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The effects of salinity on survival, growth, aerobic metabolism and ion regulation in early life stages of Pacific salmonids Morgan, John D.


A series of laboratory experiments were conducted to test the hypothesis that the energetic requirements for ion regulation in early life stages of three Pacific salmonids would be minimal at an isotonic water salinity (8-10 ppt). Hatching success, alevin survival, time to hatching, size, and metabolic rates were measured for steelhead (Oncorhynchus mykiss) and fall chinook salmon (O. tshawytscha) embryos incubated in four salinities (0, 4, 8 and 12 ppt) from the eyed stage. Egg hatchability was relatively high in all four salinities, but completion of hatch was delayed in the hypertonic (12 ppt) salinity. Newly hatched alevins were also smaller and showed decreased survival in 12 ppt salinity. Salinity effects on alevin survival and size were greater for steelhead trout than for chinook salmon. Metabolic rates of eyed steelhead trout eggs, and chinook salmon eggs and alevins were not significantly affected by the salinities tested. The metabolic rate of newly hatched steelhead trout alevins, however, was significantly elevated in 12 ppt salinity compared to the other treatments. The results indicated that a salinity of 8 ppt is the upper limit for the normal development of steelhead trout and chinook salmon eggs and alevins. The regulation of movements of ions and water in eggs and alevins of these salmonid species may rely on passive processes, such as low membrane permeabilities and tissue tolerances, rather than active mechanisms of ion and osmotic regulation. Rainbow trout, steelhead trout and fall chinook salmon fry were acclimated to a range of salinities, with one near isotonic. Survival, growth, metabolic rate, plasma Na⁺ and Cl⁻ concentrations, and seawater adaptability were measured for 5 to 12 weeks, depending on the species. Growth of all three species was highest in fresh water and declined with increasing salinity. Metabolic rates increased with salinity and were inversely correlated with growth rates. Isotonic salinity, therefore, did not offer significant metabolic or growth advantages to rainbow, steelhead and chinook fry. While plasma Na⁺ and Cl⁻ concentrations varied among groups, chinook fry tended to better maintain ionic homeostasis at higher salinities than the trout. Acclimation to the various dilute salinities did not influence the seawater adaptability of juvenile steelhead trout or chinook salmon. These results indicate that optimal salinities for growth and metabolic rates were influenced by species and life history stage. The metabolic rate data suggested that the energetic cost of ionic regulation increased with salinity, but attempts to quantify this cost were probably affected by other metabolic processes which responded to changes in salinity.

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