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Tracing Life History Patterns of Pacific Grenadier (Coryphaenoides acrolepis) off Baja California with Otolith Microchemistry


Unravelling the elemental patterns and age of fishes using otoliths allow us to better understand their life history. Pacific grenadier (Coryphaenoides acrolepis) are a species of deep-sea macrourid fish commonly found in the North Pacific along the continental slope between depths of 200 to 2000 m. Tracking the life history patterns in deep-sea fish is inherently difficult, however, examinations into otolith microchemistry offers chronological insight into potential environmental and endogenous drivers of habitat use. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to analyze the otolith microchemistry of 118 mature adult Pacific grenadier (C. acrolepis). Archived samples were collected in the Northeast Pacific Ocean, offshore of the Baja California Peninsula and nearshore to Isla Guadalupe, Mexico, between the years of 1967 and 1971. We quantified 12 elemental isotope ratios (7Li, 11B, 24Mg, 26Mg, 55Mn, 31P, 63Cu, 65Cu, 66Zn, 86Sr, 138Ba, and 208Pb) from the otolith core towards the dorsal edge for 104 samples. Based on the relationship between otolith transect ablation length and total length, ablation transects were divided into larval, settled, and mature stages. 31P, 63Cu, 65Cu, and 66Zn ratios decreased with distance from the core of the otolith, potentially reflecting a decrease in metabolic activity and somatic growth with age. 86Sr and 138Ba ratios both steadily increased across lifespan, indicating movement to saltier and higher ambient concentration waters with depth. 55Mn, 86Sr, and 138Ba showed elevated ratios within the settlement stage, suggesting a shift in environment that possibly reflects the hypoxic conditions common to the Oxygen Minimum Zone. Samples collected near Isla Guadalupe exhibited greater ratios of 7Li, 26Mg, 63Cu, and 208Pb, whereas samples found offshore of Baja were greater in 11B, 55Mn, and 86Sr, indicating differences between offshore and near island populations. Overall, element ratios varied only slightly between offshore Baja and Isla Guadalupe collection sites, with k-means clustering accurately reclassifying 62.5% of samples. In addition, age estimates for this sample set were found to be consistent with a previous age and growth study and improved estimates of Von Bertalanffy growth curve parameters with the inclusion of older and larger samples. Overall, otolith microchemistry appears to be an effective tool to detect ontogenetic and regional aspects of C. acrolepis. This information will be key to proper management of C. acrolepis with potential increased fishing pressure on deep-sea ecosystems.

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