By: Emily Meese
Hi everyone! My name is Emily and I am a Masters student in Dr. Chris Lowe’s CSULB Shark Lab. My Master’s thesis aims to characterize horn shark movements and behavior to better understand their ecological role in the kelp forest ecosystem. Horn sharks may not be the most popularly known shark, but they are a very common species in southern California, especially here around the isthmus of the island. Horn sharks are nocturnal, bottom dwelling sharks with two horns used as defense. My goals as a Wrigley Fellow are to understand where horn sharks move during their journeys at night and to quantify how active they are, or in other words how much energy they use during a night. By knowing how many calories they burn in a 24 hr period, I can estimate how much food they would need to eat, and therefore estimate their predator impacts in the kelp forest.
The intriguing thing about why we care how much these sharks eat is that they are one of the few urchin predators in southern California. Horn sharks have plates of teeth that can crush the urchin, and they have a strong suction force to extract the urchin gonads to eat. If horn sharks eat enough urchins, they may be important regulators of urchin populations, known as a keystone predator. This matters because urchins are kelp grazers. Therefore, if there are too many urchins in a population with no predators regulating them, the urchins will take over the kelp forest, creating the area into a low-diversity urchin barren. However, horn sharks may help regulate those populations, allowing the kelp forest to grow and flourish.
To find out where and how horn sharks spend their time, I created a custom tag package (Figure 1). The first part of the tag is an acoustic transmitter that pulses every 2 seconds, and allows me to follow the shark in a boat (what we call tracking) while listening to the pulse to know where the shark is. The second part of the tag is an accelerometer data logger which records the sharks 3D body movements, as well as ambient water temperature and depth. Put these two parts together and I can determine where sharks are, what they are doing in those locations (i.e., swimming, resting, eating), and also have an idea of the environment the sharks are in (Figure 2).
The most challenging part of field work is tracking these sharks at night. I follow the sharks for 48 hrs from our whaler. During the day, the task is easy since they stay under a rock sleeping, but as soon as sunset occurs, the shark is on the move and the real work begins. The summer here at Catalina has many perks, however warm summer nights are not one of them. It’s cold, it’s dewy, but it’s also a fun adventure to see where each shark will go, how far they may travel, and how fast they can get from reef to reef.
Last summer, I did nine 24 hr tracks, and this summer I have done six 48 hr tracks (4 more to go!). So far, I have learned that these sharks can move quite a bit more than expected. Many think the horn shark is a lazy shark because of their nocturnal and bottom dwelling behavior, but when they get moving they sure do know how to cruise and take off! In addition, using the acceleration data I have started to model how these sharks spend their time in regards to water temperature. Most of the sharks dive to relatively deeper, colder water before coming back to warmer, shallower water. The cold and warm temperatures can affect the sharks physiological processes (i.e., metabolism, appetite) so in order to offset the costs of traveling long distances, they may be behaviorally thermoregulating between different temperatures to speed up foraging rates, and slow down digestion rates to maximize energetic gain.
There’s no way my Master’s thesis would be possible without the Wrigley Fellowship. Having the convenience to live on the island and live in the horn sharks backyard is extremely helpful to get a sense of how these sharks move among all the different reefs in the isthmus area. I still have a few more tracks to complete before summer is over, and much to analyze, but it is rewarding to see the bits and pieces of the story of horn sharks come together and be able to better answer questions about their role in the kelp forest.