Otolith biochronology for the long-term reconstruction of growth and stock dynamics of fish

Long-term biological time series are essential to evaluate previous responses of organisms to alterations in the environment. Biochronological methods based on archival fish otoliths allow setting such time series, but their predictive potential as proxies of past environmental conditions is still underexploited. In this study, we reconstructed growth variation in European sprat (Sprattus sprattus) in the Baltic Sea from 1956 to 2020 based on measurements of the archived otoliths. We used otolith annual increment widths as a proxy of fish somatic growth. We showed significant negative relationships between sprat growth and sprat spawning stock biomass (SSB) associated with strong intraspecific competition for limited food resources. We also identified a link between sprat growth and water salinity-indicator of the ecosystem's hydrological situation. For the first time, we estimated the SSB prior to the period of available historical data based on the otolith-derived information on the past growth variation. This estimation was based on the strong relationships between SSB and fish growth, complemented with temperature and salinity variables as predictors (R2 = 0.62). A model trained on at least 40 years of data from the more recent past allows us to robustly back-estimate SSB. This study provides new multidecadal data, giving insights into environmental factors affecting the growth of Baltic sprat, and demonstrates the potential of otolith-based biochronology for the provisioning of independent indices of the historical fish stock size. The proposed methodological approach broadens the portfolio of possible applications of the biochronology time series to indicate past changes in the aquatic environment.

Automated summary

This study reconstructed the growth variation of European sprat in the Baltic Sea from 1956 to 2020 using archived otoliths, and identified significant negative relationships between sprat growth, sprat spawning stock biomass (SSB), and water salinity. By estimating SSB based on otolith-derived growth information, this study demonstrates the potential of otolith-based biochronology for providing independent indices of historical fish stock size.