This study tested the effects of very low salinity (<1 ppt) at five different levels on C. variegatus in the absence of competition with other species. Five 650 l outdoor tanks containing well-water in addition to varying amounts of added sea water were set up for each of the five salinity levels. Ten fish were placed in each of these tanks. Data were collected on overall specific conductance, major cation concentrations, and fish survival, growth in wet mass and reproduction. [Na] was significantly different at each of the treatment levels (ranging from about 400 to 1800 mM), and [Mg] was significantly different among several of the levels. There was no significant association between cation concentrations and fish survival. There was however a highly significant association among the levels of [Na], [Ca], and [Mg] and the number of offspring, and among levels of [Na] and [Mg] and growth rate. In addition, overall specific conductance was associated with both number of offspring and growth rate. A positive rate of growth (1.9% wet mass/da) was maintained at the lowest level of "salinity" (401 mM Na), but there was no reproduction at all in the two lowest levels (401 and 591 mM Na). It appears that C. variegatus is adversely affected at cation levels that are not uncommon in tidal freshwaters. It seems unlikely that competition with freshwater fish is the primary factor involved in exclusion of this species from most freshwaters since (a) it was limited in growth and reproduction in the absence of other species, and (b) it occurs widely in the Everglades of southern Florida in the presence of primary freshwater species. Instead, a direct effect of water chemistry on physiological processes involved in osmoregulation is the most likely explanation for the inhibition of growth and reproduction at low salinities. Indirect effects on the food chain remain a secondary explanation that must be evaluated by further experiments.
Freshwater flow, tidal river, abiotic effects, cyprinodontid, cations, mesocosms, salt marsh fish
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