Date of Award

1-1-1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

Stomach content analysis of 395 predatory fish caught near the point of smolt release suggested that rainbow smelt, Osmerus mordax (Mitchill), may be an important buffer of predation on stocked chinook smolts, Oncorhynchus tshawytscha (Walbaum). Numbers of chinook smolts consumed per predator were higher, while number of rainbow smelt consumed per predator were lower, then water temperatures at 5 m were greater than 17$\sp\circ$C than when they were less than 17$\sp\circ$C. Rainbow smelt distributed lower in the water column and apparently became unavailable to shallow water predators later in the season. Salmon smolts surface-fed primarily on flying insects, and were distributed near the surface regardless of when they were stocked. Numbers of smolts consumed per predator were not proportionate to numbers stocked. Predator catch per unit effort was similar following large batch stocks to that following small batch stocks.Stomach content analysis of 515 chinook salmon indicated their forage chronology consisted of 3 stages. Salmon weighing 2.4-15.0 g fed entirely on arthropods, 89% of which were insects. Salmon weighting 15-60 g fed on a combination of insects and young of the year smelt. Salmon 60-6000 g fed entirely on smelt, while salmon 6-9 kg fed on smelt and goldeye, Hiodon alosoides (Rafinesque). Only goldeye were found in salmon over 9 kg. Salmon fed heaviest in the evening between 1900 and 0100 h. Stomach content weight as a percentage of body weight was similar in 1985 and 1986. Mean August weights of salmon were lower in 1986 than in 1985 for all year classes. Sexual maturation of salmon occurred in the 4th or 5th year for females and in the 2nd, 3rd, 4th, or 5th year for males. Relative abundance of salmon was similar at a mid-reservoir site and a site near the dam, whereas smelt were over 16 times more abundant at the mid-reservoir site. Salmon and smelt were found at greater depths as the water temperature increased. Smelt were distributed highest in the water column and had the most variability in their vertical distribution during the evening and night, and were lowest in the water column and least variable in depth during mid-day.

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