The Atlantic herring is a pelagic marine teleost: a sea fish with a skeleton that lives in the water between the sea bed and the surface. This distinguishes it, in particular, from a demersal marine teleost, which lives on the sea bed. It is identified with relative ease by addressing the following three points: it has a single dorsal fin; it has a rounded belly and it has smooth gill covers.

On the fishmonger’s slab useful eliminators can include: it is not a flat fish; it isn’t gadoid (round and white like cod, haddock, whiting, hake, etc.); it’s larger than a sprat, a pilchard or a sardine, narrower than a sea bass, less deep than a sea bream and gernerally smaller than a salmon. It can be similar in size to a small trout, but it’s cheaper. It doesn’t have the blue stripy upper body of the mackerel, but you would have already discounted mackerels by reason of their two dorsal fins.

Celebrated as silver darlings in the sea, stripped of their scales they have metallic blue backs and silvery white bellies. The light brown flesh is oily.

The description by the mighty Soviet systematist AN Svetividov in Clupeidae (1952, from the series Fauna of the USSR) goes as far as most people would want to know:

Genus CLUPEA Linné – Marine or Oceanic Herrings
[morskie ili okeancheskie sel’di]

Clupea Linné, Syst. Nat., ed. X, 1758: 317 (type: Clupea harengus).
Rogenia Valenciennes, in: Cuvier et Valenciennes, Hist. nat. poiss., XX, 1847:
340 (type: R. alba = young Cl. harengus).

This genus resembles genus Sprattus in the absence of a prominent median notch in the upper jaw, lack of elongated scales (alae) on the lobes of the caudal fin, absence of two posterior, elongated rays in the anal fin, absence of the dermal bi-lobed outgrowth on the vertical portion of the cleithrum, and by the presence of a smooth operculum devoid of radial striations. The adipose eyelids are relatively well-developed and cover eyes posteriorly and partially anteriorly. The keeled ventral scales are poorly developed and clearly visible only posterior to the ventral fin. There are 24 – 32 scales anterior to the ventral fin and 11 – 18 scales posterior to it. Belly rounded anteriorly and has a poorly developed keel posterior to the ventral fins. The lower jaw-skull articulation and the posterior end of the upper jaw are located underneath the vertical of the centre of the eye; in very small fish they may be anterior to it, while in large fish they may be posterior to it. The lower jaw protrudes appreciably. The teeth on the vomer are relatively weak and arranged on the longitudinal crest of the capitulum. A few teeth are located on the premaxillary and on the anterior tip of the lower jaw. The teeth on the palatine are very weakly developed. Occasionally, teeth are found on the tongue. The middle portion of the lower edge of the upper jaw bears small teeth. There are no teeth on the mesopterygoid. The gill rakers on the upper portion of the gill arch overlap the gill rakers on the lower part of the gill arch. The gill rakers are fairly long and thin and number 40 – 51 on the lower half of the gill arch. The diverticles of the swim bladder are situated within the prootic and pterotic. The ventral fins are situated behind the beginning of the dorsal fin, usually under the anterior third of the dorsal fin, occasionally somewhat behind the middle of its base. V is usually I 8, occasionally I 7 and I 9, rarely I 6 and even I 5. The base of the dorsal fin is located in a groove formed by broad scales that are larger at the anterior margin of the fin. The number of vertebrae is 45 – 60. There are 18 – 29 pyloric caeca. There are no spots behind the operculum or on the sides of the body. The eggs are adhesive and deposited on the sea bed or on marine vegetation, lack oil globule and have a small perivitelline space. Large and medium-sized sexually mature fish measure 10 – 15 cm. Some larger specimens may measure 25 – 30 cm.

The skeleton.
The prootic bulla is somewhat larger than the pterotic bulla, forms a
prominent, strongly convex swelling on the lateral side of the skull. Pterotic bulla laterally is not prominent, but is somewhat more prominent dorsally underneath the preepiotic fossa. The diverticles of the swim bladder within the lateral occipitals are situated in osseous canals that form, one on each side of the skull, elongated, moderate-sized swellings between the fenestra auditiva and the foramen of the vagus nerve. The openings of these canals are situated near the lower borders of the lateral occipitals. The fenestra auditiva is not large, somewhat elongated, and is somewhat broader posteriorly than anteriorly. Its anterior border is separated by an appreciable space from the prootic bulla, while its posterior border is widely separated from the foramen of the vagus nerve. The preepiotic fossa is large and high. Its edges form an isosceles triangle, almost an equilateral triangle, whose base is directed dorsally while its apex is directed ventrally. The temporal foramen is relatively large and is elongated, in length almost equal to the length of the preepiotic fossa. It is however, considerably lower than the former. The articulation facet of the hyomandibular on the spenotic and prootic is continuous and barely divided into two parts. The parasphenoid is slightly curved, with its wings under the myodome not directed ventrally, wide both on the sides of the myodome, and behind its posterior opening. The anterior part of the parasphenoid bears a crest on its dorsal surface and is highest at its anteriormost edge. The process of the basisphenoid projects anteriorly from underneath the prootic. The posterior end of the supraoccipital on the top of the skull abuts the median line, forming an obtuse angle. The posterior end of the epiotic is either sharp or blunt. The pterotic terminates posteriorly in a spine that is not large and bent slightly ventrally. The anterior edge of each sphenotic extends anteriorly and laterally and is sharpened at the tip. The lateral ethmoids protrude somewhat from under the anterior edge of the frontal bones and are directed laterally and anteriorly but to varying degrees. The transverse extensions of the mesethmoid are directed somewhat anteriorly and are not sharpened at the tips. The skull is fairly broad, its width between the edges of the pterotic is almost half the length, with the maximum between the edges of the frontal bones – about 2½ to 3½ times in the length of the skull. The vomer on the ventral surface of the skull is slightly rounded, and bears a crest about a third the length of the vomer, which is more or less short and broad. The anterior portion of the parasphenoid is flattened and tapers somewhat posteriorly. The wings of the parasphenoid separate, and immediately diverge widely under the anterior portion of the prootic. The opisthotic terminates posteriorly in a blunt extension (process). The depressions of the posterior temporal fossae on the hind part of the skull are shallow. The subtemporal fossae are deep and very obvious, oval in shape, and extend horizontally. Each of them is situated above the swelling of the opisthotic in the pterotic and lateral occipital. The articulation with the skull over the hyomandibular is single and almost evenly wide along its entire length. The quadrate has no hollow on its dorsal surface. The dentary is wide. The articular is high and shorter than the dentary. Description based on examination of numerous skeletons of Cl. harengus harengus and Cl. harengus pallasii, and a number of skeletons of Cl. harengus harengus n membras, Cl. harengus pallasii n suworowi, and Cl. harengus pallasii n maris albi.

As always with Svetovidov, one is left with very little more to say, except to point out that he saw the Atlantic herring and the Pacific herring, along with the Baltic, White Sea and Chosa herrings, as all one species, sub-specied as appropriate – which is why he examined all their skeletons.

The academy went with Ponomareva on separating them all into two species, Baltic going with Atlantic, White Sea and Chosa with Pacific.

Personally, I’d go with Svetovidov, but what can you do?