The Atlantic herring, Clupea harengus, is the one of the most abundant species of fish on the planet. They can be found on both sides of the Atlantic Ocean congregating together in large schools (or swarms). They can grow up to 17.72 in (45 cm) in length and weigh more than 1.1 lb (0.5 kg). They feed on copepods, krill and small fish, and their natural predators are seals, whales, cod and other larger fish.
The Atlantic herring fishery has long been an important part of the economy of New England and the Canadian Maritime provinces. This is because the fish congregate relatively near to the coast in massive schools, notably in the cold waters of the semi-enclosed Gulf of Maine and Gulf of St. Lawrence. North Atlantic herring schools have been measured up to 2.49 cubic miles (4 cubic kilometers) in size, containing an estimated 4 billion fish. As of late the stocks of this fishery are collapsing.
Atlantic herring have an elongated body that is fairly slender and a belly that is rounded. They also have no adipose fin; this feature distinguishes herrings from the Family of salmon. The Atlantic herring are distinguished from other herring (there are close to 200 species in the family clupeidae) by their relatively small size. They have scutes without a prominent keel, and they have a pelvic fin that is located behind the dorsal fin, which is located midway along their body. Atlantic herring can be further identified from that of other herring by their cluster of small teeth that are arranged in the shape of an oval at the roof of its mouth. This feature is particular to Atlantic herring.
Atlantic herring can be found on both sides of the Atlantic Ocean. They have an extensive range that covers the North Atlantic waters such as the Gulf of Maine, the Gulf of St Lawrence, the Bay of Fundy, the Labrador Sea, the Davis Straits, the Beaufort Sea, the Denmark Straits, the Norwegian Sea, the North Sea, the Baltic Sea, the English Channel, the Celtic Sea, and the Bay of Biscay. Although Atlantic herring are found in the northern waters surrounding the Arctic they are however, not considered to be an Arctic species.
Herring are amongst the most spectacular schoolers; they aggregate together in groups that consist of thousands to hundreds of thousands of individuals these schools traverse the open oceans. A school of herring in general has a very precise arrangement thus allowing the school to maintain a relatively constant cruising speed. Schools that are made up of an individual stock generally travel in a triangular pattern between their spawning grounds e.g. Southern Norway, their feeding grounds (Iceland) and also their nursery grounds (Northern Norway). Such wide triangular journeys are probably important because herring feast efficiently on their own offspring. A school of herring can react very quickly to evade predators; they have excellent hearing. Around SCUBA divers and ROVs they can form a vacuole (“fountain effect”). The phenomenon of schooling is however, far from understood, especially the implications on swimming and feeding. Many hypotheses have been put forward to explain the function of schooling, such as predator confusion, reduced risk of being found, better orientation, and synchronized hunting. However, schooling can also have some disadvantages such as: oxygen- and food-depletion, excretion buildup in the breathing media. The school-array probably gives advantages in energy saving although this is a highly controversial and much debated field.
Schools of herring can on calm days sometimes be detected at the surface from more than a mile away by the little waves they form or from a few meters at night when they trigger the bioluminescence of surrounding plankton (“firing”). All underwater recordings show herring constantly cruising with high speeds up to 42.52 in (108 cm) per second and much higher escape speeds.
Atlantic herring are in general very tender and fragile fish. They have extraordinarily large and delicate gill surfaces, and upon contact with foreign matter they can lose their large scales. They have retreated from many of the estuaries worldwide due to high pollution content within the water although in some of the estuaries that have been cleaned up herring have been observed returning. The appearance of their larvae is used as bioindicator for cleaner and better oxygenated waters.
Because of their feeding habits, cruising desire, collective behavior and fragility they are only on display in very few aquaria worldwide, this despite their natural abundance in the ocean. Even with the best facilities that these aquaria can offer they appear slim and slow compared to a quivering school in the wild.