Eastern Adriatic Sea Research Articles

Limited population genetic variation but pronounced seascape genetic structuring in populations of the Mediterranean mussel (Mytilus galloprovincialis) from the eastern Adriatic Sea.

Population genetic analysis of variation at five neutral microsatellite loci for Mediterranean mussels (Mytilus galloprovincialis) from 18 sites along the eastern Adriatic Sea revealed little or no spatial variation. In contrast, seascape genetics analysis revealed a pronounced locus-specific gradient in allelic and genotypic frequencies across the study region. At a sixth locus, MGE7, the frequencies of two alleles, MGE7243 and MGE7249, were strongly associated, negatively and positively, respectively, with a single environmental variable - minimum salinity (minSAL). The frequency of the MGE7243/243 homozygous genotype was strongly negatively associated with minSAL, whereas the frequencies of the MGE7246/249 and the MGE7249/249 genotypes were strongly positively correlated with minSAL. Interpretation of these pronounced gradients is confounded by the fact that minSAL and another environmental variable, maximum sea surface temperature (maxSST), are highly correlated (R=-.911) and are therefore not necessarily acting independently. BLAST searches of the MGE7 locus against M. galloprovincialis whole genome shotgun sequence returned an alignment with contig mg10_S01094 (accession UYJE01010330.1) and 7 predicted M. galloprovincialis proteins VDI82194.1 - VDI82200.1. Conserved domain searches revealed a similar structure to the transcriptional regulator Msx2-interacting protein. The BLASTp search also returned significant alignments to Msx2-interacting proteins in Mytilus coruscus, Crassostrea virginica, and Haliotis rubra. The existence of the MGE7 gradient highlights the role that environmental variation may play in retarding gene flow among wild M. galloprovincialis populations, and also how the success of collection of young mussels (spat) from one site and their transfer to another site (the farm) may be influenced by a single factor such as minSAL or maxSST on a localized scale.

Revealing the Shamefaced Crab Calappa granulata (Crustacea: Brachyura) from the Adriatic Sea, Northern Basin of the Mediterranean

This study presents the first data on morphometry, length–weight relationship, diet, reproductive biology, epibionts and genetic identity of the shamefaced crab Calappa granulata from the central eastern Adriatic Sea. A total of 92 crabs were collected during 2011, 2014 and 2015, of which 64 were females and 28 were males. Overall, 11 morphometric characteristics were measured. Carapace length of sampled individuals ranged from 48.46 to 76.09 mm, and body weight from 47.06 to 221.39 g. The length–weight relationship showed negative allometry for males and isometric growth for females. Analysis of the stomach content revealed the crab’s preference for crustaceans (20.28%) and cephalopods (10.58%), less for fish (3.4%) and shellfish (0.28%). Size at first sexual maturity (CL50%) of 59.25 and 66.92 mm was estimated for males and females, respectively. Epibiotic serpulid polychaetes were recorded on the crab exoskeleton with an overall prevalence of 29.3%. Analyses of a partial sequence of mtCOI showed high haplotype (Hd = 0.964) and low nucleotide diversity (π = 0.00598). Phylogenetic inference and estimation of population differentiation (FST = 0.013, p = 0.271) with publicly available Mediterranean sequences currently imply one homogenous population unit. To the best of our knowledge, these are the first nucleotide sequences of C. granulata from the Adriatic Sea made publicly available.

