Influence of constant and periodic experimental hypoxic stress on Atlantic croaker otolith chemistry

The chemical composition of fish otoliths may provide information on the environmental exposure histories of fishes if 'vital effects' on element incorporation are minimal. In order to use redox-sensitive geochemical proxies, such as manganese, in otoliths to quantify sublethal exposure to hypoxia, the relative influence of endogenous and exogenous controls on otolith composition must first be validated. Controlled laboratory experiments were conducted on Atlantic croaker Micropogonias undulatus to examine the response of otolith Sr:Ca, Ba:Ca, Mg:Ca, Mn:Ca, and Na:Ca ratios to either constant or periodic hypoxia treatments for 4 and 10 wk, respectively. Although fish somatic growth and condition were affected by constant hypoxia, no difference in otolith chemistry relative to normoxic control treatments was detected. Similar to the 4 wk study, there was no difference in otolith chemistry between fish (males and females combined) exposed 10 wk to constant hypoxia and control normoxic fish. Periodic hypoxia significantly decreased otolith Ba:Ca and Mg:Ca in both males and females and reduced Sr:Ca in males, and there was a slight effect of sex on otolith Mn:Ca. Significant interactions between treatment and sex were detected for otolith Sr:Ca and Na:Ca, possibly related to combined stresses of gonadal development and periodic hypoxic stress. Although responses to treatments were observed for some elements, the magnitudes of responses were minimal compared to exogenous variation driven by water chemistry composition reported in previous laboratory and field investigations. The otolith chemistry of Atlantic croaker is therefore minimally influenced by endogenous factors in response to hypoxic stress, which has important implications for interpreting otolith chemical chronologies of wild fish collected within natural hypoxic regions.