Analysis of freshwater inflow effects on metabolic stresses of south Texas bay and estuarine fishes : rates of adaptability to changing salinity-temperature regimes

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The purpose of these experiments with juvenile spotted seatrout and red drum is to establish a methodology of assessing the ability of fish acclimated to estuarine salinity levels to withstand short term stresses of increased or decreased salinity at winter 15°C and summer 28°C temperatures. The experiments were conducted by determining the metabolic performance of the fish acclimated to 20 ppt salinity, which is near optimum, and then subjecting the fish to salinities of 10, 30 or 40 ppt salinity in order to follow their reactions and propensities to recover and readjust to the new salinity levels. For the red drum only a similar experiment was carried out except that the blood serum osmolality (as a measure of the degree of adaptability) was followed. For both kinds of experiments, the time course was followed by measuring at frequent intervals the metabolic or the blood osmolality levels for three-day periods. Metabolic performance was measured as the scope for activity, which is the difference between the respiratory metabolism at maximum sustained activity and at the lowest maintenance (standard) level. During the first hour or so the reaction phase to a salinity change usually was accompanied by an abrupt drop in the scope, followed by a variously extended low metabolic level stressed phase and then by a rising recovery phase at about 30 hours. By about 48 hours typically there was reasonable stabilization phase, when scope was lowered, principally by reduction in the active metabolic level and very little influenced by a change in standard level. For the first three hours after a salinity change, the smaller the fish the greater was the decrease in metabolic scope. The greater the salinity change the greater the duration of the stressed phase and the length of time for the initiation of recovery phase. Temperature is relatively not important except at the upper (28°C) levels for the small fish, and possible at salinity extremes of 10 and 40 ppt for the red drum at both 28°C and 15°C where the active metabolic rates are about equal. In general, the sudden temporary stresses caused by salinity changes are more severe than the steady state stresses at salinities far from optimum. However, this study indicates that it is possible to utilize the rapid salinity change technique with small fish to assess their capabilities to become acclimatized to salinity changes. It would appear that rapid changes toward lower salinities, which also can occur rapidly in nature, result in less stress than rapid changes to progressively higher salinities, which ordinarily occur only slowly in nature. Rapid changes to higher salinities are also accompanied by osmolarity malfunctions in regulation and death in contrast to rapid changes to lower salinities that do not usually result in osmoregulatory problems and death. With the various cautions discussed in the study, it appears that young fish do respond to rapid salinity changes in a way that is indicative of similar, but slower, responses in the natural estuaries. Therefore it would appear reasonable to suppose that the methods of this study could be developed further for different species, different size ranges, seasons, and other variables in relation to anticipated salinity changes.