Fishery bulletin of the Fish and Wildlife Service - NOAA

270 SALMON OP THE KAELUK RIVEE, ALASKA
respectively, of throe-fresh-water fish in the samples. "While the percentages of threefresh-water
fish in the small samples taken from the runs of those years are not
exactly comparable to the data under consideration, it is evident that the three-freshwater
age group was dominant.
The change in age composition might be due to any one, or a combination, of
the following causes: (1) An increase in the ocean mortality of the 3-year seaward
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PERCENTAGE OF THREE-OCEAN
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FISH IN
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ESCAPEMENT
FIODHE 7.—Percentage of three-ocean flsh in the return plotted against percentage of three-ocean flsh In escapement for the years
1922 and 1924 to 1929, inclusive. The straight lino represents a ratio of 1 to 1.
migrants or a decrease in the ocean mortality of the 4-year seaward migrants; (2) an
increase in the fresh-water mortality of the 3-year seaward migrants or a decrease in
the fresh-water mortality of the 4-ycar seaward migrants; (3) an increase in the
length of fresh-water residence.
The ocean mortality of the 4-year seaward migrants, as determined by the marking
experiments reported in a later section, is less than that of the 3-year seaward migrants.
This might be expected as they are larger at the time of migration than the 3-year
100