Cataloguing environmental influences on the spatiotemporal variability of Adriatic anchovy early life stages in the eastern Adriatic Sea using an artificial neural network

The anchovy (Engraulis encrasicolus, Linnaeus, 1758), one of the most important small pelagic fish species in the Adriatic, is currently described as a species that can be considered overfished. From 2013 to 2020, samples of anchovy eggs and larvae were collected through scientific surveys during the summer months. The collected ichthyoplankton data were combined with environmental data (measured satellite sea surface temperature and chlorophyll data, numerically simulated salinity, maps of primary production) to identify anchovy spawning habitats and environmental conditions affecting the anchovy early life stages. For this large dataset, a nonlinear method called Growing Neural Gas Network analysis was used to explain the multiple dependencies between anchovy and the explanatory environmental variables and represent them in 9 patterns called Best Matching Unit (BMU). Obtained values of anchovy early life stages abundances (eggs/m2; larvae/m2) showed a clear negative trend, which was easily observed both in the time series and in the annual spatial distributions. Among all measured environmental parameters that were previously mentioned, salinity showed a significant increase, which can be attributed to the cyclonic phase of the bimodal oscillatory system of the Adriatic and Ionian Seas. The calculated BMUs showed several interesting results that shed new light on previous findings: (a) there is a split between the richer northern and poorer southern parts of the Adriatic in terms of anchovy eggs and larvae abundances, (b) the Kvarner Bay, the west coast of Istria and the area around Dugi otok are consistently rich spawning grounds, (c) decreased abundance in the southern areas is a result of the influence of salinity, (d) an increase in chlorophyll can lead to an increase in egg count, (e) the positive effects of upwelling can be negated by an increase in salinity, (f) increased primary production is followed by increased egg count. Upwelling, as one of the factors that can influence larval and egg abundance by bringing nutrients up from the seafloor, showed increased spatial and temporal variability during the investigated period, which depended on the wind regime. Our analysis showed that neural network analysis can successfully describe the effects and interplay of environmental factors on the abundance of anchovy early life stages.

The study of European eel, Anguilla anguilla in the RiverNeretva estuary (Eastern Adriatic Sea, Croatia) using traditional fishery gear

Samples of the European eel, Anguilla anguilla from two different habitats in the River Neretva estuary region were analysed using two different traditional fishery gear, in the period from 2016 to 2019. The monthly eel catches in ten traditional fyke nets in Parila lagoon varied from 4.4 to 12.7 kg (±2.45 kg) in 2016 and 2.4 to 9.6 kg (±2.01 kg) in 2019. There were no statistical differences between monthly and total weights recorded in 2016 and 2019 (t=1.04; p=0.312). The b-coefficient of the length-weight (LW) relationship for the freshwater eel samples varied from 3,1036 to 3,3206 duringtwo-year period of the sampling. The LW relationship for the brackish population was significantly lower (b-coefficient 2,6513). The dominant eel stage in both sites was yellow eel, while silver stage was scarce. The fishery with traditional devices revealed that efficiency of these artisanal tools is low, pointing to poor abundance of eel in their common habitats. The recorded catch using bigger traditional eel trap gear during autumn spawning migrations was also dominated with yellow stage, while silver eel represented only 15,4% of the catch. The infestation of eels with the parasite,Anguillicoloides crassus was significantly higher in freshwater habitat (41% of eel specimens infested) than in brackish, Parila lagoon (7%). Poor efficiency of traditional gear, bad length and weight structure of the population and low b-coefficients of eels in their most important habitat, indicate poor status of European eel population and River Neretva estuarine ecosystems.

Hierodula tenuidentata Saussure, 1869 (Mantodea: mantidae) has settled down in Bosnia and Herzegovina

In this paper the first record of the alien mantis species Hierodula tenuidentata for Bosnia and Herzegovina is provided. This large mantis has considerably expanded its distribution range in Europe in the last years, particularly in Balkan Peninsula. We found several adult specimens on 22nd and 23rd of August 2021 in urban area of Mostar city in the south of the country. This finding confirms the species’ establishment in the western part of Balkan Peninsula and fills the distribution gap along the eastern Adriatic Sea coast.

