364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
sufficient to mix them, in lower latitudes the boundary layer and resist- 
ance to vertical mixing persist all the year round. In these latitudes 
the continual plant growth in the surface layer uses up the nutrients or 
reduces them to very low concentration. The underlying deep water 
is rich in nutrients because of the decay of sinking organic matter and 
lack of light, but the amounts of these nutrients which find their way 
to the impoverished surface layer are small because of the absence of 
mixing between the cold and warm waters. Anything which tends to 
encourage upward movement of cold water and to increase mixing 
thus leads to richer growth at the surface, and the boundary of any 
current or the site of any sharp change in speed and direction of cur- 
rent is a likely place to look for the physical processes which lead to 
greater mixing. Any boundary in the ocean, especially if the land lies 
to the left of the direction of the current in the Northern Hemisphere 
or to the right in the Southern Hemisphere (because of the earth’s 
rotation), is thus likely to be richer in food than offshore water. But 
any change of depth, particularly at the edge of the continental shelf, 
may also cause upwelling; shoals and shallow waters around islands 
generally have a much richer flora and fauna than the deep water close 
at hand. 
Far away from land, useful concentrations of fish have been found 
on boundaries of currents, such as that between the north equatorial 
current and the equatorial countercurrent. New ocean surveys have 
been planned in the light of such knowledge, and an understanding of 
the dynamics of water movements can be as good a guide in extending 
salmon and tunny fisheries into the deep ocean as a knowledge of 
shoals and currents has been to shallow-water fishing. 
But while upwelling and mixing are over-all guides to regions of 
high productivity, an understanding of the actual living and shoaling 
habits of fish also require study. ‘The areas richest in fish are not 
likely to be found where the water is coldest and contains most nu- 
trients; it may be that water coming up from below has to remain 
some time at the surface before it can support a rich growth of 
plankton and fish. Most fish are thus generally found in the boundary 
regions between colder and warmer waters. 
The distribution and shoaling habits of fish are also known to be 
affected by many other factors, such as winds, tides, currents, or 
perhaps an inlet of fresh water, and although there is often some local 
knowledge and experience available in this respect, there is too little 
systematized information on which further knowledge could be built. 
A better knowledge of water movements would also help to improve 
the operation of fishing gear and to develop new techniques, such as 
acoustic methods of finding, identifying, and counting fish, while a 
better knowledge of local movements of fish (and prawns) would in- 
