376 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



its life. Memoiy impressions of the goal or home 

 are implied and the fish is said to be seeking its 

 sjiawning area. The fish is thought to return to 

 its native stream because that was the place 

 where it was spawned rather than because the 

 stream was more accessible or made more attrac- 

 tive by immediate environmental conditions. 

 The fact that significant numbers of fish have 

 actually been observed to return to the streams 

 from which they originated is frequently used as 

 evidence for this homing viewpoint. 



The other theory is that environmental factors 

 control the direction of migration, and therefore 

 physical and chemical conditions such as tempera- 

 ture, current, amounts of dissolved gases, or odors, 

 are thought to determine the ultimate destination 

 of the fish. These factors fluctuate and are dupli- 

 cated in nature so that migratoi-y fish could go to 

 any stream with these conditions. 



Those who support the idea of environmental 

 control of the direction of migration present evi- 

 dence that the fish responds to its immediate en- 

 vironment at each point of its migration and they 

 look upon the migration itself as merely the sum of 

 the successive responses. The various species are 

 thought to arrive at their separate destinations 

 because they respond in specifically different ways 

 to the existing patterns of environmental stimuli. 

 The return of many fish to the stream of their 

 origin is to be expected, according to this view- 

 point, because the patterns of environmental con- 

 ditions which direct them persist year after year. 



Variations of these views may be found ex- 

 pressed under different interpretations of the 

 "parent stream" or ''home stream" theory devel- 

 oped by investigators working on salmon migra- 

 tion. An excellent review of major j^roblems and 

 controversial questions in salmon migration is 

 afforded by the symposium, "The Migration and 

 Conservation of Salmon," published in 1939 by the 

 American Association for the Advancement of 

 Science. 



SENSORY BASIS OF ORIENTATION 



Whether the migratory lish has purjpose and 

 seeks its native stream or whether the fish is en- 

 tirely directed by immediate envii'onmental fac- 

 tors along its route, its orientation must be 

 achieved by some sensory means. 



With fish migration, as with bird migration, 

 there has been much speculation over the possible 

 existence of a special sensory perception that is 

 unknown at present. No Jphj'sical basis for this 

 has ever been found nor have such sensory abili- 

 ties ever been demonstrated. There is evidence 

 that fish can see, hear, taste, and smell. It is 

 known that they can respond to tactile and kines- 

 thetic stimuli, react to acceleration and nonrecti- 

 linear motion, and maintain equilibrium. It has 

 been demonstrated that they can respond to tem- 

 perature and to various chemical substances by 

 means otiier than taste or smell. In the absence 

 of any evidence to the contrary, it is reasonable 

 to suppose tliat these sensory abilities are the ones 

 by which fish are guided while on their migrations. 

 Therefore, it appears logical to explore fully the 

 I'elation between these known sensory abilities and 

 prevailing environmental' patterns before consid- 

 ering hypothetical sensory abilities. 



RELATION OF SENSORY ABILITIES TO 

 ENVIRONMENTAL PATTERNS 



As the sensory abilities of fishes have been re- 

 vealed and delineated many investigations have 

 been made to examine their relation to migration. 

 Chidester (1924)^ pointed out the number and 

 diversity of these investigations as well as their 

 apparently contradictory evidence. 



Attention was turned to natural environmental 

 patterns which might have a directional influence 

 upon fish with the known sensory abilities. The 

 physical and chemical gradients which exist along 

 the migration paths of anadromous fishes were 

 examined for their possible role in orienting 

 migrating fish. 



The observations of Ward (1920) in Alaska led 

 him to believe that temperature was an important 

 factor in the choice of a spawning stream bj' 

 sockeye salmon, OncorKynchus nerka. He ob- 

 served that on the upstream migration the fish 

 I^ass from swift to slow water and vice versa, that 

 they go from shallow to deep water and vice versa, 

 and that they move from turbid to clear water 

 and vice versa. However, he found that at the 

 junction of two streams the sockeye sahiion con- 

 sistently chose the colder stream and he concluded 

 tliat temperature was the chief orienting factor. 



^ For references referred to parentheticaUy, see Literature 

 Cited, p. 395. 



