PLANKTON: LARVAE
1. Crab zoea; 2. Fish egg; 3. Sea Urchin pluteus; 4. Barnacle nauplius; 5. Fish larva; 6. Mussel larva; 7. Copepod nauplius; 8. Worm larva.
PLANKTON: UNICELLULAR ORGANISMS
1, 2, 3, 7, 10, 11, 12, 13, 16, 17, 18. Diatoms; 4, 5, 7, 9, Peridinians 8. An Algal spore; 14. Noctiluca; 15. A Radiolarian.
Fig. 6
Unlike that of the land, the productivity of the sea is greater in colder latitudes than in the tropics. This somewhat unexpected fact is attributable to the action of denitrifying bacteria which, flourishing more readily in warm, tropical waters, effectively reduce the amount of available nitrogen compounds in the water. In colder waters, denitrifying bacteria are less active, and nitrates and nitrites are available in larger quantities for the nourishment of the plankton.
All the great fisheries of the world are prosecuted in cold or temperate seas; as examples of this we have the Banks of Newfoundland, the cod fisheries of Norway, and the great trawling grounds of the North Sea and the North Atlantic.
All fish, during the larval stage of their development, feed first upon the contents of the yolk sac which, when they are hatched, is attached to their ventral surface. When the yolk is absorbed, the larvae feed upon the microscopic plankton that abound in the water on every side. The surface water, with its warm temperature, high plankton content and sunlight, forms an ideal nursery for the very young fish of all species. Demersal fish, as they complete the larval stage of their development and descend into deeper water, have to rely for their food either upon the various species of young shellfish and crustacea that drop from the surface water as they develop, or hunt for their food amongst the small fish, mollusca, crustacea, worms and seaweeds of the sea-bottom. Plaice feed chiefly upon cockles and other mollusca, which in their turn feed upon diatoms. The cod is almost omnivorous, greedily devouring small fish, crustacea, worms or mollusca; its favourite food, however, is shrimps and prawns. These, in their turn, feed upon smaller invertebrates, for example small jellyfish and larval molluscs, and these upon microscopic plankton.
Pelagic fish, herrings and mackerel, feed almost entirely upon the larger plankton, mainly copepoda (small, shrimp-like crustacea). These may be present in the surface water in enormous quantities at certain times. In many cases, shoals of herring or mackerel probably follow special swarms of copepoda. Mackerel also feed upon young fish, hermit crabs, and prawns.
With a few notable exceptions, the various species of demersal fish feed upon smaller fish. Thus—
The hake, normally a deep-water fish, ventures inshore in pursuit of herrings, pilchards, mackerel.