Impacts of Sea Bottom Temperature on CPUE of European Lobster Homarus gammarus (Linnaeus, 1758; Decapoda, Nephropidae) in the Eastern Adriatic Sea

The study describes recent decadal changes (2008–2017) in the landing biomass, fishing effort and CPUE (kg/day) data of European lobster Homarus gammarus in the eastern Adriatic Sea region, and relates these changes to increases of sea bottom temperatures detected at long-term in situ stations and modelled by an ocean numerical model (ROMS, Regional Ocean Modelling System). Modelling results were further used to quantify spatial and temporal differences of bottom temperature changes over different fishing zones. Trends of sea bottom temperature were positive and statistically significant between stations. Temporal trends of landing, effort and CPUE were also positive and significant for the northern Adriatic. Correlation analysis was used to test the relationship between winter and spring sea bottom temperatures and CPUE data of H. gammarus, separately for the northern and central Adriatic Sea, resulting in statistically significant correlations for both areas. Whether the increased CPUE in the northern Adriatic is due to increased abundance or catchability is discussed. The observed temperature changes likely reflect climate system changes recognised at the regional level and as such, lobster management measures will need to be revised and updated in the future.

Ecological quality assessment under uncertainty for transitional and coastal waters: Central Eastern Adriatic Sea study

The ecological quality assessment introduced by the European Union-Water Framework Directive (EU-WFD, 2000), based on the repeated measurements sustained over a historical time frame, are often challenged with a limited spatial and temporal frequency of data. Instead of dealing with statistical measures in terms of geometric means and/or some quantiles of quality descriptors, this paper proposes an alternative approach, which is based on quantifying the uncertainty using the full probabilistic description for selected quality indicators at the reference and impacted site. The impacted site(s) represents the water body to be evaluated for the ecological status as required by the WFD. The ecological quality ratio, representing the amount of deviation from a reference condition, is described as a probability of exceedance from the reference condition, named the risk of deviation. The ecological quality ratio is derived through the distributional approach enabling to estimate the probability of being in each of five quality classes for considered transitional and coastal water bodies. The classification boundaries for five quality classes are determined based on the trophic status and related coastal water quality conditions. The ability to estimate the probability that impacted site (or a water body under assessment) could be in any of five quality classes enables decision-makers, responsible for the WFD implementation, to select the Program of Measures, which will be specifically tailored to reduce the ecological quality ratio probability of being below a good quality class. The calculation procedure is semi-analytical and large number of sites and available data could be analyzed relatively fast. The freshwater flow over the salinity gradient in the estuary is analyzed to evaluate the impact on quality indicators in transitional ecosystems. The developed methodology is presented in the case study of coastal and transitional water bodies in the Central Eastern Adriatic Sea.

Age, Growth, and Utility of Otolith Morphometrics as Predictors of Age in the European Barracuda, Sphyraena sphyraena (Sphyraenidae): Preliminary Data

Age and growth of the European barracuda, Sphyraena sphyraena, were determined by examining sagittal otoliths belonging to fish sampled in the eastern Adriatic Sea, as the northernmost region of the Mediterranean. A total of 113 specimens (59 females, 53 males, and one individual of indeterminate sex), ranging from 23.4 to 42.5 cm in total length, were analyzed. The maximum observed age was 5 years for both females and males, and this barracuda population was dominated by 3 year old fish. Growth was described by the von Bertalanffy growth curve (L∞ = 55.58 cm, K = 0.12 year−1, t0 = −4.29 year, R2 = 0.580), and the growth performance index (Φ’) was 2.57. Otolith length, width, and mass were measured, and the utility of these morphometrics as predictors of age in S. sphyraena was evaluated. The results showed that counting otolith annuli produced a better estimation of age than proposed linear models based on relationships between observed fish age and otolith morphometrics. In comparison with age and growth data available in the literature for S. sphyraena, our results from the Adriatic Sea provide more insights into the life-history traits of this species and can be used in the future effective management and conservation.

