Evidence of exposure to hypoxia and acidification in otoliths of Atlantic Croaker from the Gulf of Mexico

• Main Author: Jiang, Shengjian
• Other Authors: Diamond, Sandra, Barnes, Melanie, Schwilk, Dylan, Grabowski, Tim, Patino, Reynaldo
• Format: Thesis
• Language: English
• Published: 2022
• Subjects: Hypoxia; Acidification; Otoliths; Gulf of Mexico; Ocean acidification
• Online Access: https://hdl.handle.net/2346/88946

Rapidly spreading hypoxia (low dissolved oxygen concentration) and ocean acidification (low pH) due to climate change are major concerns in the Gulf of Mexico. Both situations are leading to fish mortality and population decreases. Earlier studies have used fish otoliths as indicators of changes in these two environmental variables. These studies reported that low dissolved oxygen concentration (DO) can influence the chemical composition of otoliths, and that low pH condition can alter otolith size. However, laboratory experiments alone are insufficient to accurately describe impacts of hypoxia and acidification on wild fish populations. To understand the association of changes in DO and pH with fish otolith condition in the field, two hypotheses were formulated: 1) otolith chemistry will associate with natural variation in DO conditions in the Gulf of Mexico, and 2) otolith size and density will increase with acidification. Otoliths from a total of 60 Atlantic croaker (Micropogonias undulatus) were collected from 17 stations in the Gulf of Mexico in 1983, 2002, and 2016. Dissolved oxygen was reported as the value measured over a 30-min sampling period at each station. Because pH was not recorded, atmospheric CO2 pressure was used as proxy for water pH and its change over time. Twenty elements were measured in each otolith by laser-ablation inductively coupled plasma mass spectrometry, and a generalized linear mixed-effects model, with DO as fixed effect, was applied to examine the association between otolith chemistry and DO levels. Year of capture and station were considered as random effects. No samples were available for 2002 to examine the association between otolith characteristics and ocean acidification; thus, otolith mass, volume, and density were measured and compared between 1983 and 2016 by nested Analysis of Variance (fish nested into sampling station). In addition, otolith roughness was added as a potential new index of biological impacts of acidification. Roughness was classified as rough, ...