Unveiling past and present fish growth patterns using an integrated structural, microchemical and geochronological analysis of modern and fossil otoliths

Otoliths, the calcified biominerals in the inner ear of teleost fishes, provide a unique opportunity to investigate changes in growth parameters in historical fish populations as they are preserved in large numbers in marine sediments. They exhibit species-specific morphology and serve as life history archives, as they grow continuously in a concentric fashion by the accretion of alternating mineral- and protein-rich layers around a core, making them valuable for ecological and palaeontological studies. We employ sclerochronological analyses on otoliths from fish sampled alive as well as on radiocarbon-dated fossil otoliths derived from a sediment core off the coast of Piran, Slovenia. We want to verify the accuracy of historical growth patterns, which is fundamental to reconstructing reliable long-term chronologies and drawing meaningful conclusions about past fish growth dynamics. We focus here on the non-commercial but very common demersal species, the black goby (Gobius niger Linnaeus, 1758) which inhabits the Adriatic shelf regions in large numbers today and in the Holocene. We aim to test the hypothesis that growth rates and subsequently body sizes of these non-commercial fishes have undergone changes during the late Holocene and Anthropocene in the northern Adriatic Sea due to climate-induced environmental perturbations.

Before applying growth modeling to the fossil otoliths, it is crucial to ensure the preservation and reliability of the incremental record. Accordingly, we employ a comprehensive approach by comparing modern and Holocene gobiid otoliths using visual, structural and microchemical techniques. Our coupled approach includes light microscopy (LM), backscatter electron (BSE) imaging, electron backscatter diffraction (EBSD), and electron probe microanalysis (EPMA). We analyse macro- and micro-optical features, crystallographic structures, and chemical variations within the otolith incremental record. We use a novel technique to determine the geological ages of the fossil specimens in which we cut the otoliths in half, allowing sclerochronological analysis and radiocarbon dating on the same specimen.

Our visual and microchemical analyses revealed that key microincremental features are well preserved in the Holocene otoliths, enabling a direct comparison with modern counterparts. Despite occasional diagenetic alterations that potentially hinder incremental analysis in some of the Holocene specimens, bipartite incremental features, which are essential for growth analysis, are well preserved in both sets of otoliths. Additionally, visual and microchemical patterns indicate that modern otoliths exhibit partially vateritic structures.

Our study bridges the temporal gaps by directly comparing modern and fossil otoliths from the same species and study area. This approach reveals the hidden details in their microstructures and helps to establish a potential growth baseline for fish living in pre-industrial times. By examining historical fish size and growth patterns in populations living before significant human-induced environmental changes, we are providing insights into the effects of climate change on fish populations, which is important for fisheries management and conservation.