EWING ET AL.: EFFECTS OF SIZE AND RELEASE TIME ON SALMON 



reared fish were released on 3 June. These fish 

 averaged 9.7 cm and 11.2 cm FL, respectively. On 31 

 May 1978, 121,000 fast-reared fish, which had been 

 graded according to fork length, were released in two 

 groups of 95,000 and 26,000 fish to test the effects of 

 size on migration and survival to adulthood. These 

 fish averaged 10.9 and 11.8 cm FL, respectively. 

 Downstream movement in both years was monitored 

 in the Columbia River at the Dalles Dam (52 km 

 downstream from the mouth of the Deschutes River) 

 by gatewell sampling conducted by the National 

 Marine Fisheries Service and the Oregon Depart- 

 ment of Fish and Wildlife. Sampling was conducted 

 5 d a week throughout May and June. Juveniles 

 originating at Round Butte Hatchery were identified 

 by analysis of coded wire tags. 



Apparent Growth Rates 



Apparent growth rates in Pelton ladder and in the 

 Deschutes River were calculated from the size of the 

 juveniles released into the ladder or the river and the 

 size and time at which they were recaptured. Actual 

 growth rates could not be measured, because selec- 

 tive mortality of small fish or migration of larger ones 

 could not be estimated. Differences in fork lengths 

 were tested for significance at the 95% confidence 

 level using Student's t test. 



RESULTS 



Timing of Migration 



Maximum migration of chinook salmon juveniles 

 released in February and March into Pelton ladder 

 occurred between mid-May and the first of June in 

 both 1977 and 1978. There was little migration in 



these groups before or after this 4-wk period (Tables 

 1, 2). Fish released in April showed two peaks in 

 migration. A large percentage of the fish moved 

 through the ladder within 2 wk after release, while a 

 second peak of migration occurred during the last 2 

 wk of May. Fish released in early May also had a large 

 percent migration within 2 wk after release, but the 

 greatest percent migration occurred during the first 2 

 wk in June. When chinook salmon juveniles were 

 released from June to November, most of the fish 

 moved through the ladder within 7 d after release. 

 The maximum percent migration within 7 d after 

 release occurred in fish released in early June 1977 

 (Fig. 2) and in mid-June 1978 (Fig. 3). Fish released 

 in August and at later times had reduced migration 

 and had a higher tendency to become residual 

 (Tables 1, 2). Migration of slow-reared fish released 

 into Pelton ladder from May to August 1978 was less 

 than half that of fast-reared fish released at the same 

 time (Fig. 3B). 



Daily migrations of two groups released in February 

 and March 1978 were compared with those from 8 

 May to 8 June. Movement of both groups was coin- 

 cidental throughout this period (Fig. 4), suggesting 

 that environmental factors such as temperature 

 influenced migration tendency. Temperatures in the 

 ladder varied seasonally due to solar warming (Fig. 

 5). Maximum temperatures of 17°C were attained in 

 August 1977 and in July and August 1978. Tem- 

 peratures in both years exceeded 13 °C by June, sug- 

 gesting a possible temperature threshold for 

 migration. While the relationship between migration 

 and temperature was very poor (correlation coeffi- 

 cient, R 2 = 0.074), there may have been a tendency 

 for peaks in seaward migration to occur 1-2 d after 

 transient increases in temperature (Fig. 4). 



IOO 



z 

 o 



5 80-1 



a: 



o 



5 60 



S 40 



20 



IOO 



RELEASE M 

 DATE 8 



M 

 31 



A 

 12 



IOO 



H 



200 



M 

 2 



M 

 II 



J 

 14 



J 

 12 



A 

 9 





 14 



N 

 16 



X LENGTH 7 3 8 5 91 97 IOI 112 12 134 14 9 168 18 19 1 



FIGURE 2. — Percentage seaward migration within 7 d following 

 release for each group of fast-reared spring chinook salmon released 

 into Pelton ladder in 1977. Above each bar is the number of fish 

 released. Lengths are means of samples of 30 fish taken from the 

 population at the time of release. 



IOO 



80 



60 



40- 



20 



I, OOP 



RELEASE 

 DATE 



F 

 14 



M 

 15 



A 

 15 



M 

 15 



J 

 15 



J 



14 



A 



15 



XLENGTH 63 80 89 99 116 129 149 



M 

 15 



n 



j 



15 



J 

 14 



83 98 114 



A 

 15 



127 



FIGURE 3. — Percentage seaward migration within 7 d following 

 release for each group of spring chinook salmon released into Pelton 

 ladder in 1978. A) Fast-reared chinook salmon. B) Slow-reared 

 chinook salmon. Above each bar is the number of fish released. 

 Lengths are means of samples of 30 fish taken from the population at 

 the time of release. 



159 



