Hana Uvanović

The European squid Loligo vulgaris inhabits the continental shelf of the North and Central Atlantic and the Mediterranean Sea, with significant socio-economic value for the associated fisheries. Globally, the stock appears to be maintained at levels close to the optimal sustainable yield, but capture statistics indicate high fluctuations in fisheries production, and some regions might be affected by overexploitation. In this study, we used the mitochondrial marker mtCOI to investigate temporal and spatial genetic structure and variability in the European squid in the eastern part of the Adriatic Sea and put it into context with its Mediterranean and Atlantic conspecifics using data from public databases. High haplotype and low nuclear diversity of mtCOI were detected, with no significant genetic differentiation, suggesting one panmictic homogeneous population in the North and Central Adriatic Sea. The Adriatic cluster appears to diverge from its Mediterranean–Atlantic conspecifics; however, this pattern should be considered preliminary due to the limited and uneven geographic sampling available in public databases. The current dataset lacks comprehensive coverage of several Mediterranean sub-basins, which restricts the resolution of connectivity patterns and may mask subtle population structure. Despite these limitations, our results provide an important baseline for understanding the L. vulgaris Adriatic stock and for developing joint management policies among all countries that exploit this shared resource.

Archival data sources can bridge the gap between the paleoecological and ecological time-series and provide a socio-economic context for the understanding of long-term changes in populations of exploited marine species. Detailed fishery statistics from the eastern Adriatic Sea extend back to the early 1870s when the Austro-Hungarian maritime administration initiated a systematic and centralized reporting of annual landings. Here we combine these data with early naturalist accounts to track changes in the Ark shell (Arca noae) populations in that area over the last 150 years. Our results indicate that the collapse of the Ark shell fishery in the 1950s was preceded by at least 80 years of intensive exploitation. During that time A. noae fishery was one of the most important in the eastern Adriatic with the annual catch regularly exceeding 200 t and reaching as much as ~780 t in 1879. At the same time it was one of the cheapest marine products available on the market, consumed primarily by the poor. Historical testimonies indicate that by the late 19th century, fishery administrators and naturalists were well aware of the adverse effects of overexploatiation of marine populations and destructive fishing practices. However, A. noae was explicitly excluded from the regulations establishing legal size limits and no-catch periods that were introduced in the 1880s to protect the Mediterranean mussel (Mytilus galloprovincialis) and European flat oyster (Ostrea edulis), two bivalve species characterized by lower annual landings but much higher market value. Following a mass mortality event in the late 1940s, the annual catch of A. noae rapidly declined and has remained below ~30 t until today – an order of magnitude lower compared to the late 19th and early 20th century. Decades-long, intense harvesting of A. noae may be one the major drivers of the significant shift in its life history characteristics, which was previously documented by sclerochronological analyses of modern and fossil (middle to late Holocene) shells of this species.

Changes in life history of marine species in response to harvesting and human-induced environmental changes are frequently observed in modern ecosystems, but the true magnitude of these shifts is difficult to evaluate without robust information on the pre-impact state of the affected populations. The edible bivalve Noah’s Ark shell (Arca noae L.) is commercially exploited in the eastern Adriatic Sea, where its fishery rapidly expanded during the first half of the 20th century until a mass mortality in the late 1940s caused a population collapse. In spite of partial recovery, the lack of data on growth parameters of this species prior to that event complicates establishing sustainable levels of harvesting. To provide a baseline for assessment of the current state of populations of A. noae we compared modern and fossil (middle to late Holocene) specimens from the northern Adriatic Sea using samples from benthic surveys and sediment cores. Estimates of growth and longevity were based on annual shell growth lines visible in acetate peel replicas of the umbo and outer prismatic shell layer. The maximum lifespan observed in our modern samples was 35 years – a full decade longer than previously documented for this species. However, this estimate was still much shorter than the lifespan of multiple fossil specimens, among which the ontogenetically oldest one reached the age at least 85 years. Our results indicate that modern individuals of A. noae grow faster and have shorter lifespan compared to their Holocene counterparts, suggesting that harvesting pressure combined with increasing water temperatures and eutrophication of the northern Adriatic Sea had a significant impact on the population dynamics of this species.

Integrated otolith chemistry and muscle tissue stable isotope analyses were performed to allocate juvenile Diplodus puntazzo and Diplodus vulgaris to nurseries in the Adriatic Sea. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was used to quantify the concentrations of chemical elements in the otoliths. Fish muscle samples were analysed for δ¹³C and δ¹⁵N. In general, Ba/Ca and Sr/Ca ratios and isotopes varied between sites and species. Values of δ¹³C and δ¹⁵N were significantly different between species and sites. Multivariate analysis detected a significant difference in the element signature between species while there was no evidence for a significant interaction for sites. A clear pattern across the four groups of interest, D. puntazzo_Estuary > D. vulgaris_Estuary > D. puntazzo_Coastal > D. vulgaris_Coastal, following decreases in δ¹³C, and increases in δ¹⁵N were found. It seems that these species are feeding on the same local food web within more productive estuarine site while at costal site, feeding segregation among investigated species is evident. Both species were re-allocated correctly to the estuarine waters based on the otolith chemistry and stable isotopes information and higher value of δ¹⁵N. Combining otolith chemistry with tissue isotope ratios of juvenile fish provided complementary information on nursery habitat use at different spatial scales and elucidated ecological and environmental linkages.

