Graduate Thesis Or Dissertation
 

Scale patterns indicate changes in use of rearing habitat by juvenile coho salmon, Oncorhynchus kisutch, from 1955 to 1984 in the Tenmile Lakes, Oregon

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  • This study was designed to provide information about the juvenile life history of coho salmon, Oncorhynchus kisutch, in the Tenmile Lakes in Oregon by 1) classifying scales of spawning fish and outgoing smolts (1+), 2) assessing the contribution of juvenile life history types to the returns of adults (3₂) and jacks (2₂), and 3) comparing growth rates and survival of different types. Scales of outmigrating smolts from one brood year, jacks from two brood years, and scales of adults from four brood years were classified into juvenile life histories and examined for several scale characters. A linear discriminant function analysis was used to show the separation of the types and to determine the most distinguishing scale characters. Length of smolts at ocean entry back-calculated from scales of jacks and adults was compared with the length of the observed group of emigrating smolts. Based on variation in numbers and spacing of circuli and the size of the freshwater scale zone, believed to represent different residence time in the tributaries and the lakes, four types of juvenile life histories were defined. Fish classified as type 1, stream-reared, are believed to have reared in the tributaries until migrating as smolts in the following year. Type 2, stream-lake-reared, fish are thought to have reared in the tributaries for almost a year but then moved down to the lakes, where rearing continued until smolt migration in spring. Type 3, stream-lake- reared, are believed to have reared for a short time in the tributaries, then moved down to the lakes sometime in their first year of life. Rearing continued in the lakes until spring of the following year. Type 4, lake-reared, are thought to have moved down to the lakes shortly after emergence from the gravel, where they reared until migrating as smolts in the following spring. Scales of smolts, jacks, and adults were sorted into these four types. Presently, coho salmon fry and yearlings appear to be moving out of tributaries in late spring and from March to beginning of May, respectively. Migration of smolts out of the lake system to the ocean occurs mostly within the month of May. Recent habitat surveys show that dramatic seasonal changes occur in use of rearing habitat by juvenile coho salmon in the Tenmile Lakes tributaries from summer to winter. In winter juveniles appear to be using more low gradient, low velocity, off-channel areas than in summer. Fish of type 4 represented 90%, 43%, and 74% of the returns of adults in 1957- 58, 1963-64, and 1971-72, respectively, and 90% of the returns of jacks in 1962-63 and 1970-71. However, no type 4 fish was represented in the returns of adults in 1985-86, whereas type 1 fish represented 89% of the returns. Type 4 appeared to have grown better in fresh water, reached a larger size at outmigration, and have a greater relative survival than fish of type 2, when compared among the observed group of smolts, returning jacks, and adults. The large proportion of the escapement returning as jacks for some of the years may indicate good growing condition for juvenile coho salmon in fresh water. According to the classification of jack scales a large proportion of fish returning as jacks were lake-reared and were found to be larger at migration to ocean than fish returning as adults. This may suggest that fish that reared well in the lakes and reached a large size at outmigration had the tendency to mature early and return as jacks. Based on the analysis of scales, lake-reared juvenile coho salmon formerly contributed well to adult returns. The former high returns of jacks and adults reflect the importance of the lake habitat for the coho salmon populations of this system. In order to enhance this stock to a higher level, management strategies should be focused on making the lake habitat available to juvenile coho salmon for at least some part of their freshwater life.
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