DISTRIBUTION AND MIGRATIONS 



255 



By far the most important factor limiting the geographical 

 range of coastal fishes is the temperature of the sea. This 

 naturally shows some variation in different regions, as well as at 

 different seasons of the year, but it is possible to construct a 

 map to show the average annual temperature in various parts 

 of the world. Such a temperature chart of the oceans is crossed 

 by a series of wavy and irregular lines, running from east to 

 west, and known as isotherms, or to give them their full 

 title, mean annual surface isotherms, lines drawn through 

 points of equal temperature (Fig. 98). In other words, the 

 isotherm of 6° C. is a line connecting all the localities in which 

 the average annual surface temperature of the sea is 6° G. 



Fig. 98. 



Distribution of the genus Sardina. 

 The mean annual surface isotherms of 6°, 12°, and 20° C. are shown. 



The distribution of many pelagic and coastal fishes corresponds 

 remarkably closely with the temperature of the water, and it is 

 possible to divide the world into a number of zones of dis- 

 tribution, encircling the globe like a series of horizontal bands, 

 each lying between two of these isotherms. In the centre is 

 the broad Tropical Zone, limited by the isotherm of 20° C. ; 

 above and below are North and South Temperate Zones, 

 extending to the isotherms of 6° C. in the north and south, 

 each of which may be further subdivided by the isotherm 

 of 12° C. into subtropical and subarctic and subantarctic 

 zones; and finally, beyond the isotherms of 6° C. are the Arctic 



