caspian sea

There are about 55 permanent rivers in Guilan province, but there is not enough information about the distribution of their fish species. In this article, the data on the presence and abundance of non-native (exotic) fish species related to the fisheries projects of 33 rivers in Guilan during the years 1994 to 2023 were extracted and presented by river. The results showed that there are a total of 22 species of non-native fish in these rivers, and Sefidroud, Pasikhan and Siahdarvishan were the most with 22, 10 and 10 species, respectively. In all three studied areas (West Guilan, East Guilan and Anzali wetland basin), the fishes Carassius gibelio, Pseudorasbora parva and Gambusia holbrooki showed the highest percentage of presence, but the highest relative abundance was related to Chelon saliens, C. gibelio and G. holbrooki. On average, the non-native fishes in the rivers of these three regions constituted about 11.1, 19.8 and 15.0 percent of the number of fish, respectively. It seems that the most damage caused by non-native fish on the native species related to C. gibelio, P. parva, Hemiculter leucisculus and G. holbrooki species is due to their presence and abundance in these rivers and the effect of alien species with low abundance and ornamental fish species is insignificant.

Microplastics (MPs) have garnered significant attention due to their widespread presence in the environment and the potential threats they pose to aquatic organisms (Wang et al., 2021). These particles, smaller than 5 millimeters, are classified into primary and secondary types. Primary microplastics are manufactured in micrometer sizes for use in various industries, including aerospace, medicine, and cosmetics (Alomar et al., 2016). In contrast, secondary microplastics are formed from the breakdown of larger plastic debris into smaller particles (Duis and Coors, 2016). Microplastics readily accumulate in aquatic environments and, due to their resistance to degradation, are dispersed globally (Wang et al., 2021). These particles can adversely affect aquatic organisms due to their physical and chemical properties, leading to disturbances in feeding, reproduction, and immune functions (Oliviero et al., 2019). Additionally, microplastics can adsorb pollutants such as heavy metals, exacerbating their harmful effects on aquatic ecosystems. This combination presents a significant threat to marine life health (Prunier et al., 2019). Furthermore, microplastics provide a substrate for microorganisms, facilitating the formation of biofilms. These biofilms can alter the physical and chemical properties of microplastics, influencing their ability to adsorb contaminants (Tu et al., 2020). The objective of this study is to examine the correlation between microplastic pollution and potentially toxic elements in the sediments of the southwestern Caspian Sea coast and assess their impacts on the region marine ecosystem.

The Caspian Sea, the largest enclosed lake in the world, is significantly impacted by human activities such as oil and gas extraction, agriculture, and industrial development. Rivers such as the Volga, Kura, and Ural transport pollutants, including heavy metals, to the sea, posing a threat to the ecosystem, particularly along the southwestern coast (Efendieva, 1994; Simonett, 2006). Sediment sampling was conducted at three polluted stations in Kiashahr, Anzali, and Astara (Kostianoy et al., 2005). Sediment sampling was performed in the spring of 2022 using Van Veen grabs (20×20 cm) with three replicates at each statio (Claessens et al., 2011; Löder and Gerdts, 2015). After being transferred to glass bottles and sent to the laboratory. Microplastic extraction from sediments involves two essential stages. In the first stage, the organic materials in the sediments were digested using hydrogen peroxide (H2O2, 30%). The digestion time varies between 1 and 10 days, depending on the type and amount of organic material (Erkes-Medrano et al., 2015; Zhang et al., 2016). After digestion, the samples were dried at 60°C for 48 hours (Vianello et al., 2013). In the second stage, density separation was used to extract the microplastics. In this step, 100 g of dried sediment was placed in a glass beaker, and 800 mL of saturated NaCl solution (293 g/L) was added (Thompson et al., 2004). After shaking for five minutes, the beaker was left to stand for 45 minutes to allow the high-density particles to settle. The resulting supernatant, containing the floating particles, was filtered through a nitrocellulose filter (Hidalgo-Ruz et al., 2012; Wagner et al., 2014; Duis and Coors, 2016). This process was repeated three times, and the filters were dried at 60°C (Law et al., 2010). Finally, the microplastic particles were examined and counted using a 40x magnification loop, and the number of microplastic particles per gram of dry sediment was reported (Reddy et al., 2006; Morét-Ferguson et al., 2010). The polymer types of the extracted microplastics were identified using FT-IR spectroscopy with ATR, analyzing spectra in the 400-4000 cm⁻¹ range and comparing characteristic peaks with standard polymer databases (Veerasingam et al., 2021). Data analysis was performed using SPSS version 27. The Kolmogorov-Smirnov test was used to check for normality, and to compare pollution levels across stations, ANOVA and Kruskal-Wallis tests were applied. To examine the correlation between microplastic pollution and potentially toxic elements, Pearson and Spearman correlation coefficients were used. All analyses were conducted at a %95 confidence level. Graphs were plotted using Excel 2022.

