SOME FEATURES OF COHO SALMON, ONCORHYNCHUS KISUTCH, 



FRY EMERGING FROM SIMULATED REDDS 



AND CONCURRENT CHANGES IN PHOTOBEHAVIOR 



J. C. Mason^ 



ABSTRACT 



The emergence of sibling coho fry from simulated redds lasted 20-23 days during which 97-98% of the 

 fry emerged. Average size of emerging fry increased with time but the largest fry emerged during the 

 peak of emergence. No clear preference was shown for nocturnal or daylight emergence but the latter 

 increased with time. Fry showed a positive current response, 69-82% moving upstream following 

 emergence. Most fry emerged when yolk reserve was reduced to less than 10% of total dry weight. 

 Later-emerging fry did not have lower yolk reserves, but fry moving downstream had slightly more 

 yolk reserve than did fry moving upstream. Fry which were captured shortly after emergence had fed 

 actively but had not yet filled their air bladders. Chironomids composed 70% of their diet. 



Photoresponse of sibling fry denied the redd experience was studied in light-dark choice boxes with 

 reference to the timing of emergence of fry from the simulated redds. The pronounced photonegative 

 behavior of the denied fry was suddenly lessened at time of emergence but remained photonegative. 

 Weakening of the negative photoresponse was not the outcome of starvation or recent light experi- 

 ence, and was not modified by repeated testing. Retention of the photonegative response is referred to 

 hiding behavior and use of the gravel bed as a refuge. 



The anadromous female Pacific salmon, On- 

 corhynchus, usually buries her eggs in several 

 adjacent pockets in streambed or lakeshore ma- 

 terials and these egg pockets collectively consti- 

 tute a redd. The eggs hatch after several months 

 and the larvae may spend several weeks or 

 months using up their extensive yolk stores prior 

 to emerging from the redd area into open water. 



Mortality during this extended period of sub- 

 terranean life may be considerable (Royce 1959) 

 and probably routinely exceeds 70% for most 

 species of salmonids in natural habitats. Adap- 

 tion to suboptimal conditions includes physiologic 

 and behavioral responses in the embryo and larva 

 which were reviewed, especially for sockeye 

 salmon, O. nerka, by Bams (1969). 



Because destructive influences on the egg and 

 alevin stages are amenable to amelioration 

 through manipulation of substrate structure and 

 flow regime, spawning channels pioneered by 

 Wickett (1952) at Nile Creek have become a 

 major component of salmon enhancement 

 strategy. Despite these advances, we have yet to 

 define optimal redd conditions, biotic and abiotic, 

 which maximize preemergence survival of any 



'Department of the Environment, Fisheries and Marine Ser- 

 vice, Research and Development Directorate, Biological Sta- 

 tion, Nanaimo, B.C. V9R 5K6, Canada. 



salmonid. Furthermore, fry surviving to 

 emergence may face extended ecological conse- 

 quences of suboptimal conditions in the redd 

 which alter timing of, or size at, emergence (Ma- 

 son and Chapman 1965; Mason 1969). Neither 

 can we yet define for the emerging fry physiologic 

 and behavioral states which optimize survival in 

 open waters. Thus, premature emergence, imply- 

 ing underdevelopment and reduced ability to re- 

 spond adaptively is not referrable to a defined 

 state of normality. 



Alevins of Oncorhynchus , as are those ofSalmo 

 and Saluelinus (White 1915; Stuart 1953; 

 Woodhead 1957), are initially negatively photo- 

 tactic and respond to light by hiding (Hoar 1958). 

 They become positively phototactic and rheotac- 

 tic as emerged fry, orientation to current preced- 

 ing the shift from negative to positive phototaxis 

 (Dill 1969) as in Salmo (Grey 1929a; Stuart 1953) 

 but the timing of this photobehavioral change in 

 relation to emergence and remaining yolk re- 

 serve remains unknown in Oncorhynchus and 

 disagreement has arisen as to its timing in Salmo 

 (Woodhead 1957). Histophysiological studies by 

 Ali (1959) showed that only emerged fry and 

 older stages of Oncorhynchus are capable of full 

 retinomotor responses; however, partially devel- 

 oped responses have obvious survival value. 



In this paper, some features of sibling coho fry 



Manuscript accepted September 1975. 

 FISHERY BULLETIN: VOL. 74, NO. 1, 1976. 



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