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August 31, 2013

What that dorsal fin might mean

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.

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