Growth and food consumption of juvenile coho salmon exposed to natural and elevated fluctuating temperatures Public Deposited

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  • The growth and food consumption of juvenile coho salmon [Oncorhynchus kisutch (Walbaum)] exposed to natural and elevated fluctuating temperatures were studied in the laboratory with wild fish collected from a salmonid rearing stream. Control temperatures were similar to the temperature regime of a natural stream and the elevated temperatures were increased incrementally 3-4 C and 7-8 C. Short-term, 30-day experiments were conducted during five different seasonal periods in 1969 and 1970. Concurrent long-term studies were carried out during the entire experimental period. Relationships between rates of food consumption, growth and food assimilation of the fish were determined for the different experimental temperature conditions during the short-term experiments. Relationships between maintenance ration and temperature were derived from curves relating rates of food consumption and growth. The efficiency of food assimilation was determined from measurements of the quantities of the food consumed and the amounts of fecal wastes produced. Caloric measurements were obtained from oxygen bomb calorimetry of the fish and food consumed, and from wet combustion of the fecal wastes collected during short intervals within each experiment. Bioenergetic measurements were used to estimate the major fates of the energy of food consumed under the different temperature conditions during a period of fluctuating high summer temperatures. Long-term studies were conducted over eight-month and four-month periods under temperature conditions similar to those of the short-term experiments. Growth rates of the juvenile coho salmon were obtained from measurement of changes in their wet weight during 15-day intervals. The growth rates of salmon at equivalent ration levels were compared between the short- and long-term experiments. Growth rates of juvenile coho salmon kept as controls were generally greater than those of fish exposed to elevated temperatures for each of the five short-term experiments. Increases of ration size generally ameliorated the effects of elevated temperatures on the salmon growth rates. Comparisons of the caloric values of coho salmon recorded for a natural rearing stream with those of experimentally fed fish suggest that wild fish subsist on a restricted ration. Results of the long-term studies showed that salmon exposed to naturally fluctuating temperatures in the laboratory and fed restricted rations reached sizes typical of downstream smolts in Oregon coastal streams, whereas the fish exposed to incremental temperature increases of 3-4 C and 7-8 C were respectively 25 and 47 percent smaller than the control fish. Comparison of results of studies based upon naturally fluctuating temperatures with those of other workers based upon constant temperatures indicates that at average temperatures above 20 C the fluctuation of temperature is favorable for coho salmon growth. The results of the study suggest that temperature criteria for juvenile coho salmon derived from measurements based upon constant temperatures may not be satisfactory for protecting the fish during the extended period of their freshwater existence. Measurements of the growth of the young salmon during long-term experiments indicated that any substantial increases of temperature would result in a reduction in the size of smolts. Significant reductions of juvenile salmon growth resulting from elevated stream temperatures could influence the production of salmon populations through decreased marine survival. Long-term studies of the fish as well as other components of stream communities influenced by elevated temperatures are necessary for establishing meaningful temperature criteria.
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