Salinity exposure near the Critical Threshold eliminates growth
Energy homeostasis theory describes survival as the final biological capacity lost when animals are exposed to pessimum range stressors for too long [15]. Fish can sustain life in pejus range levels, but energy demands required for survival, sub-optimal enzymatic function, and/or contending with deleterious effects of increased metabolic processes contribute to reduced growth or reproduction, ultimately reducing the functional capacity of individuals and populations. This was demonstrated clearly through the 12-week exposure to 75g/kg treatment experiment, where most fish were able to survive and maintain blood osmolality, body condition, and feeding rate close to control levels, but growth was dramatically reduced to the point of no significant weight increase.
Absolute salinity and time of exposure each played a role in determining the internal level of stress, i.e. distortion from homeostasis. Physiological parameters such as blood osmolality and body condition were significantly impacted by extreme hypersalinity with greater change from controls at higher absolute salinity. However, over time the difference between fish held at 85g/kg and 105g/kg was erased when LER was reached, whereas internal levels were maintained at 75g/kg, indicating a clear external salinity threshold based on ultimate outcome. Internal blood osmolality was also higher in fish held at 85g/kg or 105g/kg versus acute exposure to 32g/kg at LER, pointing to a greater acclimatory upper limit of blood osmolality in chronic exposures versus acute salinity change. Protein abundance changes were greatest in fish with significantly higher blood osmolality, providing evidence that gill molecular phenotypes are not a direct result of specific salinity levels or exposure times, but rather that combinations of salinity, time of exposure, and rate of increase lead to different internal states and survival outcomes.