New Hope for the Critically Endangered Common Angel Shark Squatina squatina in the Adriatic Sea

Abstract Historically, two angel shark species, common angel shark Squatina squatina and smoothback angel shark Squatina oculata, were common in the upper continental shelf of the eastern Adriatic Sea. Although both species are considered critically endangered in the Adriatic and the rest of the world, there are almost no data on the current status of populations, threat assessment and species-specific in-situ conservation. Common angel shark is still present in highly fragmented areas of the eastern Adriatic with extremely rare but consistent records, while smoothback angel shark is mostly considered regionally extinct in the Adriatic due to overfishing and overuse of non-selective fishing gear. There have been only 3 published records of common angel shark this century, the last two of which were reported by fishermen. This paper presents data on 34 new finds, including neonates, juveniles, subadults and adults recorded between January 2020 and August 2021 in the Zadar-Šibenik archipelago (central Adriatic) and one in Premantura (northern Adriatic). Two are original findings during the field expedition, 9 were reported directly by local fishermen, while 23 were reported through a detailed questionnaire. Additionally, plausible breeding and/or nursery grounds between Zadar and Šibenik are discussed with their importance for revitalization and long-term conservation in situ.

Biometry of the sagittal otoliths for three demersal fish species from the Eastern Adriatic Sea (Montenegro)

The otolith morphology, biometry and otolith size and fish length relationship of three demersal fish species: red mullet Mullus barbatus, common pandora Pagellus erythrinus and hake Merluccius merluccius were examined. Based on otoliths relative sizes P. erythrinus and M. merluccius have Large otoliths, while M. barbatus has Small sagittal otoliths. The relationships between otolith variables and fish somatic growth were described by a linear function. All relationships were statistically highly significant (p<0.001). Otolith length (OL) was found to be the variable most strongly related to fish size, with 61.2 – 95.3 % of the variability, while otolith weight had the lowest coefficient of determination (r2). No differences were found neither between left and right otoliths, nor between male and female otoliths in these relationships. For all species studied the relationships of otolith length, height and weight against total fish length were all found to have positive allometry, indicating that the accretion of otoliths is relatively faster than increase in fish body size. The shape indices of form factor (FF), roundness (RD), aspect ratio (AR), circularity (C), rectangularity (R) and ellipticity (E) were calculated and the relationships between otolith length (OL) and shape indices were determined.

Reproductive traits of the European hake, Merluccius merluccius (L. 1758), in the Adriatic Sea

Within this paper, reproduction traits of the Adriatic population European hake, Merluccius merluccius (Linnaeus, 1758), were investigated. Specimens of the target species were monthly collected (N=1173) during 2019 along the eastern Adriatic Sea with commercial bottom trawlers. Overall, European hake total body length varied between 15.5 and 49.5 cm (mean±SD=27.5±5.0 cm) and domination of females was noted in larger length classes (TL>31.0 cm). The sexual ratio of all specimens was m/f=0.64; male prevalence was established over the year, except during the spawning peaks (December, June) when the sex ratio was in favour of the females. According to European hake’s maturity stages, gonad weights, gonadosomatic indexes and histological analysis of the gonad tissues, this species in Adriatic spawn twice in one year; the spawning period was from December to February, while second smaller signs of spawning were observed at the beginning of the summer, in June. 50% of the European hake male and female specimens sexually matured at 20.7 cm and 22.4 cm of total length, respectively.

Trophic status assessment of Central Eastern Adriatic Sea using water quality variables and loading capacity concept for estuaries

The temporal and spatial variability of pollution pressures of the coastal waters resulting from river discharges is the main cause of eutrophication along the coastline. This study aims to assess the water quality and trophic state of Kastela Bay and the river Jadro estuary located in the Croatian central Adriatic Sea. The trophic assessment is classified using the composite trophic status index (TRIX), originally developed for Italian coastal waters and modified for the eastern central Adriatic Sea using data series from 2012 to 2019 from 13 stations. The impact coming from the river Jadro estuary is analyzed using the loading capacity concept to examine the exchange flows and loading dynamics of nutrients discharged in the Kastela Bay. The results show that the trophic index is larger within the river Jadro estuary and maintains the increased values in the surface layer throughout Kastela Bay. The presented loading capacity concept, accounting for the exchange flows in transitional waters, may help better understand the trophic dynamics of the receiving waters. The loading capacity is changing with the river flow and daily loads from measurements can be used to assess the pattern of impairment.

