Atlantic herring migration

Due to its economic importance and regular occurrence, research on the migration of Atlantic herring has been carried out for decades, so the migration patterns of this fish are relatively well understood. In many previous writings, interesting and religious examples have indeed been recorded. The British scholar Mr. Pinat wrote in his biography: "If we look at the migration of herrings, which has a preference, from a spiritual perspective, we may think of the great power with admiration and fear, which was originally given to us. The nature of this most useful animal enables it to know and show the way, giving these islands wealth, causing them to leave the vast polar sea at certain times and within a certain period of time, and throw themselves into our waiting fishing fleet... …”.

The migration of herring is of great scale, and this large-scale migration is caused by the mixing of different races. Herring can be divided into many tribes, and each tribe has a certain distribution area and a certain reproductive period. Off the coast of the UK, herring spawn almost every month of the year. The broodstock that spawn can be roughly divided into two types, namely the winter type that spawns in shallow water and the summer type that spawns in deep water. The migrations of various ethnic groups related to reproduction or foraging are very different. Norway's spring herring spawning grounds are in the coastal waters of southwest Norway. February to March every year is the spawning period. Except for a small number of hatched juveniles that stay in the bay, most of them are passively carried to the northern seas by ocean currents, and then move south every year until they reach sexual maturity in the fifth year. Return to the original birthplace (spawning ground) and start breeding. After spawning, the adult fish will feed and migrate from the spawning ground to the northern sea every year, reaching as far as the Spitsbergen Islands. Then they return to the southern Norwegian sea area to overwinter, and in early spring they go to coastal spawning grounds to breed. The distance of a round-trip migration is more than 2,000 kilometers.

The swarming migration of marine fish, under the influence of internal factors, is also affected by external environmental conditions, such as hydrological conditions and food conditions, which are closely related to migratory movements. Atlantic herring, which inhabits the North Sea, gathers in the near-bottom zone where the water temperature is 6-7°C in summer and autumn. But in the colder, lower-salinity water flowing out of Skatalak Strait in early summer, Atlantic herring shy away from these cooler temperatures on the bottom. Obviously, temperature and plankton are related to each other, and the appearance of certain plankton is often the best indicator of the arrival of sea currents. There is a clear relationship between Atlantic herring and zooplankton. Annual fluctuations in plankton populations can shift Atlantic herring feeding grounds. Sea areas rich in zooplankton can form larger schools of fish. Schools of fish feed intensively in areas with abundant food, which can even delay spawning and migration. Long-term changes in physical conditions (such as temperature) can permanently affect the reproductive period of fish and may even fundamentally change the timing of spawning. Long-term changes in temperature can push fish out of their original habitat and affect growth and the timing of first sexual maturity.

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