The Smooth clam Callista chione is a commercially important venerid bivalve. It is widely distributed in the eastern Atlantic Ocean and the Mediterranean Sea and inhabits sandy sediments in coastal waters at depths down to 180 m. With specimens that can reach 10 cm of shell length and with a lifespan of more than four decades, C. chione represents an interesting archive for sclerochronological research. The aim of this study was to analyse possible variations in δ18Oshell and δ13Cshell values between C. chione specimens collected in different parts of the Mediterranean Sea.Callista chione shells were collected alive from three localities: (1) Caleta de Vélez in the north region of the Alborán Sea, Spain; (2) Gulf of Venice, Italy, North Adriatic, and (3) west coast of the Istrian peninsula, Croatia, also in the North Adriatic. At the first two localities, specimens were obtained from catch of the commercial fishing vessels, while in Istria they were collected by SCUBA diving. Prior to analysis, the external shell surface was physically cleaned by grinding. Shell powder for δ18Oshell and δ13Cshell analysis was then collected by milling narrow sample swaths in the outer shell surface and processed at Mainz University using a GasBench II - IRMS.Modelled daily temperature and salinity values were obtained for each locality and used for calculating the predicted δ18Oshell values. For Caleta de Vélez, daily temperature and salinity values were obtained by MEDSEA model; for the Venetian region by the AdriSC climate model, and for Istria by the 3D numerical model ROMS. Temporal alignment of measured δ18Oshell values was conducted manually in Excel by best-fitting measured isotope data to predicted δ18Oshell curves.Seasonal δ18Oshell cycles were observed in all studied specimens. Temporal alignment of measured and modelled δ18Oshell values clearly showed that C. chione grew fast during the warm part of the year, while slower growth occurred during the winter months. Samples collected in Caleta de Vélez had the narrowest range of δ18Oshell values (-0.43 to +1.73 ‰), while δ18Oshell values in C. chione from Istria showed the largest amplitudes (-1.61 to +2.67 ‰). Growth patterns varied between sampling localities.The δ13Cshell values varied strongly between localities and specimens. Highest δ13Cshell values were obtained for C. chione shells from Caleta de Vélez (0.51 ± 0.03 ‰, range -0.19 to +1.06 ‰). The broadest range of δ13Cshell values (-3.37 to -0.08 ‰) were measured in shells from the Venetian region. These samples also had the lowest mean (-1.42 ± 0.14 ‰). Shells from Istria had δ13Cshell values ranging from -1.57 to +0.38 ‰ (mean: -0.42 ± 0.28 ‰). Observed differences between localities are statistically significant (Kruskal Wallis H = 150.4, p < 0.001).  Isotope data obtained for this study were compared with data from a previous study on the same species in the Eastern Adriatic.Research was supported by the Croatian Science Foundation, research project BivACME.

Pinna nobilis is a large bivalve endemic to the Mediterranean Sea that lives in shallow coastal areas. Due to its size and relatively fast shell growth rates, it is an interesting taxon for high resolution geochemical and sclerochronological research. Subsequently to previous analyses of δ18O and δ13C in P. nobilis shells, here, we investigate nitrogen isotopes in the carbonate-bound organic matrix (δ15NCBOM) of this species. Our objectives were to test if P. nobilis shells (i) can be used as an indicator of the isotopic baseline of the system, and (ii) is a good candidate for obtaining high-resolution temporal data on environmental δ15N variability. Due to the multiple mass mortality events of P. nobilis spreading throughout the Mediterranean, including the Adriatic Sea, we also tested if (iii) P. nobilis geochemistry changes as a response to diseases.Shells were opportunistically collected by skin diving from 4 shallow coastal localities in the eastern Adriatic, as a part of a project on mortality monitoring. Specimens from Lim channel (October 2019), Kaštela Bay (January 2020) and Mali Ston Bay (November 2019) were collected alive, while in Pag Bay, shells of three recently dead specimens were collected in September 2020. Tissue and epibionts were removed and shells carefully cleaned and air-dried. Shell powder was collected by milling sample swaths by hand using a DREMEL Fortiflex drill equipped with a 300 μm tungsten carbide drill bit. For δ15NCBOM analysis, three shells from each locality were processed and three replicas were collected from each of these shells by milling shallow lines parallel to the growth axis from the internal shell surface. In addition, high-resolution δ15NCBOM data were obtained for one shell from Kaštela by milling lines (N=40) perpendicular to the major growth axis from the external shell surface. From this shell we also collected shell powder for δ18Oshell and δ13Cshell analysis to enable placing δ15NCBOM into temporal context. Isotope samples were analyzed Union College on an elemental analyzer - isotope ratio mass spectrometer.Results indicate significant differences in δ15NCBOM between sampling localities, with lowest values recorded for shells from Pag Bay (3.73±0.36‰), and highest for shells sampled in Lim channel (7.04±0.63‰). High-resolution δ15NCBOM data obtained from the shell collected from Kaštela Bay corresponded to a time interval from spring 2018 to spring 2019. These data showed relatively small variations (5.02±0.33‰). However, δ15NCBOM values increased to 8.65±1.61‰ closest to the shell margin, and were coupled with a decrease in δ13Cshell values, indicating that this animal was experiencing stressful conditions several months prior to its death. According to our findings, δ15NCBOM values serve as an indicator of the isotopic baseline of the ecosystem as well as a potential powerful tool to study bivalve physiology.Research was the supported by the Croatian Science Foundation, research project BivACME.