the immatures reaching the lake do so in their 

 first season. Development to sexual maturity 

 takes place in the lake, usually in three or four 

 years. At spawning time, the adults ascend the 

 tributary streams, the females dig the nests, 

 spawning takes place, and the survivors of 

 spawning and predation return to the lake . Of 

 those remaining alive the following spring, some 

 will again spawn that season, but a greater 

 number will spawn the year following. 



Careful measurements on these tributary 

 streams have shown that mortalities in the egg 

 stage are high, and usually less than one per- 

 cent of the eggs laid will reach the lake as fry . 

 Mortalities are low thereafter, but vigorous 

 predation accounts for many adults, both before 

 and after spawning. 



Great distances are sometimes involved 

 in migrations in the lake and in the Lower Yel- 

 lowstone River, and considerable mixing occurs 

 in the lake with fish from the several spawning 

 streams. Segregation takes place at spawning 

 time, and the spawners return to their natal 

 streams for reproduction. 



RACES IN SPAWNING STREAMS 



Homing --The study of marked cutthroat 

 at Yellowstone Lake has given an insight into the 

 existence of races that are identified with 

 spawning streams. The demonstration that 

 homing is a strong urge in these populations 

 permits us to recognize races that are distinct 

 from each other geographically, and examina- 

 tion will later show that they are ecologically 

 distinct, as well. 



Spawners ascending Pelican, Chipmunk, 

 Grouse, Arnica, and Clear Creeks were marked 

 with Peterson disks in varying numbers. Fish 

 in each of these streams were not marked each 

 year, but from 1949 to 1953, 18,836 tags were 

 applied. The fish were allowed to resume their 

 upstream migrations and spawn, and the sur- 

 vivors were allowed to descend into the lake. 

 After subsequent mortalities in the fishery and 

 from natural causes, some were still alive at 

 the next spawning time . Only 3 .2 percent of re- 

 turning spawners returned to streams other 

 than those in which they were originally tagged. 

 Table 1 summarizes the tag returns to spawning 

 streams. 



This overwhelming evidence may not be 

 proof of what is called "homing , since the fish 

 were not marked as immatures before they 

 originally left their natal streams. It does con- 

 stitute a basis, however, for the claim that each 

 of these populations uses only its own stream, 

 and can therefore be termed a local race . 

 Homing in these fish has been discussed by Ball 

 (1955). 



The question may be raised concerning 

 the lake habitat of these fish in support of the 

 view that they may return to their own streams 

 not because of their being part of a particular 

 race, but because they spend their lake existence 

 near the mouths of their own streams and use 

 these streams because they are the closest ones 

 at spawning time. This idea is partly refuted by 

 more tagging evidence. Postspawners from 

 some streams often travel considerable dis- 

 tances in the lake before returning to their 

 streams for a second time. For example, tagged 

 fish from Grouse and Chipmunk Creeks are taken 

 in some numbers in West Thumb, as well as in 

 the South Arm . Many of these are caught off the 

 West Thumb Dock, a distance of 20 miles from 

 the home streams. Fish from all streams are 

 caught in greatest numbers in their own arms 

 of the lake, but many are caught at distances . 

 The distribution of angling pressure is an im - 

 portant point in these cases, affecting the 

 localities of recovery of tagged fish. The 

 evidence is clear that many fish return from 

 great distances, and from other arms of the 

 lake to spawn in their home streams. Tagging 

 has also demonstrated that fish in the Yellow- 

 stone River ascend to the gravels near Fishing 

 Bridge and spawn there, and that some fish from 

 the lake move down to these same spawning beds 

 at the same time. Each group then returns to 

 the waters from which it started its spawning 

 migration. Movements of 10 miles are common 

 with these fish. 



Another point of importance is that the 

 postspawners from any stream are commonly 

 caught in lake areas harboring fish from other 

 streams. The lake fish are thus a mixture of 

 fish from different streams, and at spawning 

 time the various groups have equal opportunity 

 to seek the closest stream for spawning. What 

 happens, however, is that the groups segregate 

 in the lake, and each race finds its own stream . 



77 



