The factors leading to post-liberation mortalities of trout transported in tank trucks Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/v692tb63z

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  • The results of experiments conducted in 1957 and 1958 to isolate the cause of post-transportation mortalities among hatchery-reared rainbow trout, Salmo gairdneri Richardson, in Oregon are reported. Although death rate varies greatly, the average loss occurring from one to seven days following liberation has been estimated to be ten percent of the fish transported. Studies conducted over the past 15 years on problems arising from the live shipment of fishes are reviewed. General experimental procedure was to place fish in aquaria and subject them to certain environmental conditions perhaps associated with delayed planting mortalities. After a specified interval, animals were removed from test containers and placed in observation troughs. Subsequent losses were recorded and compared to the measured experimental conditions in an effort to identify causal relationships. Simulated in-transit motion, density of fish transported, and pre-hauling starvation period ha. d no apparent influence on the magnitude of delayed trout deaths. Zinc from zinc chloride and from galvanized iron plating was toxic to trout, and, at low concentrations, produced delayed mortalities characteristic of those that occurred following tank truck transportation in Oregon. The toxic effect of zinc increased with increasing zinc concentration, and with increasing length of exposure to the metal. Of the trout exposed for six hours to an initial zinc concentration of 0.10 mg/1, an average of 8.2 percent suffered a delayed mortality. When the concentration was increased to 1.00 mg/1 for the same time period, the delayed loss increased to an average of 63.2 percent. When length of trout exposure to various zinc concentrations was increased from two to eight hours, the mean delayed mortality increased from 39 to 68 percent. The toxicity of zinc increased with decreasing total water hardness. At 0.75 mg/1 zinc, mean trout mortalities increased from five to 28 percent when total water hardness was decreased from 40 to 20 p. p. m. When hardness was decreased from 20 to zero p.p.m. , mean mortalities increased from 28 to 88 percent. On the basis of research previously conducted in Oregon, it was shown that delayed trout losses were inversely correlated with total alkalinity of the transporting water. Assuming that calcium was the principal cation involved, the relationship was attributed to the effects of increased water hardness. Samples of transportation water from 32 inter-hatchery transfers of fish were taken at the completion of the station to station trips and analyzed for zinc. In every instance where delayed mortalities greater than one percent occurred, appreciable quantities of zinc were detected. When delayed losses of trout were much less than one percent, no zinc was detected in the water used during the transfer. Based on the results of the field and laboratory tests reported, it was concluded that zinc from galvanized iron surfaces was responsible for the post-transportation mortalities of trout in Oregon. Fish exposed to low zinc concentrations in liberating tanks were subjected to threshold toxic conditions in which a varying portion of the animals died a delayed death. Relatively small changes in the natural water hardness or zinc concentration could either increase or eliminate subsequent mortalities.
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