Herring

• 1. Introduction

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  Herring have been a staple food source since at least 3000 BC. That is a span of five thousand years during which one small, silvery fish shaped trade routes, founded cities, and fed empires. In 1274, a dying Thomas Aquinas lay in a monastery south of Rome and asked for a single comfort: fresh herring. What was it about this fish that made it so central to human life? The answers reach into the deep biology of the ocean, the mechanics of how herring hunt and are hunted, and the medieval economics of salt and smoke. Three species in the genus Clupea account for roughly ninety percent of all herrings caught worldwide, and the most abundant of these, the Atlantic herring, alone makes up more than half of all herring capture globally. In the United Kingdom, they were once called the "silver darlings". In Europe more broadly, herring has been called the "silver of the sea". The fish's trade has been regarded, across centuries of observation, as the most commercially important fishery in history. How one forage fish achieved that standing is a story that touches science, ecology, economics, and the founding of cities.

• 2. A Fish Without Mystery

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  The Baltic herring, known scientifically as Clupea harengus membras, grows to just 14 to 18 centimetres. Its larger relative, the Atlantic herring Clupea harengus harengus, can reach about 46 centimetres and weigh up to 700 grams. These are not imposing dimensions for a fish that underpinned so much history. Herring belong to the order Clupeiformes and to the family Clupeidae, a group that encompasses some 200 species including shads, sardines, and menhadens. The Clupeidae share a set of recognisable features: a silvery colour, a single soft dorsal fin without spines, no lateral line, and a protruding lower jaw. The Pacific herring, Clupea pallasii, grows to about 38 centimetres and its maximum recorded age stands at 19 years. Atlantic herring live up to 22 years. The two Clupea species are the type genus of their family, with subspecific divisions proposed for both but their biological basis remaining unclear. The word "herring" itself may trace back to the Old High German heri, meaning a host or multitude, a name that speaks directly to the animal's most visible behaviour: the school.

• 3. The School and the Hunt

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  Copepods are tiny crustaceans that some scientists believe form the largest animal biomass on the planet. They are also the primary food of herring, and catching them is far harder than it sounds. A single copepod can spread its antennae to sense the pressure wave of an approaching fish, then jump with great speed over a few centimetres. It can repeat this escape jump about 80 times before tiring, and after each jump it takes 60 milliseconds to re-spread its antennae. That 60-millisecond delay is the herring's opening. Herring hunting copepods swim in a precise grid where the spacing between individual fish matches the characteristic jump length of their prey. Juvenile herring hunt this way in synchronised formations; a single juvenile could never catch a large copepod on its own. When copepod concentrations reach very high levels, herring switch to ram feeding, driving forward with mouths fully open and operculae expanded, filtering plankton from the water before closing their mouths and cleaning the gill rakers in milliseconds. During daylight hours, herring remain in deeper water; they rise to the surface to feed only at night, when the chance of being spotted by predators drops sharply.

• 4. Becoming the Prey

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  Sailfish raise their sails to appear larger than they are, a tactic used when herring are compressed into a bait ball. Swordfish charge at speed through these bait balls, slashing with their swords to kill or stun fish before circling back to feed. Thresher sharks compact the school by swimming circles around it and splashing with their tails, then strike sharply with the upper lobe to stun their prey, often working in pairs or small groups. Spinner sharks charge vertically through the school in a spinning motion with mouths open; their momentum often carries them clear of the water at the end of each spiralling run. Whales use lunge feeding, accelerating from beneath the bait ball and opening their mouths to an angle that generates the water pressure needed to engulf enormous volumes of fish. This technique, used by rorquals, a family of baleen whales that includes the blue whale, has been described as the largest biomechanical event on Earth. Seals, sea lions, dolphins, porpoises, tuna, salmon, cod, halibut, striped bass, sharks, and seabirds also prey on herring. The fish's success in the face of this pressure remains, in the words of one line of scientific thinking, genuinely enigmatic, though its vast and extremely fast-moving schools are a leading candidate explanation.

• 5. Spawning and the Early Years

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  At least one stock of Atlantic herring spawns in every month of the year, with each stock choosing a different season and location. Greenland populations spawn in 0 to 5 metres of water; North Sea bank herrings spawn at depths of up to 200 metres in autumn. Females deposit between 20,000 and 40,000 eggs depending on age and size, averaging about 30,000. The eggs, each 1 to 1.4 millimetres in diameter, sink to the bottom and stick in layers to gravel, seaweed, or stones using a mucous coating. If egg layers pile too thick, they suffer oxygen depletion and die. Survival is highest in crevices and behind solid structures, where predators cannot easily reach. Incubation time depends on temperature: about 40 days at 3 degrees Celsius, 15 days at 7 degrees, and 11 days at 10 degrees. Eggs die at temperatures above 19 degrees. At hatching, larvae are 5 to 6 millimetres long and nearly transparent, virtually invisible in the water. The dorsal fin develops at 15 to 17 millimetres; by about 40 millimetres, the larva has begun to look recognisably like a herring. At one year of age a herring is roughly 10 centimetres long, and the fish does not first spawn until its third year.

• 6. The Silver of the Sea

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  During the Middle Ages, herring prompted the founding of Great Yarmouth and Copenhagen and played a critical role in the medieval development of Amsterdam. Herring has been a staple food since at least 3000 BC, prepared raw, fermented, pickled, smoked as kippers, and preserved by a range of regional techniques. The fish is very high in the long-chain omega-3 fatty acids EPA and DHA and is a source of vitamin D. Water pollution introduces a complication: large Baltic herring slightly exceeds recommended limits for PCB and dioxin, though research suggests the cancer-reducing effect of its omega-3 fatty acids is statistically stronger than the carcinogenic effect of those contaminants. The size of the fish, which reflects its age, determines safe consumption limits: Baltic herrings larger than 17 centimetres may be eaten twice a month, while smaller herrings can be eaten without restriction. Early in the twentieth century, the scientific study of herring was fundamental to the development of fisheries science as a discipline. In Scotland, the industry reached its peak in the late nineteenth century, when a herring boom turned Shetland and its town of Lerwick into what became known as the "herring capital of Europe".