Hammerschlag N & Moreiraa RG (2021). Urban living influences the nutritional quality of a juvenile shark species. Science of The Total Environment, 146025. Access online.


By Sophie Adele Maycock for SharkSpeak

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