EXTERNAL FACTORS ON THE SPAWN DATE 



133 



but, once drawn, arc quite easy to understand, for the principle is 

 identical to that of an ordinary contour map. In fact, a map could be 

 called a "joint functional regression diagram, with latitude and longi- 

 tude as the independent variables, and altitude as the dependent 



ISOPHENES IN YEAR-DAYS 



100 200 300 



RAINFALL IN MILLIMETRES 



Fig. 35. Joint Functional Regression Diagram, Showing the 



Relation between Rainfall and Temperature, and Spawning 



Date, for the Month M2 



Note that there is a strong temperature effect above about 5°C or 6°C, but 

 little rainfall effect in this region of the diagram. At lower temperatures, 

 there is a rainfall effect, but very little temperature effect. In aU these 

 diagrams, the shaded areas indicate that there are either no records at all 

 with the corresponding variables, or too few for reUable results. 



variable." It is shorter to say "map," but there is no word to corre- 

 spond when the independent variables are different, such as tempera- 

 ture and rainfall, and the dependent variable is spawn date. 



The different months were denoted Mo, Mi, M2, because the 

 month in which spawning takes place (Mo) differs so much in different 

 parts of the country that if the calendar months were used for the 



