The islands in the Eastern Tropical Pacific have many different types of sharks, and one of the most common is the scalloped hammerhead shark. These sharks usually swim in groups during the day around the islands, and at night they separate and go out into the open ocean to find food.
A lot is known about where adult scalloped hammerheads travel, especially around the Galápagos Islands in Ecuador, but not much is known about where juvenile scalloped hammerheads can be found and where they travel to.
Baby scalloped hammerhead sharks are typically found in safe, shallow bays where there are fewer predators around. Mangroves are ecosystems that are utilized by many shark species as nurseries. However, in the Galápagos Islands, most of the coast is made up of rocky areas that are hit by many waves, making it hard for mangroves to grow. As a result, there aren’t many places for baby scalloped hammerheads to grow up.
What Is A Shark Nursery?
For an area to be considered a shark nursery, it must meet three conditions:
- Baby sharks have to be seen there more often than in other places
- Baby sharks have to stay in that area for a long time
- Baby sharks have to come back to that area year after year.
Additionally, the area has to help baby sharks grow and survive better than other places where they might live.
The Discovery & Data Collection
In 2017, some fishermen informed the authors that they saw baby hammerhead sharks feeding near the shore of Puerto Grande, a mangrove-fringed bay on San Cristobal Island’s coast. This was unusual because the authors had never caught baby hammerheads in that area before. So, the authors went to the area and used a beach seine, which is a type of net, to catch them. They put the net around the shallow water near the shore and caught four baby hammerheads. After that, the authors returned to the same spot 15 times over the next three years to see if they could catch more. They used the same method each time and went to the spot during different months of the year.
To find more baby hammerhead sharks, baited underwater cameras, known as BRUVS, were used. The cameras were placed in different bays around the island where beach seines couldn’t be used. The cameras were left underwater for one hour at a time and put in the middle of each bay where the water was 3 to 4 meters deep. The bait used for the camera was either chopped mullet or skip-jack tuna.
Additionally, the movements of four baby hammerheads were tracked in the bay using tags. Information from the tags showed that the baby hammerhead sharks stayed in the bay 89% of the time, meaning that the pups were in the bay almost every day.
The Result?
The results of the study showed that Puerto Grande can be considered a nursery for juvenile scalloped hammerheads.
The beach seine surveys and underwater footage showed that baby sharks return to the area every year, which satisfies condition three of an area to be considered a nursery. The underwater cameras placed in other bays around the island didn’t detect any hammerhead pups, so the first condition that baby sharks have to be seen in one area more often than another was met. Lastly, the data from the tags showed that baby hammerhead sharks stayed in the bay 89% of the time, which meets condition two, that baby sharks have to stay in that area for a long time.
Scientific Study: Discovery of a putative scalloped hammerhead shark Sphyrna lewini (Carcharhiniformes: Sphyrnidae) nursery site at the Galápagos Islands, Eastern Tropical Pacific by Yasuní Chiriboga‐Paredes, Ángela Palomino, Lauren Goodman, Florencia Córdova, Viviana Páez, Manuel Yépez, Salvador Jorgensen, Daniel Armijos, Diana Pazmiño, and Alex Hearn.
References
1. Chiriboga-Paredes, Y., Palomino, Á., Goodman, L., Córdova, F., Páez, V., Yépez, M., Jorgensen, S., Armijos, D., Pazmiño, D., & Hearn, A. (2022). Discovery of a putative scalloped hammerhead shark Sphyrna lewini (Carcharhiniformes: Sphyrnidae) nursery site at the Galapagos Islands, eastern tropical Pacific. Environmental Biology of Fishes, 105(2), 181–192. https://doi.org/10.1007/s10641-021-01207-3
2. Gallagher, A. J., & Klimley, A. P. (2018). The biology and conservation status of the large hammerhead shark complex: The great, scalloped, and smooth hammerheads. Reviews in Fish Biology and Fisheries, 28(4), 777–794. https://doi.org/10.1007/s11160-018-9530-5