Can otolith microstructure and elemental fingerprints elucidate the early life history stages of the gadoid southern blue whiting (Micromesistius australis australis)?

Southern blue whiting (Micromesistius australis australis) once contributed to one of the largest finfish fisheries in the Southwest Atlantic. Intense exploitation began in the late 1970s with peak total annual catches attaining 258.000 tonnes in 1983. Continuing levels of exploitation hereafter resulted in a significant reduction in its abundance by the early 1990s and forced the implementation of a suite of conservation measures to rebuild these stocks. Improved recruitment levels in recent years suggests that these measures have been effective. However, more information on habitat use and behaviour during the early life history stages of this fish is required to be able to draw firm conclusions. Here, we use complimentary investigations into otolith microstructure and trace elemental composition (by LA-ICPMS) to study the early stages of ontogeny in M. a. australis. We provide evidentiary support for the formation of daily growth increments on the otoliths of juvenile M. a. australis and utilise these to estimate the age at onset and completion of metamorphosis (30.5 ± 0.3 and 60.7 ± 1.8 days after the first increment formation, respectively). The timing of a newly defined ‘juvenile transition-mark’ (corresponding to the transition from the epipelagic larval habitat to a deeper juvenile habitat) was estimated to form at 76.5 ± 2.0 days after first increment formation, suggesting that early juveniles remain in the epipelagic habitat for over two weeks post-metamorphosis. Significant shifts in element:⁴²Ca ratios were found for elements ²⁴Mg, ⁵⁵Mn, ⁸⁸Sr and ¹³⁸Ba between otolith zones corresponding to different ontogenetic stages: (A) larval-, (B) metamorphosis- and (C) juvenile zone. These shifts could be correlated with changes in behaviour and habitat. Elemental ‘fingerprints’ for each of the three otolith zones were found to be distinct through high classification accuracies of the Random Forest classifier (81.1 ± 0.1 %), with ⁵⁵Mn:⁴²Ca contributing most to the classification. This holistic approach investigating both otolith microstructure and elemental composition provided evidence that ontogeny could be a confounding factor in natal (geographic) origin studies if elements predominantly controlled by physiology are included in the respective classification analyses.