The herring fish belongs to the family Clupeidae, which contains about 200 species of fish. The name 'herring' is used for several different species of fish that belong to this family. However, the most important and the abundant species of herrings belong to the genus Clupea, and they are known as the Atlantic herring (Clupea harengus), the Pacific herring (Clupea pallasii), and the Araucanian herring (Clupea bentincki). These three species account for about 90% herrings captured in fisheries. Among these three species, the most abundant species is the Atlantic herring, which accounts for more than half of the captured herrings. Herrings are relatively small, silver-colored fish. They are abundantly found in the temperate waters of the North Pacific and North Atlantic oceans. Herring is a delicacy and used as a ingredient in number of pasta and salad dishes. All the 200 species of fish that belong to family Clupeidae share some common features and characteristics. One such important feature or characteristic of these fish is that they possess a single dorsal fin with no lateral line, and their dorsal fin does not contain spines, unlike other fish. A few species are found to have pointed scales as well. The tail of a herring fish is usually bifurcated, and it looks like a fork. They are also characterized by a protruding lower jaw, that looks like the jaw of a bull dog. Herrings have a small head and laterally flattened, slender, and sleek body. The flashing silver color of their body helps conceal them in the surrounding water, and protect them from sea predators. But the same feature helps humans catch these fish easily. The Atlantic herring can grow up to a length of 18 inches, while the Pacific herring usually attains a length of about 15 inches. The Atlantic herring is characterized by a fragile and fusiform (tapering at both the ends) body. The dorsal fin can be found midway along the body. It has a greenish or grayish-blue back, and a silvery abdomen. It can be distinguished by its distinctive circle of small teeth located on the roof of the mouth. This fish can weigh up to 1.5 pounds. The Pacific herring, on the other hand, can be distinguished by its laterally compressed body, a single dorsal fin located midway along the body, and a deeply forked caudal fin. The abdomen and the sides are silvery-white, while the back is bluish-green in color. This fish does contain scales on its head and gill, but possesses large, protruding ventral scales. The Araucanian herring, also known as the Chilean herring, is dorsally dark blue in color with a silvery abdomen. The Baltic herring is comparatively smaller than its relatives, and grows to a length of only 14 to 18 cm. Herrings are abundantly found in the temperate waters of the North Pacific and North Atlantic oceans. In the North Atlantic ocean, they can be found in Gulf of Maine, the Gulf of St. Lawrence, the English Channel, the Labrador Sea, the Bay of Fundy, the Beaufort Sea, the Denmark Straits, the Davis Straits, the Norwegian Sea, the North Sea, the Celtic Sea, the Irish Sea, the Bay of Biscay, and the Sea of the Hebrides. The Pacific herring can be mainly found along the California coast from Baja California north, to Alaska and the Bering Sea. In Asia, it can be found south to Japan. The Araucanian herring, on the other hand, can be found in coastal waters off the west coast of South America. They thrive on minute organisms, like plankton, crustaceans, and fish larvae. Phytoplankton are the main source of food for the young fish, while the adults feed on zooplankton, like copepods, pteropods, and small crustaceans, as well as krill, mysids, annelids, Calanus, fish larvae, larval snails, small fish, and even small animals. They keep their mouths open while swimming, and in the process filter the plankton which pass through their gills. Herrings attain sexual maturity when they are 3 to 4 years old. They have a life expectancy of 12 to 16 years. However, the Southern herring can live up to 23 to 25 years. The fertilization is external, as the female and the male fish release masses of adhesive eggs and milts simultaneously. Some fish travel to the coastal rivers for spawning. Atlantic herrings usually spawn in coastal waters and also in offshore banks. Mid-summer and December are the ideal time for spawning. The eggs usually take about two weeks to hatch. A female fish can lay about 20,000 to 40,000 eggs, depending on its size and age. On an average, a female herring spawns about 30,000 eggs, which sink to the bottom and stick to gravels, stones, seaweed, and other objects. The eggs are about 1 to 1.4 mm in diameter, and they cannot survive at temperatures above 66°F. The hatched larva is usually 5 to 6 mm in length with a nearly transparent body. The eyes are however, pigmented and there is a small yolk sac, that eventually gets absorbed as the larva grows. The larva looks like a small herring, when it reaches a length of about 40 mm. The young fish generally reach adulthood in about 3 to 4 years. During the day, herrings stay in deep water to avoid their predators. But at night, when the risk of being seen by the predators is less, they come to the surface and keep their mouths open while swimming. Thus, they filter the plankton which happen to pass through their gills. When the concentration of prey reaches a high level, all the members of the school swim with their mouths open and their opercula completely expanded. The juvenile fish mainly prey on copepods in a very synchronized manner. Basically, they swim in a grid by maintaining a fixed distance between them, which is equal to the length of the jump of their prey. The copepods are usually 1 to 2 mm in length, with a teardrop-shaped body, and a pair of antennae, that help detect an approaching predator by sensing its pressure waves. So, whenever a copepod senses the presence of a predator, it jumps to escape, but the length of its jump remains fairly constant, and after every jump, it takes about 60 seconds to spread its antennae and detect the presence of its predators. In the meantime, the endless stream of herrings swimming in that direction eventually allows one to catch the copepod. They have a number of predators. Marine mammals such as dolphins, porpoises, orca, whales, seals, and sea lions, as well as fish, like sharks, salmon, tuna, cod, halibut, billfish, swordfish, and striped bass, are the main predators of herrings. Seabirds are another important class of predators. Herrings have been known to be consumed by humans since 3000 B.C. They can be fermented, pickled, smoked, cured, and also eaten raw. They are also used for manufacturing fish oil. These fish are a good source of vitamin D, omega-3 fatty acids, and DHA (docosahexaenoic acid). Herrings are known to swim in large groups, which are termed as schools. They generally move in groups or schools in the same direction to the shore, in order to spawn. While swimming in vast schools, they provide food to the large predators. It was estimated that a herring school in the North Atlantic can be up to 4.8 cubic kilometers with a density of fish between 0.5 to 1 fish per cubic meter, which means that there can be billions of fish in a school. The most interesting fact about a school is that it features a precise spatial arrangement, which helps maintain a relatively constant cruising speed. It is not clearly understood why fish like herrings form schools, though several hypotheses have been put forward to explain the purpose. One such hypothesis is predator confusion. Other possible reasons include, better synchronized hunting and better orientation. However, schooling has some disadvantages as well, as it can lead to the depletion of food and oxygen, and the accumulation of excretory products in the breathing media. Herrings are one of the most abundant fish found in the oceans. They are an important source food for large predators, including large fish like dolphins, sharks, and dogfish, sea mammals like whales and seals, and seabirds. They also have immense commercial and economic importance for humans.