Application of otolith chemistry at multiple life history stages to assess population structure of Warsaw grouper in the Gulf of Mexico

Abstract

Chemical markers in otoliths have been used to assess the stock structure of many marine fishes, but these natural markers have yet to be widely evaluated or applied to demersal fishes in offshore habitats where physicochemical gradients are generally less pronounced relative to nearshore waters. To address this, we quantified trace elements (Li, Mg, Mn, Co, Cu, Zn, Sr, Ba) and stable isotopes (δ13C and δ18O) in otoliths of Warsaw grouper Hyporthodus nigritus from 4 regions in the Gulf of Mexico (Texas, Louisiana, Alabama−NW Florida, and SW Florida). Region-specific differences in otolith chemistry were observed, and notable differences in several influential markers (Mn:Ca, Sr:Ca, and Ba:Ca ratios and δ18O) were present, particularly between the most distant regions investigated (Texas/Louisiana and SW Florida). Distinct regional signatures were observed for Warsaw grouper across 3 life history stages: first year (otolith core), most recent years (otolith edge), and lifetime (whole otolith), suggesting that individuals within certain regions share common environmental histories that may represent unique contingents or sub-populations. Findings also demonstrate that spatial variability within these markers was consistent enough to overcome any temporal variability within the geographic domains investigated for all 3 life history stages, highlighting their potential value for assessing the natal origin, exchange, and population structure of this species and potentially other members of the deepwater fish assemblage.