, Newburyport, MA

August 31, 2013

What that dorsal fin might mean

By Tony LaCasse
New England Aquarium

---- — Editor’s note: the following column was provided by the New England Aquarium.

Identifying a species on the basis of a dorsal fin sighting is a challenge even for professional marine biologists. Regardless of what species might be present, they are here in almost all cases in search of food.

Even in the case of large sharks, humans are not on the menu, but we still must take all precautions to minimize any negative interaction. Often, schooling fish, such as herring, mackerel, bluefish or stripers moving through an area might attract larger predators behind them. One way to help determine this is to observe whether there is exceptional small sea bird activity just off shore repeatedly skimming the water. Often, that is an indicator of small schooling fish there. Also checking with harbormasters, bait shop owners and recreational fishermen can give an indication of the presence of schooling fish. More food or bait in the water increases the likelihood of other large predators near-by.

Likely suspects

There is significant seasonality in which species visit an area and why. For example, here in New England, there is a tremendous difference in the distribution of marine species north and south of Cape Cod. That is largely a function of water temperature.

Blue Shark – In July and August, the most common large shark by far along the local coast are blue sharks, which are exclusively fish and squid feeders. They are a summertime visitor, and some estimates are that they make up more than 90% of the large sharks north of Cape Cod from mid to late summer. (In this case, we are not considering dogfish a large shark species.)

Blue sharks are most commonly five to seven feet long but can be larger. The color of their upper body is grey with a distinctive bluish hue. Their bottom half is pale to white. They have a slender, athletic body build with a pointed snout. Their pectoral or side fins are long and thin. Their swimming pattern is usually fast and irregular. They can be found in small groups.

Shark researchers generally consider blue sharks a timid species, but they have been implicated in a few attacks involving people globally.

Basking Shark - This is the most common shark species mistaken for great white sharks, and ironically is a giant, harmless, mostly plankton feeder. It is also probably the second most commonly seen large shark in the region. Basking sharks have large prominent dorsal fins similar to great whites. They are the second largest fish in the ocean reaching up to 32 feet in length. In fact, any shark length in this region greater than 20 feet rules out great whites or any other large shark species and rules in basking sharks. If the shark is longer than the average small recreational boat, then it is a basking shark.

Basking shark swimming behavior is also very different than most sharks. They feed by opening their large mouths and then slowly swim forward allowing all of the small animal life to passively collect on its gill rakers. Typically, they are slow, relatively straight line swimmers, swimming near the surface and not bothered by the presence of boats.

Another identifying feature is that their gill slits are very long and extend almost to the top of their backs.

Great White Shark – Despite the recurring music of the Jaws soundtrack playing in our heads, great white shark sightings are extremely rare along the local coast. The area is just not prime habitat as water temperatures are generally cooler than they prefer, and most of that area does not have their preferred food, which are large seals in colonies along a sandy coastline. Great white sharks visiting here would be just transients passing through.

Mola Mola – Also called ocean sunfish, these absurdly shaped creatures are a wonder when seen. They are a thick, pancake-shaped fish with a large, often floppy top fin that are found floating near the surface of the water. This is a sunning behavior that is often seen by boaters in mid to late summer. From a distance, the very slow swimming behavior is a key clue, while close-up the animal’s bizarre appearance is a dead give-away. Mola mola look like a biological experiment gone awry.