SEA-FISITERIES LABORATORY. 329 
of our work which appeared in “ Internationale Revue 
der Gesamten Hydrobiologie und Hydrographic” for 
December, 1908, by Prof. C. A. Kofoid, the reviewer 
suggests “that possibly the two nets towed aft the boat 
were, owing to the swirl caused by the propeller, probably 
straining water which in the undisturbed sea was at 
different levels, and that their catch represents the 
plankton not of the upper two feet but of a considerable, 
and for the two nets probably different, radius below this 
level” (doc. cit., p. 846). 
In a private letter to one of us, Prof. Kofoid again 
raises the same point, and adds:—“ At a distance of 
50 feet behind the vessel it is quite possible that the 
diameter of the moving mass would be over 8 feet 
as 
surface water is stirred and presumably the deeper layers 
to a slightly less distance .... These limits are just 
such as might well make the difference between a catch 
with many worm and Crab larvae and adult Copepoda, and 
one without—under certain conditions of light or time of 
day.” 
In our steamer the diameter of the propeller is 4 feet, 
and when one of the four blades is vertical its top is 
just 2 feet below the water line in a calm sea. So the 
water is stirred up directly by the propeller blades to a 
depth of 6 feet below the surface, and indirectly, no doubt, 
for some little distance below that; but whether organisms 
would be brought up to the surface nets from a greater 
distance than, say, a fathom and a. half is probably 
doubtful; and whether stirring up to that depth will make 
a difference in the catch is also questionable. Still these 
are all points that ought to be cleared up by experiment, 
and we propose to deal with them in the coming season, 
and to compare, for example, the catches in nets towed 
some little way off the side of the ship with similar nets 
towed behind, 
