Seasonal Occurrence, Relative Abundance, and Migratory Movements of Juvenile Sandbar Sharks, Carcharhinus plumbeus, in Winyah Bay, South Carolina
(aka- The Who, What, When, Where, and Why’s of Juvenile Sandbar Sharks inhabiting Winyah Bay, South Carolina, USA).
A juvenile Sandbar shark is held right before release. The blue object in the dorsal fin is a tag for identification.
Credits: On courtesy of Dr. George Boneillo
Hello Ocean Fact Members! My name is Caroline Collatos and I am here to tell you a bit about the importance of shark research and specifically, one of my past research projects! My research focused on assessing the presence, abundance, habitat use, and migration patterns for the sandbar shark, Carcharhinus plumbeus, in Winyah Bay, South Carolina, USA. (aka the Who, What, When, Where, and Why’s of juvenile sandbar sharks inhabiting Winyah Bay, South Carolina, USA)! This paper is the first comprehensive analysis of juvenile sandbar sharks habitat use, presence, local abundance, and migration movements from one of the largest estuaries and annual habitats for sandbar sharks along the US East Coast.
Sandbar sharks are a large, coastal shark inhabiting the US East coast from New York to Florida, and throughout the Gulf of Mexico. My research focused on how juvenile sandbar sharks (i.e. sexually immature individuals) use their habitat in Winyah Bay, an estuary located in South Carolina, as well as their migration patterns from this estuary. For decades sandbar sharks were overfished in the Atlantic for their fins and meat; however due to protection and fishing regulations created in the early 90’s, the overfished NW Atlantic sandbar population has been recovering slowly. Shark species are usually slow to recover from population declines since individuals are slow growing, do not sexually mature until late in life (~15 yrs old for sandbars), have long gestation periods (~9-11 months average- some species up to 2 years to give birth to the next baby sharks!), and have small litter sizes (i.e. the number of offsprings a female gives birth to, ~10-15 pups for sandbars). Investigating seasonal presence, relative abundance, and habitat preference of juvenile sharks in estuarine habitats can help highlight factors that promote or suppress population rebounds. Elasmobranch populations (the big group that includes sharks and rays) are especially important to assess and manage since they are keystone , apex predators that are critical in maintaining healthy ocean ecosystems and balanced food-webs. As predators, shark species help control fish populations and biodiversity. Thus, of shark populations can have cascading effects on marine ecology and food-web structure. Therefore studying elasmobranch habitat use, abundance, and migration patterns is crucial in order to protect their populations, and in turn, protect our ocean ecosystems.
With the use of scientific longline surveys , tag and release, and acoustic-tag technology I surveyed the sandbar shark populations in Winyah Bay, for 3 years (2016-2019) and found that the majority of the population inhabiting the bay (98%) were all immature individuals, ranging from newborns to 10 years old. Since sandbar sharks do not sexually mature until approximately 15 years old, the results suggested the bay was dominated by sexually immature, juvenile sharks. These individuals were found inhabiting the bay during the warm water months from late spring to early fall, April – November, with some individuals residing in the bay for almost that entire time (some up to 143 consecutive days)! Like many other species of sharks, juvenile sandbars inhabiting the bay were found to use shallower, more brackish waters than deeper, saltier waters within the bay. This is most likely to access a larger variety of prey that inhabits the shallow, fresher waters of the bay, as well as gaining protection from predators (larger sharks) which usually inhabit deeper, saltier waters.
Fig. 1 Presence of acoustically-tagged individual juvenile sandbar sharks in the Winyah Bay based on acoustic detections from August 2016 to January 2019. Each black line denotes presence on that date and X denotes the start of the time juveniles could be detected. The longer the black line, the more consecutive days spent in the bay. For instance, Shark 17706 spent all its time in the bay from April till September 2017.
Credits: Caroline Collatos
Over the three years I surveyed, juveniles sandbar sharks followed this presence and habitat pattern, suggesting this is an annually used, important habitat for juvenile sandbar sharks. The overriding presence of juvenile sandbars compared to adults also suggests this area is a nursery area (areas dominated by juveniles). Nursery areas are especially important to identify because they provide areas for these juvenile sharks to grow, reach maturity, reproduce, and hopefully add to threatened populations.
In addition to surveying juvenile’s habitat use in the Bay, I was able to track some of these sharks migration movements! Using acoustic telemetry I was able to get specific locations, depth, times and dates of the juveniles migratory movements along the coastline: a 3D map of where the sharks were going! All the juveniles I tracked followed the same winter migration pattern, traveling south and spending the winter along the Georgia coastline, and then migrating back to the Bay in the spring. Three juveniles even returned back to the Bay between April 8th and April 11th each year (talk about timing)! Some juveniles remained in the bay for the entire summer, and some traveled north during the summer all the way to areas like New York. However, all juveniles within the Bay and all juveniles that traveled north in the summer returned to Winyah Bay by late fall, and followed the same migration pattern south for the winter. This migration route and overwintering area differs from juvenile sandbar sharks inhabiting the Delaware and Chesapeake estuaries, which overwinter off the coast of Cape Hatteras, North Carolina.
Overall, the information collected from this research shows Winyah Bay acts as an annually utilized, important habitat for juvenile sandbar sharks and can be classified as a secondary nursery for this species. Additionally, this is the first study to document this unrecognized southerly winter migration route of juvenile sandbar sharks, which is critical information for fishery management to protect sandbar shark habitats and subsequently, aid in the recovering of this species.