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.