Hammerhead’s Night Out: The Day-to-Day Life of Hammerhead Sharks
This post belongs to a series written by students in the Conservation Biology course BSC4052 at the University of South Florida. This course provides an overview of major themes in conservation practice and related applied problems in biology, including: population ecology in the context of conservation, patterns of diversity, valuing diversity, threats to diversity, management actions and strategies for preserving diversity.
This guest post is authored by Juan Cortes, a third-year undergraduate student at the University of South Florida (USF) in Tampa, FL. He will receive a Bachelor’s degree in marine biology in August 2020. Thanks to his upbringing in the community surrounding the Tampa Bay estuary, Juan is now specializing in wetland ecology. He currently volunteers at the Florida Aquarium and as a research assistant in USF’s marine ecology lab. After graduation, he plans to pursue employment with Florida Wildlife or one of the great conservation organizations in the Tampa Bay area and beyond.
Article Reference: Aldana‐Moreno A, Hoyos‐Padilla EM, González‐Armas R, Galván‐Magaña F, Hearn A, Klimley AP, Winram W, Becerril‐García EE, and Ketchum JT. 2020. Residency and diel movement patterns of the endangered scalloped hammerhead Sphyrna lewini in the Revillagigedo National Park. Journal of Fish Biology 96:543–548. https://doi.org/10.1111/jfb.14239
We all recognize this fearsome, seafaring shark for its unique look. Hammerheads (Sphyrna lewini) are, understandably, one of the most charismatic species in the world of marine biology. However, these sharks have suffered major declines in recent years due to direct hunting and overfishing of their prey. As a result, they have secured a spot on the endangered species list. Now, researchers like Alejandro Aldana-Moreno and his colleagues are springing into action to better understand and protect these large, beautiful sharks.
Addressing the Problem
In order to properly protect and restore hammerhead populations, scientists must first understand where they live and how they move around their habitat on a daily basis. These day-to-day movement are known as diel movement patterns and can be instrumental in understanding an organism’s behavior. When Aldana-Moreno and his team noticed a lack of information concerning the diel movement patterns of hammerheads, they set course for Revillagigedo National Park, the largest marine reserve in North America and a well-known hammerhead hot spot. The reserve consists of a small, four-island archipelago off the western coast of Mexico. The researchers placed a total of 12 acoustic receivers on the coasts of the archipelago’s various islands. Acoustic receivers work by collecting signals when tagged sharks pass within a certain distance. The researchers gave special attention to a known hot spot of hammerhead activity: the archipelago’s easternmost island, San Benedicto. Here, they deployed receivers at four distinct sites and got in the water with the sharks to tag them by hand.
Tag! You’re it!
In 2013, the researchers ventured out into the waters surrounding San Benedicto. While scuba and free diving, they tagged 11 adult sharks, each between 200 and 300 cm (6.5 to 9.5 feet) in length. Then, they tracked the sharks using acoustic detections over the next one to two years. They paid close attention to the times of day when detections occurred. The end result was a complete time table of detections of the eleven sharks from the four receivers surrounding San Benedicto.
Hammerheads Working the Night Shift
Researchers found that the tagged sharks preferred to spend their mornings (from 6:00am to 12:00pm) moving between two distinct sites on the island. Next, they traced them to two separate sites in the afternoons (from 12:00pm to 6:00pm). They observed very high numbers of detections at these sites (up to 40,000 at one site!), confirming that the sharks move between these areas on a habitual, day-to-day basis. Perhaps the most important finding from the study was that researchers observed almost no detections at any of the sites at night (between 8:00pm and 6:00am).
They predicted that the sharks were visiting the sites at San Benedicto Island during the day for cleaning and social behaviors and leaving overnight to forage and feed offshore. The data collected confirmed this hypothesis. Some of the researchers involved with the study have observed “cleaning stations” in which small reef fishes like hogfish and angelfish will eat parasites and debris off the skin of the hammerheads, although exactly how and why they do this remains unclear. The researchers used their observations to illustrate a pattern of diel movement that can then be used to create new policies for the protection of the hammerhead species.
A Safer Commute
The results of the study show a small congregation of hammerhead sharks in shallow waters that inhabit different locations in the mornings and afternoons. So how can all of this information help us better protect the sharks? Aggregations like these can be dangerous to the sharks because it makes them more vulnerable to capture by poachers.
Legislation is a great way to combat this issue: by creating policies that protect small, inshore shark aggregations, we can give the hammerheads a fighting chance. The establishment of marine protected areas has proven an effective measure for the conservation of populations like these. As citizens interested in conservation, it is imperative that we use our collective voice to speak up for these endangered sharks.
Groups like Project AWARE and the Shark Trust are actively working to maintain and restore shark populations around the world. Donating to these organizations and encouraging your legislators to enact shark-friendly policies are great ways to support our wide-faced neighbors to the west.
Featured Image: A scalloped hammerhead seen off Mexico’s West Coast. Image courtesy of Richard Hermann and NOAA.