Salinity Tolerance
Non-lethal endpoints indicative of a fish approaching a moribund state
are necessary to avoid unnecessary physical distress to animals and for
the collection of useable tissue samples. A preliminary experiment
conducted with six individuals acclimated using a continuous increase of
6 g/kg/day indicated that loss of equilibrium (LOE), a common endpoint
for stress tolerance, did not precede more severe impacts including
pronounced lethargy, cessation of feeding, and death. Therefore, we
aimed to establish a more humane and consistent endpoint indicative of
severe stress. The most consistent behavior was found to be
non-reactivity to a potential threat, specifically a dip-net. Normally
functioning fish will evade capture in a net, and though many fish
appear to move less frequently under salinity stress, they still
approach food pellets and avoid nets. When O. mossambicus no
longer evades the net, this was the most consistent indicator that they
will very soon become moribund, and for this reason we introduce the
term “Loss of Escape Response” (LER) as an alternative and more humane
endpoint to LOE. Two fish in the preliminary experiment which reached
LER were rescued, showing that this state is not terminal if the
salinity stress is ameliorated. In trials with an LER endpoint, constant
fish monitoring allowed for capture of fish before death as this was
necessary for acquiring usable serum and tissues for proteomics.
High rates of salinity increase induced acute effects, resulting in
lower CSMAX than rates below 12g/kg/day (Table 1). Based
on the CSMAX for the 8 g/kg/day and 6 g/kg/day rates
the acclimatory CSMAX is approximately 115g/kg for this
population and experimental set-up. This result defines the upper end of
the pessimum salinity range. For long-term challenges, decreasing
increments of 10g/kg lower than the CSMAX (115g/kg) were
used until a salinity was found in which fish were able to survive
indefinitely to determine the critical salinity threshold. The mean
times until LER at these salinity levels were as follows: 263 hours at
105g/kg, 326 hours at 95g/kg, and 559 hours at 85g/kg (Table 1). 79% of
individuals survived at 75g/kg for 10 weeks.