The average concentration of elements at the three stations revealed that the highest and lowest average concentrations of elements in the sediment were for manganese (Mn) with 760661.80±53.41 µg/kg dry weight at the Anzali station and cadmium (Cd) with 41.44±1.93 µg/kg dry weight at the Astara station. The results of the Kolmogorov-Smirnov test for the distribution of potentially toxic elements in the sediment samples from the stations indicated that some elements did not follow a normal distribution (p˂0.05). To compare the average concentrations of elements across the stations and examine the correlation between elements, parametric tests (one-way analysis of variance and Pearson correlation coefficient) were used for normally distributed data, while non-parametric tests (Kruskal-Wallis and Spearman correlation coefficient) were employed for non-normally distributed data. One-way analysis of variance (ANOVA) results for comparing the average concentration of elements across the stations showed that manganese and zinc had significant differences at all stations (p˂0.05). The cadmium element showed no significant difference between the Kiashahr and Astara stations, but significant differences were observed between Kiashahr and Anzali, as well as Anzali and Astara stations (p˂0.05). The Kruskal-Wallis test results also indicated significant differences for arsenic, cobalt, chromium, copper, iron, mercury, nickel, and lead among the stations (p˂0.05). The Anzali station, with 67±4 pieces per 300 grams of dry sediment, exhibited the highest contamination, while the Kiashahr station, with 45.33±2.30 pieces per 300 grams of dry sediment, showed the lowest contamination. One-way analysis of variance (ANOVA) revealed that the Anzali station had a significant difference when compared to both Kiashahr and Astara stations (p˂0.05). However, no significant difference was observed between the Kiashahr and Astara stations. The microplastics extracted from the sediment samples were categorized into two color groups: blue and red. At the Kiashahr station, red microplastics accounted for 53%, representing the highest abundance, while blue microplastics constituted 47%, representing the lowest abundance. At the Anzali station, blue microplastics were the most abundant, comprising 75%, while red microplastics represented the lowest abundance, constituting 25%. At the Astara station, red microplastics accounted for 53%, representing the highest abundance, while blue microplastics constituted 47%, representing the lowest abundance. A total of 496 microplastic pieces were extracted from the sediment samples at the three stations. All the extracted microplastics were fiber type. The microplastics found in the sediment samples were classified into seven categories: less than 0.5 mm, 0.5-1 mm, 1-2 mm, 2-3 mm, 3-4 mm, 4-5 mm and greater than 5 mm. At the Kiashahr station, the highest abundance of microplastics was found in the 4-5 mm range, followed by the 3-4 mm range. The lowest abundance was observed in the 1-2 mm particles. At the Anzali station, the highest abundance was found in particles larger than 5 mm, followed by the 4-5 mm range, while the lowest abundance was observed in particles smaller than 0.5 mm. At the Astara station, the highest abundance was found in particles larger than 5 mm, followed by the 4-5 mm range, while the lowest abundance was observed in the 1-2 mm particles. The extracted microplastics from the sediments of the southwestern Caspian Sea coast were identified using FTIR-ATR spectroscopy. Five different polymers were identified, including polyethylen (PE), polypropylene (PP), polyester, polystyrene (PS) and nylon. Overall, polyethylene was the dominant polymer in the extracted microplastics from the sediments. The correlation analysis results between the abundance of microplastics and the concentration of potentially toxic elements in the sediments from the Kiashahr, Anzali, and Astara stations indicated no significant correlation between these two variables at the stations under study. Manganese, zinc, and cadmium had a normal distribution in all stations, thus Pearson's correlation coefficient was used to assess the correlation between microplastic pollution and potentially toxic elements. For other elements, whose data did not follow a normal distribution and successful normalization techniques were not applied, the Spearman correlation coefficient was employed. 

