Turtles in the harbour of Argostoli are often observed close to the harbour wall, regardless of external disturbances such as humans and loud noises. It seems as though many turtles have become habituated, slowly decreasing their levels of fear towards humans. The turtles that do venture towards the harbour walls are regularly seen feeding on foods that are added to the water by humans and are not always a part of their natural diet. These food items include, are however not limited to, several species of fish, rays or even bread. Food sources other than those part of their natural diet can have negative impacts on the turtles.
Through the regular feeding of turtles by humans, turtles start to associate the two. They begin to spend more time near regions with more boats and people with the hope of getting food, which is easier than their natural foraging behaviours. Turtles may also gather around fishing nets or fish farms to catch easily accessible fish. This behaviour consequently leads to a larger number of injuries through entanglement in fishing lines, ingestion of hooks, or by boat and propeller strikes. Furthermore, turtles have been solitary animals for over 200 million years. By looking at our data we can see that anthropogenic pressures increase the frequency of aggressive events between individuals, thereby changing their natural behaviours on a very short timescale.
Feeding the turtles also creates a range of biological and nutritional issues. A few studies have assessed the blood biochemical markers of green sea turtles (Chelonia mydas) in regions in which supplemented food is added, in a very similar setting to the one observed in our harbour. Turtles regularly seen feeding on fish scraps were observed to have higher levels of certain enzymes, some of which were related to the early stages of liver disease. Liver disease was not the only worrying observation as fed turtles also showed signs of cardiovascular disease as well as differences in certain vitamins and minerals. Similar effects would likely be observed in loggerheads (Caretta caretta).
These are some of the reasons why it is so important to not feed the turtles in the harbour and rather just observe and admire their unique beauty!
For more information visit these papers:
Monzón-Argüello, C., Cardona, L., Calabuig, P., Camacho, M., Crespo-Picazo, J.L., García-Párraga, D., Mayans, S., Luzardo, O.P., Orós, J. and Varo-Cruz, N. 2018. Supplemental feeding and other anthropogenic threats to green turtles (Chelonia mydas) in the Canary Islands. Science of the Total Environment, 621: 1000-1011.
Stewart, K., Norton, T., Mohammed, H., Browne, D., Clements, K., Thomas, K., Yaw, T. and Horrocks, J. 2016. Effects of “swim with the turtles” tourist attractions on green sea turtle (Chelonia mydas) health in Barbados, West Indies. Journal of Wildlife Diseases, 52(2s): 104-S117.