A first qualitative and quantitative study of marine cave fish assemblages of intracave cavities

This study aims to describe for the first time the diversity and abundance of “cave within cave” fish species assemblage and to study the effects of habitat variables on these assemblage patterns. The term “cave within cave” is used to describe small, enclosed cavities and other enclosed spaces regularly present on the inner surface of large marine caves. The quantitative study was conducted from October to November 2019, in a large marine cave under Lučice Bay, on the south side of Brač Island in the eastern Adriatic Sea. In total 48 one-square-meter surface squares were sampled, extending 40m inside from the cave entrance, and down to a depth of 26m. A total of 18 fish species were found in the studied cave, of which 15 were recorded on the square frames. Nine species were found in intracave cavities, including five species which occurred exclusively in this highly cryptic habitat. The three strictly “cave within cave” fish species (Corcyrogobius liechtensteini, Didogobius splechtnai and Zebrus zebrus) prevailed by abundance and by frequency of occurrence over other twelve species recorded on the squares. The total fish abundance significantly increased with increased light intensity, with steeper surface and from the bottom to the top of the cave, while the total biomass and species richness significantly increased only with the increased light intensity. The assemblage structure of “cave within cave” fishes differed significantly with light intensity, among different surface inclinations and among different cave positions. The “cave within cave“ fish species differ in habitat preferences, some species inhabit the entrance of the cave with high light conditions and rich zoocover, and others inhabit further inside the cave with low light conditions.

A Comparison of Traditional and Locally Novel Fishing Gear for the Exploitation of the Invasive Atlantic Blue Crab in the Eastern Adriatic Sea

The Atlantic Blue Crab Callinectes sapidus has been recognized as invasive in the Mediterranean Sea, where it now provides a significant contribution to artisanal fisheries. In this study, we compared the efficiency, selectivity, and productivity of American wire crab traps and traditional fyke nets for the capture of Blue Crabs in a study conducted from June to December 2019 in the Parila Lagoon (River Neretva Estuary, Croatia). A total of 7707 specimens were caught in 15 wire traps, comprising 6959 males and 749 females. The total catch using 50 traditional fyke traps was 1451 crabs, of which 1211 were males and 240 were females. In general, wire crab traps showed a higher capture selectivity and economic performance compared to fyke nets. The catch per unit effort (CPUE) was 102.76 kg for the American wire crab trap and 5.96 kg for the traditional fyke net. The CPUE of gravid female Blue Crabs was lower for the wire traps than the fyke nets. Herein, the utility of crab traps as selective fishing gear for the capture of Blue Crabs, and management tools for control of the reproductive and recruitment phases of the crab, are discussed, with the perspective of future exploitation of the species as a commercially valuable shellfish product in the Mediterranean.

Mediterranean fanworm, Sabella spallanzanii (Gmelin, 1791), as a potential biomonitor of trace metal pollution in the marine environment

