CRONE and BOND: LIFE HISTORY OF COHO SALMON 



tion studies of juvenile coho salmon in Sashin 

 Creek suggest that no large-scale reentry of coho 

 salmon fry occurs. 



Coho salmon fry from an Oregon coast stream 

 adjusted to water of moderately high salinities in 

 laboratory tests (Conte et al. 1966). Our field 

 observations, live-box experiments, and bioassays 

 at Little Port Walter confirm that ability for fry 

 from Sashin Creek. In July 1964, after about 44,000 

 coho salmon fry had migrated from Sashin Creek, 

 schools of fry were seen near the surface of the 

 inner bay. Most of them appeared to be in water of 

 low salinity above a density interface about 30 cm 

 deep, but they retreated to deeper more saline 

 waters when disturbed. 



To study the ability of coho salmon fry to adjust 

 to the saline conditions in the Little Port Walter 

 estuary, some fry were confined in live-boxes in 

 the inner bay during the summer of 1964. Two 

 sizes of live-boxes were used: six small boxes (86 by 

 86 by 122 cm deep) were arranged so that the 

 water depth in the box was 81 cm, and two large 

 boxes (122 by 122 by 244 cm deep) were suspended 

 from a floating frame so that water depth was 235 

 cm. The small boxes were arranged in three sets of 

 two boxes each, and 60 fry were placed in each box; 

 the fry were from the weir trap, the inner bay, and 

 Sashin Creek. Twenty-five fry from the inner bay 

 were placed in each of the large boxes. 



Initially, the Sashin Creek and weir trap fry in 

 the live-boxes entered high-salinity water for 

 short periods only, whereas some inner bay fry 

 remained in the high-salinity water for long 

 periods. Most of the fry stayed at or near the 

 density interface close to the top of the box where 

 the salinity was 14"/oo or less; but some, especially 

 those from the inner bay, swam for extended 



periods near the bottom of the box where salinity 

 was 28 to 29"/oo. 



Survival and growth of the fry seemed to be 

 related more to the size of the live-box and the 

 resulting competition for food than to the fry's 

 ability to adjust to the saline water of the bay. In 

 the small live-boxes, during the first 5 days 3% of 

 the fry died and in 30 days 26% had died; in the 

 large live-boxes in 35 days only 8% of the fry died 

 (Table 12). The general comparison is true both for 

 the entire small-box group versus the large-box 

 group and for the small-box group of fish from the 

 inner bay versus the large-box group (also from 

 the inner bay). No supplemental food was provid- 

 ed, and the fry in the small boxes grew very little 

 or not at all, whereas those in the large boxes grew 

 about 5 mm (Table 12). 



Tests were conducted in July 1964 to measure 

 the ability of coho salmon fry to survive abrupt 

 transfer to higher salinity waters. Salinities were 

 determined with hydrometers. Plastic buckets 

 were used as test containers, and as in the live-box 

 studies, coho salmon fry were taken from Sashin 

 Creek, the weir trap, and the inner bay. For each 

 test, 10 fish were abruptly transferred from their 

 source water to the test water. The fry from all 

 three sources survived 48 h in salinities up to 

 23.5/00 (Table 13). In 29"/oo water, the fry from the 

 inner bay lived for 48 h, but none of those from the 

 weir trap and only 50% of those from Sashin Creek 

 survived 48 h. Of seven fry from the weir trap that 

 survived 96 h in 17.6"/oo salinity, three were trans- 

 ferred to 31"/oo for 48 h, and one survived; four were 

 transferred to 23.5"/oo and three survived for 48 h. 

 Seven fry that survived 96 h in 23.5"/oo salinity 

 were transferred to 31"/oo-four of these fry were 

 still alive 48 h later when the experiment was 



Table 12.-Mortality and growth of coho salmon fry held in live-boxes in the inner bay of Little 



Port Walter in the summer of 1964. 



'Experiment terminated after 30 days. 

 ^No observations until day 35. 



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