KEOKUK DAM 
105 
SIGNIFICANCE OF THE BARRIER 
TYPES OF MIGRATORY MOVEMENTS 
The term “barrier,” as applied to any fixture in the course of a stream, implies a 
necessity for some sort of movement of fishes from place to place. If any group of 
fishes were found to live continuously in the same place with no necessity or habit of 
removing therefrom, a structure of any kind placed above or below that place could 
not be termed a barrier for that group. No matter if the structure be impassable, it 
is not an obstruction to movement if there be no necessity or inclination for the fish 
to pass. 
If another group of fishes is accustomed to range indiscriminately back and forth 
over an extended region, then a construction that checks these movements at a cer- 
tain point becomes in a definite sense a barrier, but perhaps not a significant one. 
If, again, some fishes are habituated to spend certain periods of their lives below 
a given point in a stream and other periods above that point, an impassable obstruc- 
tion becomes a barrier of a significance that is greater or less according as the periodic 
migrations are essential for the continued existence and abundance of the species on 
one side or the other of the barrier. 
The significance of the Keokuk Dam as a barrier depends, then, upon what species 
of fish may inhabit the Mississippi River at Keokuk in any season and what may be 
the migratory movements habitual or essential to those species. These are questions 
into which we must inquire as closely as possible. 
The most familiar, because the most extreme and the most conspicuous, instances 
of a migratory tendency in fishes are those of the Atlantic shad, the alewives or river 
herring, and the salmons, all of which at certain definite seasons leave their accus- 
tomed waters to pass in definite migration up the courses of the rivers to spawning 
grounds that may be hundreds of miles from the point of departure in the sea. Such 
fishes are termed “anadromous,” a word of Greek derivation meaning “uprunning.” 
While none of the familiar species mentioned are found at Keokuk, it does not follow 
that there may not be other species there that are also of anadromous habit. 
Another type of migratory habit equally pronounced, though less conspicuous and 
familiar, is just the reverse of the anadromous habit and is called “catadromous.” 
This is manifested by the common eel, which spends the greater part of its life in rivers 
and lakes but which, on the approach of sexual maturity, abandons the fresh waters 
and journeys down to the sea to give rise to a new generation in the depths of the ocean. 
The young eels born in the sea find their way into the mouths of the rivers, which they 
ascend gradually and adopt as their home until they, in turn, must return to the sea 
to accomplish the ultimate end of their existence. 
A third and very familiar form of migration is that characterizing the majority 
of the common fresh-water fishes, which at the time of spawning find their way into 
the shallower waters along the shores, in the outspreading water of spring floods, or 
in the upper portions of the smaller tributary streams. Very few of the common fishes 
are known to form nests or deposit eggs in the deeper waters of the rivers and lakes. 
There are yet other manifestations of a migratory habit, which may be less regu- 
lar or definite in character. Such are the movements governed by the search for food, 
the seeking of protection from extreme temperatures, the avoidance of enemies, or 
the perhaps involuntary drift with the current. 
These, then, are the types of migratory movements that we must have in mind 
in our consideration of the fishes of the Mississippi as they may be affected by the dam 
