Method's in Plankton Investigations. 11 



5 



by the operator, who regulates the speed by the meter knots 

 on the draw-line and the counting of the oarsman, one count 

 being given every other second by the watch. By this method 

 a uniform velocity for the plankton net at all parts of its 

 ascent and in different hauls is assured, and a very important 

 source of error in the vertical haul, as it is usually made, is 

 avoided. The oblique haul is not so complicated as it may 

 perhaps seem to the reader. With a little practice the whole 

 operation may be completed in less than twenty minutes. 



Whenever a current is present in the body of water exam- 

 ined, it has been our custom to make the oblique haul across 

 the current, thus eliminating, so far as may be, its effect upon 

 the coefficient or straining capacity of the net. In water in 

 which there is little or no current it is of advantage, in work- 

 ing in a skiff, to set the apparatus "with the wind." In 

 waters abounding in vegetation, channels of the length and 

 width requisite for making the oblique haul were opened from 

 time to time by cutting out and removing the rank growth of 

 aquatic plants. 



5. Difficulties encountered. — Certain difficulties attended 

 the operation of the oblique haul. Owing to the turbidity of 

 the water, in many situations it was practically impossible to 

 place the apparatus so as to avoid vegetation. Whenever the 

 plankton net or the ropes strike a submerged plant, a cloud 

 of flocculent debris is set free in the water and the collection 

 is fouled. It was only with the greatest labor that channels 

 could be kept open in the vegetation, for its rapid growth and 

 its shifting by the wind soon closed any opening that had 

 been made. Again, the manipulation of the apparatus in 

 rough weather is somewhat difficult, the waves at times tear- 

 ing loose the stakes before the completion of the collection. 

 At periods of high water the strong current and the increased 

 depth made it impossible to set the apparatus or keep it in 

 place. This necessitated the substitution of a series of ver- 

 tical hauls from a floating boat for the customary oblique 

 method of collection. 



The plankton at Havana is subject to extreme local and 

 seasonal variations, not only in volume but also in composi- 



