This is a guest post by graduate student Amanda Ingram. Amanda is currently a Masters of Marine Affairs Graduate student at the University of Rhode Island. She graduated in 2010 with a double Bachelors degree in Environmental Science and Communications Journalism from California Lutheran University. Her research interests include science journalism and the economic and environmental impacts of Shark conservation policies in the Caribbean.
Sulikowski, James A. et al. Seasonal and life-stage variation in the reproductive ecology of a marine apex predator, the tiger shark, Galeocerdo cuvier, at a protected female-dominated site. Aquatic Biology. Vol. 24: 175-184. February 22, 2016. DOI: 10.3354/ab00648
Establishing a Safe Haven
In a world where migratory marine apex predator populations are in steep decline due to harvesting and overfishing, the Bahamas’ Tiger Beach provides a safe haven for female Tiger Sharks to mature, gestate, and give birth.
Bahamian waters provide protection for sharks through multiple policies. The Bahamas’ Economic Exclusive Zone prohibits long-line fishing, a process by which a long fishing line is used with baited hooks along the line at intervals. This fishing technique tends to catch a lot of bycatch (species other than the one being targeted), including sharks. The Bahamian government also passed a 2011 amendment to the Fisheries Resource Act which established all Bahamian waters as a Shark Sanctuary. This Shark Sanctuary prohibits the fishing of any shark species by any type of fishing technique. These laws are a huge contributor to the conservation of shark species in the Caribbean and create protected areas for species such as the Tiger Shark to thrive.
Tiger Beach is a renowned ecotourism dive spot off the coast of Grand Bahama Island, where thrill-seeking divers can dive cage-free with many different species of sharks, including large female Tiger Sharks, which tend to congregate in this area. Researchers speculate this area serves as a refuge from harassment by mature male Tiger Sharks, as mating rituals are typically violent and some females are still immature. The gestation period of Tiger Sharks is typically about 15 months long with only a few pups produced each time. The shallow, warm, and calm waters of Tiger Beach provide ideal conditions for gestation, which may be a reason why these female Tiger Sharks gather seasonally and return to the same area year after year.
Researchers set out to establish whether or not Tiger Beach is largely occupied by mature, pregnant females, and the potential effects this has on the conservation of Tiger Sharks. The scientists conducted five research expeditions over the course of three years to look for seasonal variations.
The researchers used a passive fishing technique in which sharks were caught using a circle-hook drumline. They were slowly brought onto a platform partially submerged in water to promote vitality and limit stress. Once captured, the scientists performed several sampling techniques to get an accurate representation of maturity and gestation. First, they sampled blood from the sharks’ caudal fins to measure reproductive hormone levels within the plasma. They also measured the length of each shark and recorded the gender. In later stages of the project, they also examined mating scars and used a portable ultrasound transducer to scan the belly of the shark to assess whether or not the shark was with pup(s).
The researchers analyzed the data to determine whether the majority of the sharks found in these waters were mature, gestating females, or if it was just a popular site for females of all ages.
What They Learned
In the early stages of the project, the researchers found that there were no differences between the hormone levels of immature females and mature, pregnant females, and so these measurements provided inconclusive results. However, sharks could be differentiated by length. According to previous studies, female tiger sharks are generally mature at 300 cm or greater in length. Therefore, length data was used to figure out if the females were mature or not. This, combined with the hormone analysis, provided the scientists with enough information to estimate the probability that mature females were pregnant using statistical analysis. In addition, the addition of ultrasonography provided a much more comprehensive and definitive method to establishing gestation in the sharks. They were able to see the shark pups on the ultrasound to confirm that the shark was indeed pregnant (Figure 1).
The scientists caught a total of 65 sharks, 59 of which were female: 19 (32%) immature females, 17 (29%) mature non-pregnant females, and 23 (39%) mature pregnant females. This revealed a gender ratio of 1 male shark for nearly 10 females, which led them to the conclusion that this site is predominantly occupied by females and the males here are quite outnumbered. This is particularly important as most areas do not have such a disparity in the number of males versus females, leading researchers to speculate that this particular location may be essential for gestation and birthing of these species, or even possibly a nursery for newborn sharks to thrive.
The researchers also found that there were more mature non-pregnant sharks during the months of May, October, and November, and more pregnant sharks found in the months of October and December. The same sharks were found seasonally, so researchers concluded that this location is a regular stop in their migration patterns and perhaps the Autumn is a popular time for this species to come here to gestate (Figure 2).
Many species of sharks are endangered or threatened with extinction due to the shark fin trade. Shark-finning is the process of reeling in sharks, cutting off their fins, and either throwing the bodies back into the ocean or leaving them to decay on shore. These fins are used to make shark fin soup, which is considered a delicacy in Chinese cuisine. Not only is the shark-finning process wasteful, it is also not sustainable. Sharks are fished out of the oceans at a current rate of about 70-100 million per year due to shark-finning and bycatch. If humans were killed at that rate, the United States would be devoid of human life within 4 years.
This study suggests that Tiger Beach provides a crucial habitat for female Tiger Sharks of all ages, especially pregnant and mature females, which are vital to the regeneration of populations to keep them from going extinct. Bahamian waters provide a protected zone for the reproduction of this species, thanks to current legislation, which will likely have a significant impact on the conservation of these species in the years to come.
This is your Ocean: Sharks https://www.youtube.com/watch?v=K9bjTk5Hszg
I am the founder of oceanbites, and a postdoctoral fellow in the Higgins Lab at Colorado School of Mines, where I study poly- and perfluorinated chemicals. I got my Ph.D. in the Lohmann Lab at the University of Rhode Island Graduate School of Oceanography, where my research focused on how toxic chemicals like flame retardants end up in our lakes and oceans. Before graduate school, I earned a B.Sc. in chemistry from MIT and spent two years in environmental consulting. When I’m not doing chemistry in the lab, I’m doing chemistry at home (brewing beer).