More On How To Identify Herring
Herring Biology: What is a herring?
There are several species of fish in the herring family. Typically, herring are small, streamlined, schooling "planktivores," or plankton-feeders. The nearly 200 true herring species in the family Clupeidae share several distinguishing characteristics. Herring are silvery fish with a single dorsal fin, no lateral line, and a protruding, bulldog-like lower jaw.
Their pelvic fins are situated on their abdomen well to the rear of their pectoral fins. Unlike many other fish, true herring have soft fins that lack spines, although some have pointed scales that form a saw-toothed "keel" running along the belly.
Streamlined for swimming, the herring body is relatively deep and flattened laterally (side-to-side), with a distinctly forked tail (caudal fin). Turn an Atlantic herring sideways and you could probably slide it under your closet door. The compressed body and silvery scales serve as camouflage in the open waters of the ocean, scattering light and helping to conceal herring from predators attacking from the deep. 
Silvery scales, however, are of no help during attacks from above. Even in murky water, the flashing of silver alerts fishermen to the herring's presence. Anglers searching for tarpon, a tropical herring-like fish, scan the water for that distinctive silver flank and single dorsal fin breaking the surface. The long, slender, highly-prized tropical Tarpon is herring-like in appearance but weighs over one hundred and sixty times more than an average Atlantic herring and can grow almost 80 inches longer, up to eight feet in length.
In general, species of the herring family are characterized by large spawning migrations, with schools of fish traveling round trip distances of up to 3000 km.  Within the boundaries of these common traits, the many species of the herring family are actually quite distinct from one another in terms of size, appearance, behavior, and distribution.
The Atlantic herring is a relatively small fish that schools in waters of northern latitudes, filtering plankton from the water. In contrast, the tropical wolf herring grows up to a meter in length and is a voracious predator of fish, including other herring species.
In other cases there are subtler differences between herring species. Bluebacks (Alosa aestivalis) and alewives (Alosa pseudoharengus), for example, are so uniform in size and appearance that even experienced fishermen have difficulty telling them apart. In fact, it is almost impossible to distinguish these two fish without the aid of a dissecting knife.
In some cases, the range, natural history, and behavior can be unique to a single species, making its identification a simple matter. If you stand at the river flowing out of Damarascotta Lake in mid-coast Maine in May, you will see thousands of fish traveling upstream from the Great Salt Bay. No need for a dissecting knife in Damariscotta. The journeying fish are alewives faithfully completing their annual spawning migrations.
For the most part, herring in the Gulf of Maine are similar in appearance. Upon closer examination, there are some notable differences among these species; for example, the narrow Atlantic round herring is 1/6 deep as long while the deep-bodied Hickory shad can be 1/3 deep as it is long.  Some are entirely marine species while others are anadromous river herring. In total, there are nine herring species in the Gulf, including the commonly found Atlantic herring, blueback herring, alewife, American shad, and Atlantic menhaden. Less common are the Atlantic thread herring, round herring and hickory shad. Gizzard shad have invaded some rivers and possibly estuaries in the Gulf of Maine region. According to Bigelow and Schroeder, co-authors of Fishes of the Gulf of Maine, hickory shad were at one time caught in a number of rivers in the Gulf of Maine, which is the northern extreme of their range.
Atlantic menhaden migrate north from the Mid-Atlantic states in the summer and, in some years, are very abundant in the Gulf of Maine. Schools of menhaden can be so abundant that when they crowd into warm, shallow, inshore waters, or are forced in by predatory bluefish, they use up all the oxygen in the water and die. The last time this happened in Maine was in the early 1990's.
In the North Pacific Ocean the Pacific herring, Clupea pallasii, closely resembles our Atlantic species, Clupea harengus. While morphologically similar, there are some differences in their life histories. Atlantic herring spawn in the spring and fall whereas Pacific herring are strictly spring spawners. Pallasii is the Latinized last name of Petrus Simon Pallas, a Russian naturalist and explorer who first described the Pacific species during his travels in the North Pacific.
The Atlantic Herring (Clupea harengus)
The Atlantic herring is a small, pelagic plankton-feeder that grows to a maximum of 17 inches and 1.5 pounds. Distinguishing characteristics include a dorsal fin located midway along the body and a weak saw-toothed keel along the belly. The fish is iridescent, greenish or grayish blue dorsally with a silvery abdomen and sides. The "pearl essence" of the scales was extracted by the Englehard Corporation of Eastport, Maine for use as a pigment in cosmetics and paints.
This type of coloration ("countershading") is common in pelagic species of fish, as it provides a degree of camouflage in open waters. If viewed at close range, the Atlantic herring can be positively identified by its conspicuous cluster of small teeth arranged in an oval shape on the roof of its mouth. No other herring species possesses this distinctive circle of teeth.
What distinguishes Atlantic herring from all other herring and, in fact, all other fish species in the Gulf of Maine, is their great abundance. Linneaeus (the father of modern classification) referred to the herring as "copiosissimus piscis," or, in other words, the most prolific of fish.  Count the individual fish in the Gulf of Maine - (a task akin to counting the ants in Portland, Maine) - and the Atlantic herring vastly outnumbers the other species.
Herring are pelagic, fish that inhabit the open sea and offshore banks for most of their lives. Young juveniles ("brit") are numerous in inshore waters along the Maine coast in the spring and summer. Adults migrate across hundreds of miles of ocean during their life span. In the winter, schools of migrating Atlantic herring can join forces, forming massive expanses of fish as far as the eye can see. In the North Atlantic, people have observed herring schools measuring up to 4.5 billion cubic meters (over 4 cubic kilometers) in volume, with densities of up to 1 fish per cubic meter. 
In a wonderful passage from Fishes of the Gulf of Maine, Bigelow and Schroeder provide perspective on the historical abundance of Atlantic herring.
"To list the localities where herring have been recorded would be to mention every hamlet along our coasts whence fishing boats put out, for more or less herring, large or small, appear at one season or another around the entire coast line of the Gulf of Maine, and on the offshore fishing banks as well." 
Due to their great abundance, the Atlantic herring became one of the most important and sought after fish species in the Gulf of Maine. They still are.
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 Waller, G., Ed. (1996) SeaLife: A complete guide to the marine environment. Smithsonian Institution Press, Washington, D.C. 504 pp.
 Collette, B.B. and G. Klein-MacPhee, eds. (2002) Bigelow and Shroeder's Fishes of the Gulf of Maine. A complete guide to the marine environment. Smithsonian Institution Press, Washington, D.C. 748 pp.
 Stephenson, R.L. (2001) The role of herring investigations in shaping fisheries science. Herring: Expectations for a New Millenium. Alaska Sea Grant College Program. AK-SG-01-04.
 Radakov, DV. (1972) Schooling in the ecology of fish. Wiley, New York. 173 pp.
 Bigelow, H. B. and W. C. Schroeder. (1953) Fishes of the Gulf of Maine. Fishery Bulletin Of The Fish and Wildlife Service. 53:1-557 pp.