One of the islands of Tahiti, Mo’orea (also spelled Moorea) is a haven for those who love a varied coral reef ecosystem and colorful mix of marine life. This includes the local stingrays and reef sharks, who are known to swim around the sand bars, reefs, and lagoons in this jewel of the South Pacific island. An island of volcanic origin, it is famous for its lush green cliffs, inspiring waterfalls, and baby sharks.
It’s the baby sharks that has brought scientist Dr. Ian A. Bouyoucos, a post-doctoral fellow at the University of Manitoba, here.
Like many fish species, sharks often use coastal and estuarine areas as nurseries for their babies (called ‘pups’) due to their shallow protected waters and high abundance of prey. According to Bouyoucos, the ecological benefits of shark nursery areas are well studied, yet the physiological mechanisms that enable sharks to exploit coastal habitats, especially those that experience extreme and dynamic temperatures, remain understudied. He further explained: “We wanted to understand how some sharks use hot, shallow waters as nursery areas for their young, while others do not. Additionally, we wanted to know whether the physiological mechanism we tested, temperature tolerance, could be acted on by climate change.”
Past research has dictated that in order for an area to be labeled a ‘shark nursery,’ it must follow three rules: (1) the relative abundance of sharks is greater on average here than other areas; (2) sharks exhibit site fidelity, returning or remaining in the area for extended periods of time; (3) the sharks use this area repeatedely over time. While baby sharks were seen darting in and out of hidden pockets within the Mo’orea coastline, Bouyoucos and his team determined whether ten sites around the island of Moorea, French Polynesia, satisfied nursery area criteria for neonate populations of blacktip reef sharks (Carcharhinus melanopterus) and sicklefin lemon sharks (Negaprion acutidens) using five consecutive years of abundance surveys. They found several potential blacktip reef shark nursery areas, but during different sampling years, and identified a single sicklefin lemon shark nursery area that remained consistent during the entire five-year study.
Both of these sharks get their common names from their looks. Found in shallow marine waters around coral reefs throughout the tropical Indo-West and Central Pacific, the blacktip reef shark (Carcharhinus melanopterus) sports a distinct black tip on their dorsal fin. Meanwhile, the sicklefin lemon shark (Negaprion acutidens) is a stocky, yellowish shark found in the same region, common on coral reefs as well as in shallow, sandy-bottom lagoons, and mangrove swamps. Younger sicklefins and blacktip reef sharks are often observed on reef flats, where water is so shallow that their infamous dorsal fins are exposed.
Once the nurseries of these two young predators had been identified, the researchers defined patterns of their nursery area use and temperature-dependent physiological performance. “We know that warming temperatures can speed up the metabolism of cold-blooded animals, and, depending on how hot it gets, can speed up – or slow down – growth rate. What we wanted to measure was just how strongly temperature affected these processes – growth and metabolism – in our shark study species,” the authors explained in their publication regarding this work. “To look at growth, we measured water temperatures in the sharks’ habitat for almost five years and, over the course of the study, caught, measured, and recaptured most sharks after up to a year in the wild. By comparing the measured temperatures experienced by sharks with their growth, we were able to test whether sharks grew more than expected for the time they were in the wild based on the water temperatures they experienced. To look at metabolism, we tested sharks in the lab. Using specialized chambers called respirometers, we measured sharks’ oxygen uptake rates (this is like measuring VO2max in athletes) during simulated exposure to a daily temperature regime. That way, we knew how much the metabolic rate of a resting shark changed as the daily temperature went from a daily low to a daily high.”
The team was elated to find that growth and metabolic performance were not strongly affected by temperature in either species, as they had previously hypothesized. “Thus, thermally insensitive physiological performance may be a trait that [sharks and their relatives] exhibit in thermally variable coastal habitats, including shark nursery areas,” said Bouyoucos. “[To be honest,] our most surprising finding was that, for one species we studied, different habitats played different roles as potential nursery areas during different years of our study!”
While not all young sharks use nursery areas, Bouyoucos wants to stress that “these young shark habitats are still ecologically important!” They are vital for many to have ‘la vie heureuse,’ as they say in Tahiti – a happy life.