Atlantic herring

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Clupeidae

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Atlantic herring Clupea harengus is the most abundant species of fish in the world (Guinness Book). They can be found on both sides of the Atlantic Ocean congregating together in large schools or (swarms). They can grow up to 45 centimeters (approximately 18 inches) in length and weigh more than half a kilogram. They feed on copepods, krill and small fish, 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 4 cubic kilometers in size, containing an estimated 4 billion fish.

Morphology

Atlantic herring have an elongated body that is fairly slender, a belly that is rounded (compared with that of a sprat, Sprattus sprattus), they also have no adipose fin this feature distinguishes herrings from the Family of salmon. The Atlantic herring are distinguished from other herring (their are close to 200 species in the family clupeidae) by their relativley small size, they have scutes without a prominent keel and they have a pelvic fin that is located behind the dorsal fin, the dorsal fin 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.

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

*Clupea harengus* distribution on a NASA SeaWIFS image - the main concentrations are in the North Atlantic at the North Sea

Biological specialities

Herring are amongst the most spectacular schoolers ("obligate schooler" under the old definitions), 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 trianglular 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 offsprings. A school of herring can react very fast to evade predators they possess an excellent hearing capacity. 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-energetics. Many hypothesis have been put forward to explain the function of schooling (for an overview see Pitcher/ Teleost behaviour) such as: predator confusion, reduced risk of being found, better orientation, 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 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

transparent eggs with the eyes visible, one larvae hatched - observe the yolk.

freshly hatched larvae in a drop of water besides a match to demonstrate how tiny the larvae are - visible are the black eyes and the yolk

very young larvae imaged *in situ* in the typical oblique swimming position - the animal in the upper right in the classical S-shape of the beginning phase of an attack of probably a copepod - the remains of the yolk and the long gut are very well visible in the transparent animal in the middle

juvenile herring. Length ca 38 mm, ca. 3 months old - still transparent, visible are the otoliths, the gut, the silvery swimmbladder and the heart - click twice into the image for high resolution

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

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school of juvenile herring close to the surface

Underwater video (looping) of a school on its migration to their spawning grounds in the Baltic. With such high speed they can migrate over thousands of kilometers. In the North Atlantic they cruise between Norway and Greenland every year.

Feeding

In this sequence a herring attacks four times in a row (50% timelag, looping, each image shifted to compensate the rolling microturbulences from the waves). In the third attack the copepod is visible between the wide opened sides of the mouth. The opercula are spread wide open to compensate the pressure wave which would alert the copepod to trigger a jump.

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") (Kils 1992), 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").

herring ram feeding on a school of copepods, all fish have the opercula wide open all at the same time (the red gills are visible) and the mouth wide open (click to enlarge) - the fish swim in a grid with a distance of the jumplength of their prey, as indicated in the animation below

juvenile herring hunt for the very alert and evasive copepods in synchronization: The copepods can sense with their antennae the pressure-wave of the approaching herring and react with a fast escape jump. The length of the jump is quite similar. The fjish arrange in a grid of this caracteristic jumplength. The copepods can dart ca. 80 times before they tire out. It takes 60 milliseconds to spread out the antennae again, and this timeslot is utilized often by the herring to snap finally a copepod. A single juvenile herring would never be able to catch a large copepod ("Synchropredation" - results from *in situ* videos taken from the ATOLL laboratory).

References

Guinness Book of World Records

Kils, U., The ecoSCOPE and dynIMAGE: Microscale Tools for in situ Studies of Predator Prey Interactions. Arch Hydrobiol Beih 36:83-96;1992

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