School of fish: Studying sand tiger sharks' complex migration patterns
Sand tiger sharks have already started migrating through the Delaware Bay and a new study in the journal Scientific Reports looks into how they take their southern path.
In 2012, University of Delaware and Delaware State University researchers implanted transceivers in 20 sand tiger sharks to study their movement and interactions. Previously, around 300 acoustic transmitter tags were deployed around the Delaware area and nearby over the last several years.
They were wondering if groups of sharks that formed in Delaware stayed together throughout the whole year or if the sharks separated from one another when they left the Delaware Bay.
Danielle Haulsee, one of the researchers involved in the study, said it appears that both are true. The sharks then came back together around Cape Hatteras. The group then spread out again and the sharks dispersed to be near different sharks in the late winter/early spring. In the late spring/early summer, they came back together and reformed a group and came back to the Delaware Bay.
"At times the groups dispersed and sharks formed smaller groups or were solitary, while at other times groups of sharks joined together," Haulsee said.
Sand tiger sharks are listed as a species of concern by the National Marine Fisheries Service. Johnny Moore from Delaware’s Department of Fish and Wildlife said they’re also a prohibited species — if caught fishing, they must be released into the water.
“There’s a lot of uncertainty about population status, at least in the Delaware Bay,” Moore said. “We’re trying to limit the impact humans have on them.”
They’re one of the largest shark species that come to the area, and the Delaware Bay is important for their growth, Moore said. It acts as a nursery area where sand tiger infants grow and develop.
Haulsee said researchers are beginning to realize the importance of the Delaware Bay to this species.
“I think we’re finding out more and more that the Delaware Bay is an important area for them to come in the summer time, rest and recover from migration and there’s a lot of food resources for them. There are habitat spaces they prefer. I think it’s important for the juveniles to come in and have shallow water, there’s lots of fish for them to eat and grow.”
Wanting to learn more about their habits is why the researchers felt it is important to dig deep into their social behavior, Haulsee said. Data they accumulate can be used in informing a conservation plan in the future, especially with the dwindling number of sharks.
The low numbers are in part due to low reproductive rates: Female sharks give birth to one or two viable baby pups every other year due to intra-uterine cannibalism — the ability of the strongest fetus to eat the others. Sand tiger pups are born with a well-developed ability to defend themselves against predators, but due to the lack of breeding, it takes a long time for the population to rebound.
But Haulsee said researchers are limited in determining how social the sharks are. The technology they used - tags that detect soundwaves - only showed that sharks were traveling close to each other. They did not reveal specific types of social interaction.
“Some of the techniques we use to look at the data are taken from social network analysis. Right now, we’re inferring what could be going on behaviorally based on where the sharks are what they could be doing during the different times of the year. We don’t have video feedback on what they’re doing near each other so we can’t really say to each other that they’re close to each other because they wanted to be close to each other, or they were both feeding on a food resource or ideal water temperature.”
Haulsee added when the group continues the study in the future, they hope they can tag and recapture more sharks to create a larger pool of data.
Journal reference: Danielle Haulsee, Dewayne Fox, Matthew Breece, Lori Brown, Jeff Kneebone, Gregory Skomal, & Matthew Oliver. Social Network Analysis Reveals Potential Fission-Fusion Behavior in a Shark. Scientific Reports 6. 2016: 10.1038/srep34087.
Ed. note: This story was edited to clarify that the groups of sharks disperse and come back together around Cape Hatteras; they disperse again and come back together at the Delaware Bay.