by Dr. Sarah Treanor Bois, PhD
Director of Research & Conservation at the Linda Loring Nature Foundation
Great white sharks get all the publicity. Especially this summer with the 50th Anniversary of Jaws, great whites are having a moment. But there are other sharks in the sea, and we’re starting to see more of them than ever. One group in particular— hammerhead sharks—are becoming more of a regular summer visitor than an anomaly.
Back in the summer of 2022, hammerhead sharks were seen several times in the shores of Nantucket causing multiple beach closures. Recently, on July 13, half a dozen hammerhead sharks were seen in waters just off of Cuttyhunk. Last week, hammerheads were seen south of Nantucket and off of Falmouth. In fact, in the past several years, hammerheads have been seen coming closer into shore throughout southern New England. One was even seen off the coast of Maine in waters once considered to be inhospitable to these more tropical species.
The increase in local sightings has coincided with increases in water temperatures. And it’s not just hammerheads that are being seen: blacktip sharks, bull sharks, and spinner sharks are also expanding their ranges as the southern waters become less hospitable.
Hammerhead sharks, of which there are many species, are more typically found in warmer, tropical-like waters. With the gulf stream extending further north, hammerheads are being seen with increased frequency along our coast where, previously, it would have been too cold. Unlike great white sharks, hammerheads and other warm water species like bull and tiger sharks cannot regulate their body temperatures by generating heat. Cold water shuts them down. The warmer water is extending their range.
Water temperatures in the Gulf of Maine, including the water around Nantucket, are warming at a faster rate than any other body of water in the Atlantic. For Nantucket, the Gulf Stream typically brings warmer waters to our southern shoreline. As the waters further south become too warm, the prey species move northwards and so do the predators. With food and favorable conditions, the hammerhead sharks and others may become a more frequent sight. This is potentially the beginning of a range shift for these species.
The hammerhead we are typically seeing is the smooth hammerhead (Sphyrna zygaena). Sporting a wide and smooth cephalophoil or “hammerhead” this seasonal migrant is the second largest of the hammerheads, growing up to 12 feet in length. Preferring shallow waters less than 65 ft., smooth hammerheads live close to shore over continental shelves and inshore waters such as bays and estuaries. Smooth hammerheads are also the most tolerant of temperate waters. They are more likely to be seen sometime from July through September. Nantucket waters have always been at about the northern limit for their range. However, with warming waters, we’re seeing more and more hammerheads come into our shallows to feed.
Of course, it’s not just the water temperatures. As a top predator, sharks, including the hammerheads, follow the food. For smooth hammerheads, that means squid, smaller sharks, and bony fish. As the prey move, so do the sharks. For hammerheads in particular, they follow the fish close to shore and force them into the shallows. That’s how we can get those great images of hammerheads from drone footage or spotter planes: shallow clear water and the distinctive silhouette of the hammerhead shark.
Despite the increase is sightings, smooth hammerheads are not aggressive sharks. There is little danger to people from these relatively docile sharks. We should treat them more with respect than fear. Smooth hammerheads are a protected species. In the US, smooth hammerhead sharks are grouped with large coastal species, a group that biologists consider being most vulnerable to overfishing. The species is caught in gillnets, longlines, and as bycatch in driftnet fisheries. Due to their high fin ray count, they and other hammerheads are among the most desired species in the Asian shark fin market. The ICUN currently lists them as “Vulnerable.”
One purported reason for the recent increase in shark sightings is that there are more sharks due to conservation efforts. Many scientists believe there are more sharks around Nantucket in the last 5 years partly because the populations are recovering after an 80% decimation from being overfished in decades past.
While shark populations are still considered critical globally, marine biologists are beginning to see the effects of effective fisheries management, which is then leading to population increases across a number of species that provide food for sharks.
John Chisholm, adjunct scientist at the New England Aquarium’s Anderson Cabat Center for Ocean Life, is interested to hear about shark sightings. He has studied sharks for more than 40 years and is using community science to keep track of shark sightings. Hammerheads and others are the subject of research projects with some efforts to tag individuals. But community science uses the “on the water” community to collect the data. By collecting sighting info from those already on the water, recreational, commercial, and charter boats, researchers are gaining insight into potential range expansion of many species.
To submit sightings to John Chisholm, you can tag him on Instagram @ma_sharks or send an email to masssharks@gmail.com.
As for those species that are newer to our waters? As one Florida marine biologist noted, “To see lots of sharks means you have a robust ecosystem that can support them.” And that is a good thing.
