Salinity exposure near Critical Threshold eliminates growth
Energy homeostasis theory places survival as the final biological
capacity lost when animals are exposed to pessimum range stressors for
too long (Sokolova et al., 2012). Fish can sustain life in pejus range
stress 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 MP
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 at MP versus acute exposure to 32g/kg at MP, pointing
to a greater acclimatory upper limit of blood osmolality in chronic
exposures versus acute salinity change. Protein abundance changes were
most highly correlated with blood osmolality, providing evidence that
gill molecular phenotypes are not a direct result of specific salinity
levels or exposure times, but rather that specific combinations of
salinity, time of exposure, and rate of increase lead to different
internal states and survival outcomes.