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.

Population Structure and Regional Connectivity of Young Snowy Grouper in the Gulf of Mexico and Western Atlantic Ocean

“…Our finding of elevated otolith Mn:Ca and Ba:Ca in the nwGoM is consistent with levels found in Warsaw Grouper (Sanchez et al. 2020), suggesting that regional differences are not species‐specific but a result of large‐scale geographic dynamics in physicochemical conditions of the GoM.…”

“…However, elevated Sr:Ca was also detected in Warsaw Grouper in the nwGoM (Sanchez et al. 2020), thereby suggesting factors other than water chemistry (e.g., diet, temperature, membrane transport regulation) are influencing Sr:Ca deposition rates (Campana 1999; Sturrock et al. 2012).…”

“…Following a preablation procedure to remove surface contamination (Sanchez et al. 2020), elemental concentrations were determined for a series of 70‐μm‐diameter spots on each otolith, with 150‐μm spacing between centroids (80‐μm distance between consecutive spot edges). The series of ablation spots ran from the otolith primordium to the otolith margin (most recent deposition) up the growth transect along the sulcal groove on the dorsal face of the otolith section (Figure 2).…”

“…Element concentrations (ppm) were then converted to element : Ca ratios (μmol/mmol) using 44 Ca as the Ca standard at Texas A&M University at Galveston and 43 Ca as the Ca standard at University of Texas at Austin as described in Sanchez et al. (2020).…”

“…For the only species in this deepwater grouper complex on which a population structure analysis was conducted, Warsaw Grouper, results indicated the presence of distinct population contingents within the Gulf of Mexico (hereafter, "GoM"; Sanchez et al. 2020), potentially increasing their vulnerability to fishing pressure.…”