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).