Revealing Ontogenetic Vertical Migration in Deep-Sea Grenadiers (Macrouridae) from the Southwestern Atlantic Through Otolith Microchemistry

Understanding the life-history strategies of deep-sea fishes is essential for improving ecological knowledge and informing conservation efforts. Using otolith microchemistry, this study reconstructed the ontogenetic movement patterns of four grenadier species (Nezumia aequalis, Hymenocephalus billsam, Coelorinchus marinii, and Malacocephalus occidentalis) caught in the continental slope off southern Brazil (Southwestern Atlantic). Elemental signatures (Ba:Ca, Sr:Ca, Li:Ca, Mg:Ca, Mn:Ca, Ni:Ca, Cu:Ca, and Zn:Ca) were quantified along core-to-edge transects of sagittal otoliths using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Ontogenetic shifts were identified using change-point detection (PELT). A general decline in elemental incorporation with age was observed across species, consistent with ontogenetic physiological regulation. Species-specific multi-elemental patterns suggest distinct ecological strategies. Nezumia aequalis exhibited an abrupt decline in Ba:Ca, indicating an early-life environmental shift. Hymenocephalus billsam showed increasing Ba:Ca and Sr:Ca profiles, consistent with continued use of pelagic-associated water masses. Coelorinchus marinii and Malacocephalus occidentalis displayed more complex patterns, with the latter showing pronounced Ba:Ca and Zn:Ca peaks that may reflect mid-life habitat shifts or physiological events. Mn:Ca ratios differed between pelagic and demersal species. Otolith microchemistry combined with change-point analysis could provide insights into deep-sea fish ontogeny, although interpretations should consider both environmental and physiological influences.