The mackerel is a southern fish, and prefers the warm water of the Mediterranean and West African coast. In spring, as the wave of rising sea temperature travels northwards, it migrates to the English Channel and the North Sea. This migration is often directly associated with the presence, in large quantities at that season, of a particular kind of copepod in the surface water of the English Channel. =Phosphorescence.= Many marine creatures, ranging from deep-sea fish living in the dark abysses of the ocean to various species of the minute plankton drifting in the surface water, possess phosphorescent organs, which emit light of low intensity similar to that of a glow-worm and firefly. In many cases the light appears to possess some important function, and highly specialized organs are developed. In such cases the light is only emitted in response to some stimulus—thus, the phosphorescence of the surface water of the sea, when disturbed by the blade of an oar, is due to the disturbance of myriads of minute planktonic organisms, equipped with phosphorescent organs, either protozoa or protophyta; many pelagic copepods are phosphorescent. In other cases, phosphorescence appears to be a more or less accidental by-product of some other process, and of little or no significance. The substance which produces the glow is contained in the slimy secretion produced by the epidermal glands of the fish, and, as phosphorescence can only occur in the presence of oxygen, it is evident that the light is produced by the slow oxidation of this substance. The colour of the light emitted by marine organisms is generally blue or light green, but red and lilac also have been observed. The distribution and colour of the light or lights produced by individual fish vary with the different species. In many cases it would appear that these points of light provide the means by which fish recognize each other in the dark depths of the ocean. Some fishes possess highly developed phosphorescent organs known as photophores, consisting essentially of a group of gland cells that secrete the phosphorescent fluid. These organs are generally distributed in rows along the sides and ventral surface of the fish. Some fishes possess more complex and highly developed organs containing, in addition to the gland cells, a system of blood vessels and nerves, a transparent, protecting membrane and reflector, an iris-like diaphragm and a lens. These more complex organs are generally larger and less numerous than the simpler ones. Possibly they are used to search for, or to attract, prey. The phosphorescence of decaying fish and meat is due to the presence on the fish or meat of certain bacteria of putrefaction, which are themselves phosphorescent. When seen under the microscope, the individual bacteria appear as shining points of light. CHAPTER III METHODS OF FISHING