Salinity tolerance
Physiological tolerance limits of fishes to environmental stressors are
species-specific and sensitive to time course of exposure. Common
assessments of upper salinity limits such as LOE may not be applicable
to all species [12]. Therefore, here we introduce an endpoint based
on non-reactivity to a dip-net (LER), a threat which all normally
functioning fish recognized, to determine approaching morbidity. Several
fish in this state were rescued through transfer to improved conditions,
suggesting that they were still functional with physiological and
molecular phenotypes representative of living individuals, not
pre-mortem internal processes.
An exposure protocol was developed to account for salinity level, time
of exposure, and rate-of-change. Salinity tolerance is often assessed in
a binary fashion between acute exposure i.e. direct transfer from
initial to final salinity, and chronic exposure which involves gradual
salinity change to the endpoint [12]. These assessments do not fully
capture the dynamics of salinity acclimation, as a high rate of salinity
change may outpace the necessary alterations in phenotype required for
acclimation. Additionally, without duration at a final salinity one
cannot assess the breadth of the zone of resistance (pessimum range).
Previous assumptions about survival for a specific amount of time
representing long term-survivability may be inaccurate, as O.
mossambicus can survive in salinity above the incipient lethal salinity
for up to six weeks.
O. mossambicus has been recorded in nature in salinities up to
120g/kg, with historical data indicating that they can remain in extreme
hypersaline conditions for weeks and even months [3]. This data
aligns with our results, with fish able to survive for several weeks
above 100g/kg, and for months at 75g/kg. Experimental conditions are of
course different from natural conditions, and impacts of predator
avoidance and foraging can increase the effects of environmental stress
[37]. Nevertheless, extreme hypersaline ecosystems have greatly
reduced species diversity and thus O. mossambicus is likely to
experience fewer predators and greater food availability due to fewer
competitors [3].