Salinity tolerance
Physiological tolerance limit of fish species to environmental stressors
is 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 (Schultz & McCormick, 2012), and in this experiment the
use of an endpoint which involved non-reactivity to a dip-net, a threat
which all normally functioning fish recognized, was used 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 post-mortem internal processes. The term
Morbidity Point (MP) was used to describe this endpoint.
An exposure protocol was developed to account for salinity level, time
of exposure, and salinity 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 (Schultz & McCormick, 2012).
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 (Whitfield et al.,
2006). This data aligns with our results, with fish able to survive for
several weeks above 100g/kg, and for months at levels near 75g/kg.
Experimental conditions are of course different from natural conditions,
and impacts of predator avoidance and searching for food can increase
the effects of environmental stress (Davis et al., 2019). 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 (Whitfield et
al., 2006).