RUGGERONE and ROGERS: ARCTIC CHAR PREDATION ON SOCKEYE SALMON SMOLTS 



u 



QC 



a. 



A 6 8 10 12 



NUMBER OF SMOLTS ( X 1 4 ) 



FIGURE 6. — Percent mortality at various levels of smolt abun- 

 dance. (A) Entire population of 1,100 Arctic char fed each 

 day; (B) number offeeding char equaled 1,100(1 - e - 000004M ), 

 where M = number of migrating smolts. 



distribution of smolts in the migration indicates 

 the vulnerability of the smallest and largest 

 smolts (Fig. 7). The large peaks represent age I 

 smolts and the smaller peaks to the right are age 

 II smolts. In each of the three time periods, the 

 distribution of smolts from full char was consis- 

 tently broader than the distribution of smolts in 

 the migration, indicating that smolts average in 

 length have a greater probability of escaping pre- 

 dation. The length distribution of smolts con- 

 sumed by less full char was also broader than the 

 distribution of smolts in the migration, but was 

 skewed to the right. Thus, a greater proportion of 

 age II smolts was consumed by less full char and to 



a lesser extent by full char than proportionately 

 available in the migration. 



When smolts consumed by full and less full char 

 were combined, the average consumed smolt 

 length was significantly larger than the average 

 length from the migration {d = 0.1-1.1 mm, a = 

 0.05); however, the length distribution of the con- 

 sumed smolts was similar to the smolt length dis- 

 tribution from the full char. This was due to the 

 large proportion of smolts consumed by full char. 



DISCUSSION 



Char Functional Response 



The functional response of char at Little Togiak 

 River was similar to those reported for other 

 salmon predators in one aspect (Ricker 1941; 

 Cameron 1958; Hunter 1959; MacDonald in 

 Foerster 1968). Because smolt abundance was 

 usually low, char normally operated at the low end 



MIGRATION X=76.1mm N=801 



FULL CHAR X = 76.1 mm N=385 



LESS FULL CHAR 7^781 mm N- 86 



MIGRATION X=75.5mm N=1421 



FULL CHAR X = 75.3mm N= 371 



LESS FULL CHAR X=77.5mm N= 284 



57 



62 



67 



72 77 82 87 92 



SMOLT FORK LENGTH Imml 



107 



FIGURE 7. — Length distribution of smolts in the migration and 

 in Arctic char stomachs according to stomach fullness. Solid line 

 refers to smolts in the migration, dash line refers to smolts from 

 full char, and dotted line refers to smolts from less full char. 

 Sampling period from 9 to 16 June (A), 16 to 20 June (B), and 20 

 June to 17 July (C). 



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