The fact that the salmon comes from the sea and spawns in the river has induced the supposition that this fish ascends the river only for the purpose of spawning. That the supposition is not always justifiable seems, however, sufficiently shown by the fact that clean-run salmon with undeveloped reproductive organs may be found in fresh water during the spawning season, and indeed at any season of the year, and also by the fact that a spring run of fish is usual. “When we regard the head waters of many of our rivers as localities for the natural propagation of the salmon, we find, however, that, except at the spawning season, adult fish are never present. When, at the same time, we remark that the fish which ascend to those waters during the winter months are all sexually ripe, we may fairly conclude that the fish are impelled to migrate to those head waters for the express purpose of spawning.

It has been said that the shads (Clupea allosa, and C. finta) and the sea lamprey (Petromyzon marinus) are examples of fishes which have a spawning habit analogous to that of the salmon, since they also ascend rivers for the purpose of propagating their species. Their habit is, however, more analogous to that of the comparatively few salmon which penetrate at once to head streams and tributaries, than to the fishes which inhabit the lower reaches of a salmon river. They ascend for a limited period only, and seek again the salt water whenever the operation of spawning is completed. They are marine fishes which spawn in fresh water. The common eel may be taken as an example of a fresh water fish which spawns in sea water.