322 



THE SEAS 



method, it is necessary now to put to sea to these grounds. 

 The plaice egg we have said is pelagic and drifts about in 

 the water layers above the bottom. To catch these eggs 

 therefore a tow-net must be used. The area to be searched 

 is divided up on the chart into a number of small sections 

 in each of which a point or " station " is marked, at which 

 hauls with the tow-net are to be made. Generally, when a 

 research vessel is on a " plaice-egg cruise," a vertical 

 tow-net is used ; that is, a net which is hauled up vertically 

 through the water from bottom to surface. The depths 

 being known, it thus becomes possible to compare with 

 accuracy the actual numbers of eggs caught in different 

 localities. At the end of the cruise the numbers of eggs 

 taken at each station are marked out on the chart and 

 contours drawn through the stations where approximately 

 the same numbers of eggs were caught (Fig. 61). In 

 this way, if the whole area in which the eggs were actually 

 present has been studied, a map should be produced showing 

 the largest numbers of eggs caught near a centre, the 

 numbers becoming less and less the further one goes from 

 this centre. Now the worker, who was counting the eggs, 

 at the same time made notes as to the stage of development 

 of the young fish within the egg. By the use of drift 

 bottles and current metres, as described in a previous 

 chapter, a rough measure can be obtained of the move- 

 ments of the water in the regions examined. It has been 

 worked out how long it takes a plaice egg to develop under 

 different temperature conditions, and the temperature 

 of the water being known the investigator can immediately 

 reckon how many days have elapsed since the actual 

 spawning of the eggs which he has taken. Knowing also 

 the rate and direction of the water movements he can lay 

 his finger on the spawning grounds. 



Other methods need to be resorted to in a search for the 



