24 Univcrsitij of California Publications in Zooluyij [Vol. 15 



blades, permits the waves to strike with suffieieut force, in a moderately 

 rough sea, to lift the stern entirely out of the water. Even when fairly 

 (juiet, considerable surging- is caused in this manner. Now. if .sucli 

 a surge occurs after a haul has been made, say from fifty to tweut\- 

 five meters and the net is at rest awaiting the descent of the messenger, 

 it is clear that the net will be lowered very suddenly, sometimes as 

 much as eight or ten feet. Virtually, therefore, a current of water 

 passes upwards through the net which, while of brief duration, has a 

 comparatively high velocity. Assuming that it takes two seconds for 

 the boat to drop say six feet from the crest to the trough of the sea, 

 a temporary current of about three feet per second would thiLs be 

 generated which, if flowing horizontally, would be capable of trans- 

 porting stones the size of hens' eggs. When one considers that, though 

 the lifting power of such a current is much less than the transporting 

 power of its horizontal equivalent, the specific gravity of most plankton 

 organisms being only slightly greater than tliat of sea-water, it is at 

 once evident that .such a vertical current has more than sufficient 

 lifting power to throw out everything that the net might catch. 



However, the extent of such expulsion would vary with a number 

 of conditions. In the first place, it would depend upon the height of 

 the sea for, if any of the organisms were in the net-bucket, the momen- 

 tary effect of the surge would be insufficient to lift them above the 

 rim unless the net dropped a distance at least equal to its own altitude. 

 Secondly, the large surges occur onlj- when the waves follow each 

 other and .strike the boat in a particular way. Thirdly, violent surging 

 might occur but would have little or no effect unless it happened while 

 the messenger was making its descent. Fourthly, as the depth of haul 

 increases, the effect of surging decreases because of the inertia of the 

 cable which tends to respond by sagging, so that below a certain depth 

 the cable would be straightened out before the surge could eiJect the 

 net. Finally, those organisms would be expelled first whose exposed 

 surface was relatively large or whose specific gravity was relatively 

 small, so that a comparatively slight surge might have a selective effect 

 in expelling certain organisms while not affecting others. It is thus 

 evident that, if expulsion occurs at all, it must be very erratic, and 

 probably selective in its effect. 



As .stated above, we have as yet no definite proof that such ex- 

 pulsion occurs. One fact, however, pointing in this direction is that 

 open vertical hauls, say from fifty meters to the surface, usually (but 

 not always) catch many more organisms than an equivalent series of 



