Distribution by Race | index

When the number of each race captured in different parts of the tributaries is examined, it becomes apparent that the fall chinook juveniles were generally captured in downstream reaches while winter chinook were more likely to be captured further upstream. (See Table 3 and Figure 16.)

Stream
Km		 Fall Winter Spring Late Fall Combined Races
No. % No. % No. % No. % No. %
0-0.5 629 20 12 7 19 11 2 67 662 19
0.5-3 1667 54 22 12 68 38 0 0 1757 51
3.0-10 606 20 90 49 74 42 1 33 771 22
>10 173 6 59 32 16 9 0 0 248 7
TOTAL 3075 100 183 100 177 100 3 100 3438 100
Table 3. Races of juvenile chinook captured in the 1999 sampling season.

Figure 18.
Percent of different races captured closer to or further from the river in the 1999 sampling season.

The differences are highly significant and it is doubtful that they arise merely from different capture efficiency for different size fish since winter chinook as small as 51 mm were captured in December. In 1999 we focused much of our sampling effort on the lower ends of tributaries so it is not surprising that the majority of juveniles were captured there. We compared the area of capture in 1998 when our sampling focus was broader and found a similar discrepancy in distribution of the races (Figure 19.)

Figure 19.
Percent of different races captured closer to or further from the river in 1998.

Perhaps the winter chinook forage further upstream because food is scarcer when they arrive in tributaries that have just begun running. By February, when most fall chinook juveniles arrive, aquatic insect populations have exploded and there may be little incentive for foraging far upstream. Upstream distribution of spring run juveniles, like their time of entering the rearing tributaries, is intermediate between the other two races.

Conclusions