Sharks and BATs: How Scientists are Tracking Marine Apex Predators
Written by Spence Cox, this article was selected as a winner of our 2023 High School Science Communication Challenge. Cox is a student at Academic Magnet High School in Charleston, South Carolina.
Despite technological advances, comprehensive tracking of marine apex predators has eluded scientists who seek to better understand the life cycles of these animals. Apex predators are highly vulnerable to anthropogenic climate change and pollution, and marine species are at an even greater risk. The future conservation of these species — specifically sharks — depends on a greater understanding of their preferred habitats and movement through the ocean. Additionally, accurate information about sharks’ reproductive cycles can improve our understanding of their population and biology in the wild. Unfortunately, traditional methods of data collection on the reproductive cycles of marine animals, which usually involve killing parent and newly-born specimens for investigation, are unsuitable for sharks. Researchers James Sulikowski from Arizona State University and Neil Hammerschlag from the University of Miami have developed and deployed a tracking device for marine predators which seeks to resolve these issues and break ecological barriers.
The researchers’ device, created with wireless technology company Lotek, is a tag placed within the uterus of a pregnant animal. When implanted correctly, the device will remain inside the animal until it gives birth, at which point it is deposited into the ocean with the newborns. After the tag exits during birth, it will relay location data periodically to ARGOS (Advanced Research and Global Observation Satellites) until its battery depletes roughly 12 days later. The scientists’ specialized device varies significantly from terrestrial designs in order to function correctly within large marine predators. Designed to be egg-shaped, 6 cm in length by 2.5 cm in width, and weigh just 42 grams, the intrauterine tag conforms with the shape of the embryos and does not disturb the mother.
Above: (A) Image of the intrauterine tag. (B) Insertion of the tag into the female tiger shark. Adapted from Figure 1 in Sulikowski et al., 2023.
These devices, termed Birth-Alert-Tags (BATs), have been implanted and tracked in two individuals since 2019: one tiger shark and one scalloped hammerhead shark. The tiger shark was caught on December 15, 2019, offshore of the Bahamas. Once the specimen was confirmed to be pregnant, the tag was safely inserted. A satellite tracker was also attached to the shark’s fin to alert the researchers when the shark surfaced, providing a map of her movement during pregnancy. After 157 days, the BAT signaled that the tiger shark had given birth near the southern Georgia coast.
The scalloped hammerhead was caught offshore of North Carolina on March 29, 2022, and researchers completed a similar procedure to attach the tag. However, unlike the first trial, the researchers used an improved fin tag, which relayed location data for the shark daily before the BAT was ejected. The hammerhead moved south near Hilton Head, South Carolina, for 46 days before giving birth. The location of birth was marked by the BAT, allowing the researchers to study the area in the following days.
Above: Maps of the movement of the tiger shark (B) and scalloped hammerhead shark (C). The ejection point of the BAT is marked with a yellow star and the red points plot the movement of the sharks before giving birth. Adapted from Figure 2 in Sulikowski et al., 2023.
The findings of this study are promising for the future of reproductive tracking in ocean predators. Intrauterine tracking device technology continues to improve, and scientists are eager to use the methods to study whales, dolphins, and other shark species. Specific reproductive cycle location data allows scientists to test long-held theories, such as whether sharks return to previous birthing sites for future pregnancies. Ultimately, widespread deployment of these devices offers rewards of greater protection and understanding of large marine species.