Water strontium concentrations influence strontium concentrations in the pre-hatch otolith region of capelin (Mallotus villosus)

Unique otolith chemistry signatures have been useful in reconstructing environmental histories of teleost fish, including their use as a natural tag of natal habitat. The ability to use otolith chemistry as a chronological measure of life history, however, assumes that the concentrations of trace elements in the ambient environment are taken up and calcified on the otolith in proportion to their environmental availability. Capelin (Mallotus villosus) is a key forage fish species on the Newfoundland Shelf that migrates from offshore to coastal regions during the summer to spawn. Capelin eggs adhere to the sediment at spawning sites, where they remain throughout incubation. As capelin larvae disperse from spawning/rearing sites immediately upon hatch, the pre hatch region of capelin otoliths, formed during incubation, indicates natal origin. Chemistry in the otolith pre hatch region of capelin displays high variability, even in environmentally stable lab-rearing experiments and among individuals within the same family, potentially confounding the ability to characterize otolith chemical signatures from different regions. Here, we aimed to determine whether trace element concentrations in the pre hatch region of capelin otoliths were influenced when naturally fertilized capelin eggs were reared to hatch under varying concentrations of Barium (Ba; 1x, 6x, 7x, 16x, 27x), Strontium (Sr; 1x, 2x, 4x, 5x, 9x), and Ba+Sr (1x, 5x/1.5x, 5x/3x, 10.5x/3.5x, 19x/5x ambient concentration). Naturally fertilized capelin eggs were collected at a beach spawning site in July 2019 and incubated in control or enriched water treatments until hatch. Sagittal otoliths were removed from larvae from each treatment (n = 20) and analysed for their trace element concentrations (e.g., Mg, Mn, Sr, Ba) using Laser Ablation Inductively Coupled Plasma-Mass Spectrometry (LA ICP-MS). Otolith Sr concentrations from 1 to 2 day old larvae increased significantly with increasing ambient Sr concentrations, and the Sr partition coefficient (0.37 +/- 0.01) appeared to be concentration-dependent, decreasing non-linearly with increasing ambient water Sr concentrations. In contrast, otolith Ba concentrations remained consistently high despite environmental enrichment. We estimated high Ba partition coefficients (3.67 +/- 0.52), which were calculated using the limits of detection for Ba (i.e., 0.0025 mg/ L), due to low ambient Ba concentrations. These high estimated Ba partition coefficients suggested that the pre hatch otolith Ba concentrations may be primarily maternally derived. Overall, findings suggested that regions varying in ambient water Sr concentrations during capelin egg incubation could result in distinct chemical signatures in the pre-hatch region of capelin otoliths, but variability in water Ba concentrations will not. Future studies could use this variation to investigate population connectivity and productivity, which is currently unknown for capelin.

Automated summary

Unique otolith chemistry signatures can be used to reconstruct environmental histories of teleost fish, but the variability in otolith chemistry of capelin may complicate the characterization of chemical signatures from different regions. The concentrations of trace elements in the pre hatch region of capelin otoliths may be influenced by environmental factors, with Sr concentrations increasing significantly with ambient Sr concentrations while Ba concentrations remain high despite enrichment.