Measurement of thyroxine concentration as an indicator of the critical period for imprinting in kokanee salmon (Oncorhynchus nerka): Implications for operating Lake Roosevelt kokanee hatcheries. Supplement: Lake Roosevelt Monitoring Program -- 1991 annual report

摘要

Previous investigations have determined that thyroid hormone surges activate olfactory imprinting in anadromous salmonid smolts. The mechanism of action appears to require binding of thyroid hormones to receptors in brain cell nuclei, which stimulates neuron differentiation and wires a pattern of neuron circuitry that allows for the permanent storage of the imprinted olfactory memory. In this study, thyroxine concentrations (T(sub 4)) were measured in 487 Lake Whatcom stock and 70 Lake Roosevelt stock Kokanee salmon to indicate the critical period for imprinting. Eggs, alevins and fry, reared at the Spokane Indian Kokanee Hatchery, were collected from January through August 1991. Sampled fish were flash frozen on dry ice and stored at (minus)80(degrees)C until T(sub 4) was extracted and concentrations determined by radioimmunassay. Mean concentration (+-) SEM of 10--20 individual fish (assayed in duplicate) were determined for each time period. T(sub 4) concentration peaked on the day of hatch at 16.8 ng/g body weight and again at swim-up at 16.0 (+-) 4.7 ng/g body weight. T(sub 4) concentration was 12.5 to 12.9 ng/g body weight in eggs, 7.1 to 15.2 ng/g body weight in. alevins, 4.5 to 11.4 ng/g body weight in 42 to 105 day old fry and 0.1 to 2.9 ng/g body weight in 112 to 185 day old fry. T(sub 4) concentrations were highest in eggs at 13.3 (+-) 2.8 ng/g body weight, then steadily decreased to 0.1 (+-) 0.1 ng/g body weight in older fry. Fry were released in Lake Roosevelt tributaries in July and August 1991, at about 170--180 days post hatching, in order to imprint them to those sites. The results of this study indicate that the time of release was not appropriate for imprinting. If T(sub 4) levels are an accurate guide for imprinting in kokanee, our results suggest that the critical period for imprinting in kokanee is at hatching or swim-up stages.