FISH MIGRATIONS

Abstract

SUMMARYA knowledge of the migratory movements of fish is of great practical importance in the commercial fisheries, and accordingly much research has been carried out on the subject by the maritime nations of Europe and by the United States, largely by means of marking experiments. The general schema of the migrations, which holds good for most species, is (1) a spawning migration of mature or maturing individuals towards a definite spawning area, generally upstream or up‐current, (2) a dispersion of the spent fish, generally downstream or down‐current, in search of food, leading sometimes to definite feeding migrations. The general effect of this cycle of migration is to keep the species within its ecological norm.These principles are illustrated by reference to the cod and the plaice in the North Sea, Norwegian coast and Barentz Sea, Iceland and Greenland. It is shown that in each of these areas there is a definite migration cycle, and evidence is brought forward that in most cases the spawning migration takes place against the prevailing current. The recent spread of cod from Iceland to Greenland and their return to the Icelandic spawning grounds is described in some detail.In cod and plaice the eggs and larvae are pelagic and are at first distributed passively by the currents. It seems impossible that they should retain any individual memory of their spawning places. In the case of the salmon, however, the eggs are deposited in the upper waters of rivers and streams and the young fish spend one or more years of their life in the vicinity. It has been shown for the chinook salmon of the Pacific that the fish returning from the sea to spawn remember and return to the stream in which they were brought up, even though they had been spawned in a stream hundreds of miles away. Hence individual and not ancestral memory is the dominant factor in the so‐called “homing instinct”.To provide a contrast with the salmon and other anadromous forms, which run up rivers to spawn, brief reference is made to katadromous forms such as the flounder, the eel, and the New Zealand “whitebait” (Austrocobitis). The last‐named appears to spawn just after the height of spring tides, as is also the case with the Californian grunion (Leuresthes), whose very remarkable spawning habits are discussed in detail.In conclusion, an account is given of the response of the local anchovy to the cutting off of its main spawning ground through the reclamation of the Zuider Zee; it has adopted new spawning grounds outside and has spread further north.The article is not to be regarded as exhaustive, but merely as supplementary to the comprehensive surveys of the subject by Meek and Scheuring.