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The role of the marine food chain in herring distribution

The wealth that man derives from the sea is ultimately dependent on tiny plants called phytoplankton that float near the surface of the sea. In the sea, as on land, green plants need some basic conditions to allow them to live, grow and multiply. They need sunlight; carbon dioxide, water and certain mineral salts, all of which are available near the surface of the sea. These tiny plants form the starting point of marine food chains at the other end of which are larger creatures and eventually man. The phytoplankton are grazed by masses of tiny animals called zooplankton. These in turn provide food for herring and other fish that swim in the upper layers of the sea.

The mixing between the deep and surface water is very important as it leads to a continued supply of nutrients. In the surface layer of the sea, nutrients are quickly used up by plants which then fall to the seabed as they die. Bacteria act on these so that they are brought back to the surface. This happens best in shallow coastal waters. Within coastal waters there is also variation, with colder waters being more productive. This is because surface water cools in winter, and cooler water becomes denser and sinks causing surface and deep water to mix. Storms also speed up the mixing process.

In the waters around Scotland there are strong seasonal trends in plankton production. It is lower in winter and rises in spring and early summer as levels of sunlight increase causing phytoplankton (plant plankton) levels to rise. This in turn leads to an increase in zooplankton (animal plankton). The growth of plants gradually uses up the nutrients in the upper layers, so that by midsummer levels drop. However, levels rise again in autumn as storms mix the layers before the onset of winter cools the water again. The seasonal rhythm means that summer is the most important time for herring feeding and growth.

The movement of the herring is influenced by this seasonal rhythm, with the numbers of fish rising as the density of plankton in a particular area increases. This means that summer is the most important time for herring feeding and growth.

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