Abstract

This study was designed to evaluate the tolerance of Clarias gariepinus juveniles to a gradual and abrupt increase in salinity over time. To this effect, C. gariepinus juveniles were exposed to three salinity incremental protocols namely 1g L-1day-1, 5g L-1day-1, and 10g L-1day-1. Changes in the hematological parameters and the gill histology of fish were analyzed to determine the impact of osmotic stress on the health status of the fish and its osmoregulatory ability. The result obtained showed that juveniles of C. gariepinus can tolerate salinity stress up to 14g L-1. At 15g L-1 and beyond, all samples died regardless of gradual (i.e., 1g L-1day-1 administered for 15days) or abrupt salinity exposure (i.e., 5g L-1day-1 administered for three days and 10g L-1day-1 administered for two days). Interestingly, more than 90% of the fish survived a direct 10g L-1 exposure for 24h without prior acclimation. The hematological parameters accessed in the fish exposed to 10g L-1 (either gradually or abruptly) showed a significant increase in the white blood cells and a decrease in the red blood cells, packed cell volume, hemoglobin concentration, and all derived blood parameters. The results of the serum biochemistry show a lower total protein and albumin in the salinity-treated fish compared to the control group. However, the serum glucose and the plasma electrolytes (i.e., K+, Na+, and Cl-) were higher in the former group than in the latter. Aside from the stress response expressed in the blood parameters, severe gill degenerations were seen in the histological micrograph obtained for the salinity-treated fish, while the control had a near-normal gill architecture. It was concluded that C. gariepinus could tolerate salinity exposure of 10g L-1day-1 (administered gradually or abruptly) and below without killing the fish within 24h.

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