Levels of trace metals (Cd, Cr, Cu, Fe, Ni, Pb, Mn, and Zn) were measured in soft tissues and external tubes of the Mediterranean fanworm, Sabella spallanzanii (Gmelin, 1791), to evaluate its suitability as a biomonitor of metal pollution. Study was conducted in the coastal area of the eastern Adriatic Sea. Polychaetes were collected from two locations, a site located in a local marina and an unpolluted control site. Results indicate that S. spallanzanii has the capacity to accumulate metals in soft tissues as well as in tubes. The levels of most metals were considerably higher in tubes compared to soft tissues, indicating long-term accumulation of metals in tubes. High levels of several metals (Cu, Zn, Cr, Pb and Fe), found in soft tissues and tubes sampled at the marina site, reflect the elevated contaminant levels in the marina, associated with the use of antifouling agents, paints, petrol additives, and ship maintenance activities. Metal concentrations in tissues and tubes generally decreased with size, indicating the influence of growth and metabolic rates on trace metal uptake. Trace metal levels found in S. spallanzanii tissues were comparable to the levels found in Mytilus galloprovincialis, a species commonly used in biomonitoring studies, in areas exposed to similar levels of anthropogenic stressors. According to the results of the present study, S. spallanzanii has been identified as a suitable biomonitor of metal pollution due to its sedentary nature, widespread distribution, abundance and the ability to accumulate high levels of metals within its tissues and tubes.

Age, growth and maturation of Illex coindetii (Cephalopoda: Ommastrephidae) in the eastern Adriatic Sea

Analysis of statolith growth increments was used to determine age and describe growth pattern of broadtail shortfin squid Illex coindetii in the central eastern Adriatic Sea, an important demersal resource in the Adriatic basin. Size-at-age data were obtained from a large sample (N = 476, including 249 males and 227 females) of individuals with dorsal mantle length (ML) from 55 to 216 mm and body weight (BW) from 5.7 to 356.9 g. Estimated age ranged from 52 to 186 days, with an average of 108 days, suggesting the species has a life span of six months. No statistically significant difference in statolith length was found between the sexes. The back-calculation analysis showed that Adriatic I. coindetii hatches continuously throughout the year, with two alternative spawning periods, in summer and winter. Therefore, a generation born in summer reproduces in winter, and generation born in winter reproduces in summer. Hatchlings of the autumn–winter period were significantly larger and heavier with slower growth than their offspring hatched in summer and spring that exhibited faster growth and matured at younger age. Estimated age at first maturity were 137.4 and 109.2 days, for females and males, respectively. Growth was best described by Schnute and logistic model that display asymptotic tendencies by the end of the life span. There were no major differences in growth pattern between different cohorts of each sex, with faster growth recorded at younger age.

Plastic Responses of Gilthead Seabream Sparus aurata to Wild and Aquaculture Pressured Environments

Fish farms, as artificial marine structures with a constant food supply, have a high capacity to attract surrounding wild fish. Different phenotypes of abundant gilthead seabream have been recorded in natural and aquaculture pressured environments in the eastern Adriatic Sea, where the influence of tuna farming on plastic traits associated with habitat use and foraging behaviour remains largely unknown. Several traits, such as body and otolith shape, external colouration, diet preference, and proximate tissue content, were analysed to examine habitat selection of the tuna farm-associated phenotype in comparison to wild and farmed phenotypes. Foraging behavioural shifts in prey selectivity, from hard-shelled bivalves towards soft textured baitfish feed, and local hydrodynamic conditions have initiated plastic responses in farm-associated seabream. Consequently, morphological traits connected with feeding and swimming performance and paler vs. vivid body colouration patterns differed between wild and farm-associated fish, highlighting the existence of resource polymorphism in gilthead seabream. While otolith shape proved to be a reliable phenotypic tracer in distinguishing farmed from wild and farm-associated fish, reduced sensitivity was found between individuals residing in the vicinity of tuna farms and wild ones. To fully understand the impact of fish farms on associated gilthead seabream and its plastic response with a distinctive morph outcome, the underlying molecular mechanisms involved in the regulation of alternative phenotypes needs to be investigated.

