MASON: FEATURES OF EMERGING COHO SALMON FRY 



groups LC and DC showed higher counts 

 (P<0.01) than did their experimental counter- 

 parts LE and DE tested on day 17 (Table 3). 

 Nonsignificant differences in subsequent tests 

 suggested that frequency of testing may have de- 

 pressed the magnitude of photoresponse change. 



Fry receiving supplemental food (LC-l-F) made 

 scores similar to DC and LE groups (P<0.01) 

 when tested on day 18, but lack of homogeneity in 

 the data from one of the three tests performed 

 precluded further evaluation. 



Light history and recent feeding did not sig- 

 nificantly affect response level when the four pre- 

 viously unfed groups were tested on day 20 {t = 

 1.3 with 158 df, P<0.020). 



Differences in average length among the four 

 unfed groups of fry 1 day after the last tests were 

 not significant (Table 4, F = 0.33 with 3, 96 df) 

 but fish in the LE and DE groups weighed sig- 

 nificantly more and therefore had higher K val- 

 ues. Their heavier weight is attributed to feeding 

 on natural drift foods available only in the choice 

 boxes. The control group given supplemental food 

 from day 9 onward were significantly longer than 

 the other four groups of fry in average length {F 

 = 11.4 with 4, 122 df, P<0.01) and weighed con- 

 siderably more. 



Table 4. — Average lengths, weights, and condition factors {K) 

 of samples of 25 coho fry used in the photoresponse experiment, 

 measured 1 day after final testing. 



'K = W X 10^//-^ where W is weight in milligrams and/, is length in millimeters. 



The average length of fry emerging from the 

 redds (Table 1) did not differ significantly from 

 that of the unfed siblings used in the photore- 

 sponse tests (Table 4). However, the emerging fry 

 weighed somewhat less than fry of the experi- 

 mental groups but more than those of the control 

 groups (X = 425.7 mg) and were in similar condi- 

 tion to the experimental groups (K = 0.766). 



DISCUSSION 



Emergence from these simulated redds in- 

 volved several differences from that reported by 

 Koski (1966) for natural redds of coho salmon. 



Fry from individual natural redds took from 10 to 

 47 days {X = 35 days) to complete emergence 

 which peaked 8-10 days after first emergence, 

 and size of fry decreased as emergence proceeded. 

 In the simulated redds, duration of emergence 

 was 20-23 days peaking at 12-13 days and size 

 increased with time although yolk reserve re- 

 mained nearly constant. The physical structure of 

 the natural redd, particularly the proportion of 

 smaller particle sizes, restricted permeability 

 and impeded emergence. Low permeability re- 

 duced size of fry and increased mortality, later- 

 emerging fry and those failing to emerge that 

 were excavated from redds were emaciated, 

 weight loss indicating exhaustion of yolk prior to 

 emergence. 



As yolk reserves remained fairly constant 

 throughout emergence from the simulated redds, 

 the larger, later-emerging fry probably developed 

 from larger eggs. Koski (1966) found that large 

 female spawners produced large fry at emer- 

 gence, but large size of progeny did not alleviate 

 physical hindrance to emergence, typifying the 

 majority of redds, leading to decreasing size of fry 

 as emergence progressed. 



The strong upstream response shown by fry 

 emerging from the simulated redds is charac- 

 teristic of coho fry emerging in natural streams. 

 Apart from counteracting downstream transport, 

 upstream movement provides for the seeding of 

 upstream rearing areas unavailable to, or not 

 used by, spawners. The small but significant dif- 

 ference in yolk reserve between fry moving up- 

 stream or downstream may reflect, rather than a 

 minor difference in swimming ability, behavioral 

 differences associated with rising aggression, 

 onset of territoriality, and commencement of 

 feeding on the invertebrate drift. 



The lack of preference for nocturnal emergence 

 is in contrast to findings for sockeye salmon; pink 

 salmon, O. gorbuscha; and chum salmon, O. keta, 

 fry which emerge primarily at night (Neave 1955; 

 Heard 1964). But like these other species, the 

 coho salmon fry retained a photonegative re- 

 sponse at emergence of potential survival value, 

 e.g. escape from predators. Stuart (1953) also re- 

 ported that fry of brown trout, Salmo trutta, re- 

 mained photonegative during their ascent in 

 simulated redds, even upon reaching positions 

 only 1 or 2 inches from the gravel surface. For 

 several days after emerging, fry of coho salmon 

 and cutthroat trout, S. clarki, will bolt back into 

 the gravel bed when disturbed (pers. obs.) and 



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