Manganese, magnesium, and micro pogonias undulatus: Identifying growth and hypoxia exposure histories of fish in the northern Gulf of Mexico using otolith microchemistry
Hypoxia is a worldwide natural phenomenon that is becoming increasingly more severe as anthropogenic high-nitrogen nutrient loads enter marine systems. In the northern Gulf of Mexico (nGoMex), seasonal hypoxia occurs during the summer months due to stratification and lack of mixing events. These events have a range of ecophysiological effects on demersal and pelagic organisms, from forcing individuals out of their natural habitat to altering species niche widths and stifling growth rates. Atlantic Croaker (Micropogonias undulatus) is a demersal Sciaenid species abundantly found in the nGoMex. While hypoxia exposure is known to have many detrimental and sub-lethal effects on croaker for early life history stages, quantifying growth effects for the entire lifespan of croaker is critical for the understanding of how hypoxia affects metabolism and growth. Otolith chemical signatures can reflect the composition of the ambient water a fish resides in as well as the internal physiological status and growth rate. These signatures provide detail about hypoxia exposure histories and how abiotic stress affects the biology of the fish. To quantify hypoxia exposure, metabolic activity, and hypoxia corrected for growth, I used the microchemical otolith markers Mn:Ca, Mg:Ca, and Mn:Mg, to assess lifetime hypoxia exposure histories and growth responses. Age-0 croaker were revealed to have the highest Mg:Ca duration fraction, as well as the highest Mn:Ca duration fraction of all ages analyzed, indicating that a large proportion of juvenile croaker are exposed to hypoxia within the first year of life, but their growth is not directly affected by this exposure. Age-1 and Age-2 croaker that experienced high levels of hypoxia were found to have high mean Mg:Ca values. This result may indicate that hypoxia-exposed fish gain a growth advantage through enhanced foraging for benthic prey. Alternatively, differential mortality may have selectively removed slow growing individuals from the high hypoxia exposure groups and only faster growing individuals survived this stressor. Neither Mn:Mg nor Mn:Ca showed consistent trends with Mg:Ca, possibly due to the repetitive use of Mg in the chemical metrics, or due to the fact that Mg:Ca is not the best proxy to identify growth dynamics in Atlantic Croaker. A further understanding of how hypoxia affects the growth and trophic interactions of demersal fishes is critical for managing and protecting vulnerable fishes.