Saturday, January 4, 2020

Thresher Shark Strandings on Cape Cod this Fall and early winter.

Thresher Sharks in Cape Cod, MA
By: Cory Farrelly
UMASS Dartmouth Intern

At the end of 2019, there were a number of Thresher Sharks washing ashore on Cape Cod beaches. Thresher Sharks are very unique sharks in the order Lamniformes, which are commonly called Mackerel Sharks. Threshers are easily identified by their extremely long caudal or tail-fin, which they use to stun their prey, making their prey easy to consume. Threshers can be found in the Atlantic, Pacific, and Indian Oceans. These species are highly migratory and can cover vast distances in a relatively short period of time. There are three species of Thresher Sharks: The Bigeye Thresher, Common Thresher and Pelagic Thresher. The Common Thresher is the largest species, reaching a length of 20ft and is the species that is found in New England waters. 

Dead Common Thresher Shark found in Wellfleet, MA
In the fall of 2019, I have responded to 2 stranded Thresher Sharks along the shores of Cape Cod. One shark was found in Uncle’s Tim’s Bridge on Duck Creek in Wellfleet. This shark was unfortunate enough to get trapped in a salt marsh during high tide. When the tide receded, the water most likely got too low for the shark and it could not survive. A local resident had actually seen the shark alive struggling to get out of the salt marsh a few days before. The shark was 6 feet 10 inches long. When we found the carcass, it was partially buried in the mud.
The second Thresher Shark carcass was found on Burton Baker beach in Wellfleet. The carcass was very fresh looking and had bled significantly through its gills. In addition to the blood from the gills, the shark had several marks along its body from Sea Lampreys. It also had interesting marks that appeared to be healed scrape marks on its side. The pectoral fins were completely buried in the sand, which suggested that it washed ashore then thrashed around and buried itself in the sand. This shark’s body was 7 feet long and with its tail it reached a length of 13 feet. 
All three species of Thresher Sharks are classified by the International Union for Conservation (IUCN) as vulnerable. Due to their low fecundity (the amount of offspring they can produce), it is difficult for these sharks to rebound from losses to their population. Thresher Sharks are vulnerable to fishing, both commercially and recreationally. Commercially, their livers are used for oil and their fins are used in shark fin soup. By-catch is also an issue. Every year, 50 million sharks are unintentionally caught and often killed by the fishing industry. Recreational fishing also has negative impacts for the act of being caught can cause undue stress and extreme exhaustion that the fish may not be able to recover from. Many shark populations struggle to rebound from fishing pressures due to their low fecundity and long gestation periods so killing millions every year is detrimental to their populations. Since threshers are found worldwide and are highly migratory, regulations on this species are nearly impossible to enforce because it would require cooperation among many countries. 
To help Thresher Sharks and many other shark species, become more knowledgeable by learning more about them. Limit your seafood consumption, especially various tuna species, and be a choosy consumer using the Seafood Watch app from Monterey Bay Aquarium. Also, do not purchase any shark products, including shark fin soup.  

References for Further Reading