The sublethal effects of zinc at different water temperatures on selected haematological variables in Oreochromis mossambicus

Abstract

Trace metals such as zinc play an important role in the normal metabolic functioning of all organisms. However, metals can become toxic if background concentrations are exceeded in the environment. This study investigated the sublethal effects of zinc on the haematology of Oreochromis mossambicus at different water temperatures. Fish were exposed to sublethal zinc (Zn) concentrations of 40µg/l (mean Zn concentration measured from the Mhlathuze River) for a period of 96h at different water temperatures representing the seasonal temperatures in the Mhlathuze River (18ºC, 24ºC, 28ºC and 32ºC). Exposure to zinc at 28ºC and 32ºC resulted in the most pronounced haematological changes in O. mossambicus. The erythrocytotic conditions found following exposure to Zn could be attributed to damage to the gill surface. The ensuing hypoxic conditions are alleviated through the release of large amounts of immature RBC into the circulatory system. The leucocytotic conditions support the findings of damage to the gill surface. The increased glucose concentrations were attributed to a general adaptation response whereby metabolites are mobilised to meet increased energy demands during periods of stress. The changes in Na⁺ and K <sup>levels in the plasma of O. mossambicus can be attributed to a combination of stimulation of Na-K ATPase activity, reduced membrane permeability in the gill tissue and leakage from cells into the blood, respectively.


Keywords: sublethal stress; zinc; temperature; haematology; osmoregulation; freshwater fish


(Afr J Aqua Sci: 2000 25: 146-151)</sup>