Marine litter on the shallow seafloor at Natura 2000 sites of the Central Eastern Adriatic Sea

Shallow seafloor marine litter (ML) is the least studied component of ML, and a better understanding of its distribution and accumulation patterns is needed to develop efficient management practices. Abundance and composition of seafloor ML were quantified in two eastern Adriatic Natura 2000 sites (Vis Island and Pakleni Islands), applying a structured sampling design to test whether the intensity of nautical tourism or exposure to prevailing winds and currents are significant drivers determining the ML distribution on shallow, coastal seafloor. ML was widespread and present in considerable amounts across the entire studied area, regardless of the remoteness from most known potential ML sources. Tourism intensity and exposure seem not to be significant predictors of ML accumulation in the investigated Natura 2000 sites. ML abundances displayed large small-scale spatial variation, while for ML composition at a larger scale, regional differences between the two Natura 2000 sites were detected. The Pakleni Islands are mainly polluted by glass items, suggesting a local vessel-based source. The ML composition on Vis Island, with a predominance of plastics, is more determined by long-distance transport. The study indicates the complexity and multi-layeredness of processes driving the patterns of seafloor ML accumulation. Hence, it is suggested to mitigate the issue by addressing the root causes: inadequate waste management and lack of public awareness.

Microeukaryotic and Prokaryotic Diversity of Anchialine Caves from Eastern Adriatic Sea Islands

Anchialine ecosystems in the eastern Adriatic Sea are diverse both morphologically and biologically. In this study, for the first time, we explored the microeukaryotic and prokaryotic community of anchialine caves in the Mediterranean region using high-throughput sequencing. Four anchialine caves located on nearby islands with a well-pronounced salinity gradient were sampled at the surface freshwater area, halocline area, and seawater area. Sequencing revealed a surprisingly wide diversity of the microeukaryotic and prokaryotic community with the relative abundance of major phyla differing within the salinity gradient and between the caves. Interestingly, microeukaryotic and prokaryotic communities clustered into four groups based on location, pointing out that sampled anchialine caves have different microbial community patterns and high microbial endemism. Our results indicate that even with the halocline acting as a selecting barrier, the salinity is not the only community structuring factor. Despite the short geographical distance, the isolation of anchialine caves facilitated high microbial community adaptation and endemism. Our study suggests that anchialine caves represent reservoirs of new biodiversity, maintaining unique and complex microbial diversity influenced by biotic interactions and abiotic environmental conditions.

Analysis of Manganese Bioaccumulated in Mediterranean Blue Mussel (Mytilus galloprovincialis) from the Bay of Mali Ston (Adriatic Sea, Croatia) during Diarrhetic Shellfish Poisoning Toxicity

Diarrhetic Shellfish Poisoning (DSP) toxicity was revealed in the Mediterranean blue mussel (Mytilus galloprovincialis) from the Bay of Mali Ston, in the south part of the Eastern Adriatic Sea, through the Croatian National Monitoring Programme in the period from January until June of 2011. A survey of DSP toxicity within the frame of regular controls carried out through the mouse bioassay (MBA, at the time the official method for DSP toxins) demonstrated that in some incidents, positive MBA, which manifested by the atypical symptomatology of the animals, dominated. Additional studies were done to explain the atypical results of the conducted biological tests at the time. In the current study, the bioaccumulated manganese concentration in the soft tissues of mussels was measured to investigate its influence on the MBA results. In both DSP negative and DSP positive samples, which were prepared for the analysis according to the modified US EPA 3052 method, the concentration of the bioaccumulated manganese was performed on the atomic absorption spectrometer using flame atomic absorption spectroscopy technique. The analysis revealed higher concentration of manganese in 87% of DSP positive samples and the expressed per wet weight ranged from 0.15 to 5.38 mg kg−1. The mean concentration of manganese for all DSP positive samples was 1.78 mg kg−1, while for DSP negative samples, it was 48% lower (0.93 mg kg−1). The highest concentration of manganese in DSP positive samples was measured in February 2011. Since the low concentrations of lipophilic biotoxins gymnodimines (GYMs) and spirolides (SPXs) were also detected in the analysed DSP positive samples in the parallel studies, the results obtained in this study suggest future investigations of the connection between the concentration of manganese and lipophilic biotoxins.