Abstract
The chemical composition of fish otoliths may provide information on the environmental expo- sure histories of fishes if 'vital effects' on element incorporation are minimal. In order to use redox-sensi- tive 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 vali- dated. Controlled laboratory experiments were con- ducted 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, respec- tively. Although fish somatic growth and condition were affected by constant hypoxia, no difference in otolith chemistry relative to normoxic control treat- ments 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 con- stant 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 treat - ment and sex were detected for otolith Sr:Ca and Na:Ca, possibly related to combined stresses of gona - dal development and periodic hypoxic stress. Although responses to treatments were observed for some ele- ments, the magnitudes of responses were minimal compared to exogenous variation driven by water chemistry composition reported in previous labora- tory 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.
Highlights
IntroductionHypoxia (dissolved oxygen < 2 mg O2 l−1) is increasing in frequency and severity in coastal ecosystems worldwide due to human activities (Diaz & Rosenberg 2008). Fish exposed to hypoxia have exhibited negative sublethal physiological effects including reduced growth (McNatt & Rice 2004), shifts in habitat (Craig 2012), and reproductive disorder (Thomas & Rahman 2012), yet less is known about natural exposure levels that elicit damaging organismal and ecological responses, especially for mobile species capable of sensing, tolerating, and avoiding hypoxia (Froeschke & Stunz 2011)
Hypoxia is increasing in frequency and severity in coastal ecosystems worldwide due to human activities (Diaz & Rosenberg 2008)
No significant difference in otolith Sr:Ca, Ba:Ca, Mn:Ca, Mg:Ca, or Na:Ca ratios were detected between control and hypoxia-exposed fish when accounting for otolith mass variation using ANCOVA (Fig. 1, Table 2)
Summary
Hypoxia (dissolved oxygen < 2 mg O2 l−1) is increasing in frequency and severity in coastal ecosystems worldwide due to human activities (Diaz & Rosenberg 2008). Fish exposed to hypoxia have exhibited negative sublethal physiological effects including reduced growth (McNatt & Rice 2004), shifts in habitat (Craig 2012), and reproductive disorder (Thomas & Rahman 2012), yet less is known about natural exposure levels that elicit damaging organismal and ecological responses, especially for mobile species capable of sensing, tolerating, and avoiding hypoxia (Froeschke & Stunz 2011). A validated chronological geochemical proxy indicating hypoxia exposure in fishes is needed to assess the long-term sublethal effects of hypoxic exposure on aquatic communities. HIF-α is expressed in a wide range of fish tissues and codes for genes related to red blood cell production, vascularization, apoptosis, and carbohydrate metabolism. Short-term biomarkers, such as HIFα, provide reliable molecular indicators of exposure to hypoxia in several fish (Thomas & Rahman 2009) and crustacean (Kodama et al 2012) species on time scales of hours to days. HIF-α mRNA expression returns to basal levels within 24 h after recovery from hypoxia, offering no information on lifetime patterns of exposure and the potential for long-term sublethal effects (Rahman & Thomas 2007, Kodama et al 2012)
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