Introduction
Mediterranean mussel Mytilus galloprovincialis is one of the most important aquaculture species in the Mediterranean and Adriatic Seas. Over the last decade, its production has faced several challenges, including high mortalities linked to marine heat waves and extreme summer temperatures (Lupo et al., 2021). Despite its commercial importance, relatively little is known about the spatial and temporal variations in growth rates and condition index of M. galloprovincialis in the Adriatic Sea, or the factors that influence them. A recent study by Peharda et al. (2024) documented cessation/slowdown of M. galloprovincialis growth during periods of high temperatures and reduced salinity in the Krka River estuary. However, that study was spatially limited and did not include an analysis of the condition index. Thereby, within the framework of the research project BivalveSPEECH, which uses sclerochronological methods to obtain insights into bivalve growth patterns and the environmental and biological factors influencing them, we conducted research at three aquaculture sites in the eastern Adriatic Sea.
Material and methods
Monthly samplings were conducted between March 2025 and March 2026 at three sites in the coastal area of the eastern Adriatic Sea, including Rovanjska (Velebit Channel area), Novigrad Sea and Bistrina (Mali Ston Bay). At each site, temperature and conductivity were measured at hourly (Rovanjska and Novigrad Sea) or 10-minute (Bistrina) resolution. Each month, 30 specimens of Mytilus galloprovincialis were collected from each sampling location at a depth of 4 m and transported to the lab. Condition index was expressed as the ratio of dry tissue weight (after 48 h at 60°C) to total dry shell and tissue weight.
For the growth analysis, the experiment was conducted in the Novigrad Sea. Shells (N=60) were measured in the field using a digital calliper and individually marked using Hall print shellfish tags in October 2025. They were randomly divided into two groups: the first was placed in a basket at ~1.5 m depth, while the second was placed at ~4.5 m depth. Specimens were left in place until early March 2026, when they were collected, transported to the lab, and their growth was analysed.
Results and discussion
Hydrographic conditions varied by sampling site and season. The lowest salinity conditions were observed at the sampling site in the Novigrad Sea, lasting for several months, from late November 2025 to the end of the experiment in March 2026. Temperature in general followed annual cycle, with the lowest values observed in the Novigrad Sea during winter months, particularly above the halocline, near the surface. Maximum summer temperatures surpassed 26 °C in July 2025 at all three sites. Condition index values had a clear seasonal pattern only for specimens from Bistrina (Mali Ston Bay), with the highest values observed during the summer months. Overall, analysed specimens from Rovanjska and the Novigrad Sea had higher condition index values than those from Bistrina, and these values were relatively constant throughout most of the year. This indicates variations in reproduction between sites, with Bistrina characterised by a single major spawning peak, while at Rovanjska and the Novigrad Sea several spawning events occurred. The majority of tagged shells in the Novigrad Sea survived the experimental period, even those exposed to low salinity. Shells placed deeper grew faster than those placed near the surface, which were exposed to less saline water. The obtained results are compared to the previous studies, including Peharda et al. (2007) on the growth of mussels in experimental integrated aquaculture. Our results contribute to understanding the ecology of M. galloprovincialis in aquaculture settings and provide important parameters for high-resolution shell geochemical studies. Additionally, the results indicate that farm management should be adapted in terms of cultivation depth due to the severe impact of low salinity periods on the growth and condition index of the mussels. Further research includes stable isotope analysis (δ18O and δ13C) in shell carbonate material to obtain insight into the seasonality of growth and the effects of reduced salinity on the growth dynamics of M. galloprovincialis.
Acknowledgment
Research has been conducted in the framework of the BivalveSPEECH project (HRZZ-IP-2024-15-2190).
References
Lupo, C., Bougeard, S., Le Bihan, V., Blin, J.L., Allain, G., Azema, P., Benoit, P., Bechemin, C., Bernard, I., Blachier, P., Brieau, L., Danion, M., Garcia, A., Gervasoni, E., Glize, P., Laine, A., Lapegue, S., Mablouke, S., Poirier,. L., Treilles, M., Chauvin, C., Raymond, J.C., Le Bouquin, S., 2021. Mortality of marine mussels Mytilus edulis and M. galloprovincialis: systematic literature review of risk factors and recommendations for future research. Reviews in Aquaculture 13, 504-536. https://doi.org/10.1111/raq.12484
Peharda, M., Župan, I., Bavčević, L., Frankić, A., Klanjšček, T. 2007. Growth and condition index of mussel Mytilus galloprovincialis in experimental integrated aquaculture. Aquaculture Research 38, 1714-1720. https://doi.org/10.1111/j.1365-2109.2007.01840.x
Peharda, M., Schöne, B.R., Markulin, K., Uvanović, H., Tanaka, K., Shirai, K., Goodwin, D., Mihanović, H. 2024. Mytilus galloprovincialis shell growth – Insights from shell geochemistry. Palaeogeography, Palaeoclimatology, Palaeoecology 650, 112367. https://doi.org/10.1016/j.palaeo.2024.112367