52 
BULLETIN OF THE BUREAU OF FISHERIES 
planktonic animals, so often recorded and occasionally so extensive, are invariably 
the result of actual and corresponding movements of the water masses in which they 
live. Utterly at the mercy of tide and current, they drift as helplessly as buoys with 
the latter, able to escape from an unfavorable environment only by swimming up 
or down in response to light or to gravity. For them there is no such thing as the 
geographic migration in the true sense, with which we are familiar among birds and 
fishes. 
It follows from this that to state the currents or the more diffuse movements of 
water that enter the Gulf of Maine is to list the sources from which occasional visitors 
can reach it. These are, first, but least important, the surface stratum of tropical 
water, popularly known as the Gulf Stream, lying close outside the continental 
edge, proverbial both for high temperature and salinity and for the tropical pelagic 
fauna it carries with it, and which enters the gulf regularly, though in small amounts, 
as a component of the general surface indraught into its eastern side, besides flowing 
directly across Georges Bank on rare occasions. Second, and equally characteristic 
both hydrographically and biologically, is the ice-cold water of the Cabot or Nova 
Scotian current that flows past Cape Sable in considerable volume in spring, carry- 
ing arctic inhabitants. Greater in amount than either of these, though not always 
so clearly characterized by its plankton, is the complex mixture between coastal, 
northern, and tropical oceanic waters, which is constantly being manufactured 
along the outer edge of the continental shelf and over the upper part of the 
continental slope, and which composes the major part of the influx into the 
eastern side of the gulf. To this the name “cold wall” has often been applied. 
Finally, the mid-depths of the Atlantic basin contribute an occasional straggler, 
which must enter via the deepest trough of the Eastern Channel. None of these 
sources, except the third, adds appreciably to the gulf plankton, in which, as I have 
pointed out, endemic animals are overwhelmingly preponderant; but so important 
are the exotic forms as indicators of the respective waters that give them birth that 
they deserve more attention than their numerical strength of itself would warrant. 
Several of the commonest and most characteristic inhabitants of the different 
ocean currents are among the largest and most easily recognized. For example, the 
presence of a Salpa or of a bit of gulf weed (Sargassum) anywhere in the Gulf of 
Maine is as sure evidence of an actual influx of Gulf Stream water as if the latter 
could actually be seen, and the same is true of the Arctic pteropod Limacina helicina 
for northern waters. Note, also, that whatever the origin of an exotic immigrant, 
whether Tropic or Arctic — or any driftage, for that matter— it travels the same route, 
once it is caught up in the inflow into the eastern side of the gulf, a fact well illus- 
trated by the striking resemblance between the distribution (within our limits) of 
the cold-water Aglantha, on the one hand (p. 353), and the whole category of tropical 
organisms, on the other (fig. 31). So close, in fact, is the parallel, that the one chart 
might almost be substituted for the other, so far as the inner parts of the gulf are 
concerned, were the seasonal element ignored. Immigrants in the upper strata, 
whatever their source, rarely reach the central part of the gulf unless their numbers 
be fortified and their period of existence within our limits lengthened by local repro- 
duction; but those entering in the deeper strata of water do follow the troughs (p. 64). 
