|Abstract or Summary
- The abundance of the 1982 brood of juvenile coho salmon
(Oncorhynchus kisutch) was determined in August 1983, and January
and April 1984 at 20 study sites spread throughout Knowles Creek,
an Oregon coastal watershed. The timing of emigration of juvenile
coho from the watershed was monitored from October 1983
through June 1984. Condition factor, fork length, and gill
(Na+K)-ATPase activity were measured in migrants, a captive group
of Knowles Creek juvenile coho held in the laboratory, and nonmigrant
fish periodically sampled from the stream. Skin guanine
levels were also measured in migrant and nonmigrant groups.
Juvenile coho abundance in January was significantly correlated
with abundance in August. Wood volume and amount of undercut
streambank were the pair of physical variables that best
explained variation in the number of fish per square meter or per
cubic meter in January. Two debris torrent ponds in the middle
of the watershed contained large amounts of woody debris and were
the most heavily used overwintering habitats for juvenile coho in
the Knowles Creek. Few juvenile coho overwintered in the lower
half of watershed, an area lacking woody debris.
Peaks in outmigration occurred in November and May. Approximately
24% of the total number of migrants emigrated in November.
Fish that reared in two of three third-order areas in
summer, together with fish from the lower (fifth-order) half of
the mainstem, were the first to leave the watershed. While
lack of winter habitat may have been the cause of migration from
the lower mainstem, low summer streamflows may have caused early
migration from the low order sites.
Gill (Na+K)-ATPase activity of migrants rose gradually
from a low in January to a peak at the end of the study in June.
Mean gill (Na+K)-ATPase activity of nonmigrants was only significantly
lower than that of migrant fish during April. Gill
(Na+K)-ATPase of captives was similar to that of nonmigrants
until it peaked during the last two weeks in April, after which
the activity fell below that of migrants or nonmigrants. Condition
factor of nonmigrant fish was higher than either migrants or
captives throughout the study. Migrant skin guanine levels rose
sharply during the first two weeks in April and continued to rise
until the end of the study in June.
Approximately 8,300 juvenile coho, 44% of the estimated
number of juvenile coho present in Knowles Creek in August,
migrated from the watershed by the following June. An estimated
9% of the August population migrated as smolts after April 1.