Atlantic herring

Atlantic herring Clupea harengus is the most abundant fish species of the world, living in large schools (swarm). Growing up to 45 centimeters (approximately 18 inches) in length and more than half a kilogram in weight, they feed on copepods, krill and small fish and are food for 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, because the fish congregates relatively near 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 4 cubic kilometers in size, containing an estimated 4 billion fish.

Systematics and Diagnostic Features

Family: Clupeidae

Order: Clupeiformes

Class: Actinopterygii (ray-finned fishes)

Body elongate and fairly slender, belly rather rounded (compared with sprat, Sprattus sprattus), scutes without prominent keel. No adipose fin (distinguishes all the herrings from any of the salmon tribe), pelvic fin insertion behind vertical from dorsal fin origin (below or in front in Sprattus sprattus), origin of dorsal fin at 50 % of the length of the body.

Ecological importance

Herring-like fish are the most important fish group on the planet, Clupea harengus the most frequent fish (Guiness Book of Records). They are the dominant converter of the enormous production of zooplankton, utilizing the biomass of copepods, mysids, euphausiids in the pelagial. They are on the other side a central prey item for higher trophic levels. The reasons for its success is still enigmatic; one speculation is attributing their dominance to the outstanding way of living in huge, extremely fast cruising schools.

Geographical Distribution

Biological specialities

Herring are amongst the most spectacular schoolers ("obligate schooler" under the old definitions), voyageing oceanwide in groups of thousands to hundredthousands. A school is generally in very precise arrangement, constantly cruising. Of an individual stock the schools generally travel in a triangle between their spawning grounds (southern Norway, for example), feeding grounds (Iceland) and nursery grounds (northern Norway). Such wide triangular journeys are probably important because herring feast efficiently on their own offsprings. Herring schools react very evasive and fast, possessing excellent hearing capacity. Around SCUBA divers and ROVs they form a vacuole ("fountain effect"). The phenomenon of schooling is far from being understood, especially the implications on swimming- and feeding-energetics. Many reasons are discussed for function of schooling (overview see Pitcher: Teleost behaviour): predator confusion, reduced risk of being found, better orientation, synchronized hunting. However, schooling has also some disadvantages like oxygen- and food-depletion, excretion-buildup in the breathing-media. The school-array probably gives advantages in energy saving (a very controversly debated field).

Schools can on calm days sometimes be detected at the surface from more than a mile distance, by the little waves they form, or from a few meters at night, when they trigger bioluminescence of surrounding plankton ("firing"). All underwater recordings show herring constantly cruising with high speeds up to 108 cm per second, and much higher escape speeds.

Habitat requirements

Herring are very tender and fragile fish. They have extraordinary large and delicate gill surfaces, and upon contact they loose their large scales. From polluted waters they have retreated in many estuaries worldwide. Into some cleaned-up estuaries they are recently returning, where the appearance of their larvae is used as bioindicator for cleaner and better oxygenated waters.

Because of their feeding habits, cruising desire, collective behaviors and fragility they are at display only in very few aquaria, despite their dominance in the world, and even at the best facilities they appear slim and slow compared to a quivering school in the wild.

Life History

There is at least one herring stock spawning in any one month of the year, each race having a different spawning time and place (spring, summer, autumn and winter herrings) in 0 to 5 m off Greenland down to 200 m in autumn (bank) herrings of the North Sea. Eggs are laid on the sea bed, on rock, stones, gravel, sand or beds of algae. " .. the fish were darting rapidly about, and those who have opportunity to see the fish spawning in more shallow water ... state that both males and females are in constant motion, rubbing against one another and upon the bottom, apparently by pressure aiding in the discharge of the eggs and milt" (Moore at Cross Island, Maine).

A female herring may deposit from 20000 up to 40000 eggs, according to her age and size, averaging about 30000. In sexually mature herrings, the genital organs are so large just before spawning commences that they make up about one-fifth of the total weight of the fish.

The eggs sink to the bottom, where they stick in layers or clumps to gravel, seaweeds or stones, by means of their coating mucus, or to any other objects on which they chance to settle.

If the layers get too thick they suffer from oxygen depletion and often die, entangled in a maze of fucus. They need a fair amount of water microturbulence, generally provided by wave action or coastal currents. Survival is highest in crevices and behind solid structures, because many birds and other prey feast on openly disposed eggs. The individual eggs are 1 to 1.4 mm in diameter, depending on the size of the parent fish and also on the local race. Incubation time is about 40 days at 3°C (38 F), 15 days at 7°C (45 F), 11 days at 10°C (50 F), they die at temperatures above 19°C (68 F).

The larvae are 5 to 6 mm long at hatching, with a small yolk sac that is absorbed by the time a length of 10 mm is reached. Only the eyes are well pigmented (a camera works only with a black housing) the rest of the body is as transparent as possible, and virtually invisible under water and natural luminance conditions.

The dorsal fin is formed at 15 to 17 mm, the anal fin at about 30 mm - the ventral fins are visible and the tail becomes well forked at 30 to 35 mm - at about 40 mm the little fish begins to look like a herring.

Larvae diagnostics: The larvae of the herring family are very slender and can easily be distinguished from all other young fish of their distribution range of similar form by the location of the vent, which is so far back that it lies close to the base of the tail. But it requires critical examination to distinguish several clupeoids one from another in their early stages, especially herring from sprats.

At the age of one year they are about 100 mm long, first spawning at 3 years.

Schooling

Feeding

Herring is a pelagic feeder - their prey consists of copepods, amphipods, larval snails, diatoms (only herring larvae below 20 mm), peridinians, molluscan larvae, fish eggs, euphausids, mysids, small fishes, herring larvae, menhaden larvae, pteropods, annelids, tintinnids (only herring larvae below 45 mm), Haplosphaera, Calanus, Pseudocalanus, Acartia, Hyperia, Centropages, Temora, Meganyctiphanes norvegica.

Young herring capture copepods predominantly individually ("particulate feeding" or "raptorial feeding"), a feeding method also used by adult herring on large prey items like euphausids.

If prey concentrations reach very high levels, like in microlayers, at fronts or directly below the surface, herring ram forwards with wide open mouth and far expanded opercula over several feet, then closing and cleaning the gill rakers for a few milliseconds ("sift feeding" or "filter feeding").

External links

• Information on Fishbase (http://www.fishbase.org/Summary/SpeciesSummary.cfm?ID=24&genusname=Clupea&speciesname=harengus%20harengus)
• Information on clupea.de (http://clupea.de/biology/index.html)da:Sild

de:Atlantischer Hering sv:Sill