Discussion and conclusion: 

In this study, the Anzali station exhibited the highest contamination, with an average of 67 ± 4 microplastic pieces per 300 grams of dry sediment. This finding is consistent with the study by Rasta et al. (2020) which reported a high concentration of microplastic contamination in the sediments of Anzali Wetland. All the microplastics extracted from the sediments in this research were of the fiber type, with blue being the most dominant color, accounting for 58%. Similar results were found in the study by Zhang et al., (2020) in the Shengsi region of China, where fiber-type microplastics were the most abundant, with blue identified as the predominant color. Microplastics at the Kiashahr station were most abundant in the 4-5 mm size range, while the Anzali and Astara stations exhibited the highest abundance in microplastics larger than 5 mm. Similar findings were reported by Kühn et al., (2018) on the coasts of the Netherlands, where microplastics in the size range of 500-2000 micrometers were predominant. In this study, no significant correlation was observed between microplastics and the concentration of elements in the sediments. This lack of correlation may be attributed to differences in the sources of microplastics and elements, the physical and chemical properties of these pollutants, and the varying environmental conditions (Napper and Thompson, 2016). Finally, it can be concluded that the Anzali station has the highest microplastic pollution, primarily composed of fiber type and secondary microplastics, with blue being the predominant color. Anthropogenic sources, such as laundry runoff, fishing gear, and the release of plastic packaging by tourists, contribute to the spread of this pollution in the marine environment. Correlation analysis revealed no significant relationship between microplastics and elements, likely due to differences in sources and the physical and chemical characteristics of these pollutants. A comparison of element concentrations with global standards indicates that the pollution levels are within safe limits; however, continued monitoring and management are crucial to mitigate pollution levels.

The aim of this study was to determine the optimal salinity for rearing the juveniles of Starry Sturgeon (Acipenser stellatus) in enclosed environments. Sturgeon with an average initial weight of 54.11 ± 1.54 g were raised in 500-liter fiberglass tanks (average temperature 22.58 ± 0.14 °C)in a control group (freshwater) and two treatment groups (6  and 12 ppt) in 3 replicates for 60 days. They were fed with commercial feed (Faradaneh) twice a day. At the end of the rearing period, specific growth rate (SGR), weight gain (WG), condition factor (CF) and hepatosomatic index (HIS) in experimental fish (12 ppt) were significantly elevated compared to control fish (p≤ 0.05). Food conversion ratio (FCR), on the other hand, was significantly reduced in the experimental fish (p≤ 0.05). The survival rate of fish was 100%. Intestinal villi length and internal space were significantly elevated and reduced in 12 ppt salinity, respectively (p≤ 0.05). The results suggest that the optimal salinity at this point of Starry Sturgeon life is the Caspian Sea water salinity (12 ppt) which provides the conditions for introducing this species as a suitable species for rearing in enclosed environments in natural water resources. Considering that A. stellatus lives in waters with diverse salinities at different stages of its life, it is important to know the appropriate weight and salinity for releasing in order to increase the percentage of survival and improve growth.

Plastics, despite their widespread applications and advantages across various industries, have become a significant environmental challenge due to their durability and persistence. Incomplete degradation of these materials in nature leads to the production of microplastics (plastic particles smaller than 5 millimeters), which pose severe risks to the health of living organisms, particularly aquatic species (Arthur et al., 2008). These particles, due to their physical properties such as density, size, and shape, disperse unevenly in aquatic environments, with some floating on the water surface while others sink to deeper layers (Anderson et al., 2017). Microplastics are transferred through the food chain, with planktonic organisms playing a pivotal role in their transfer to higher trophic levels (Setӓlӓ et al., 2014). Accumulation of microplastics in the bodies of organisms can lead to physiological damages such as digestive and reproductive disorders (Farrell and Nelson, 2013). These particles are classified into primary (e.g., microbeads in cosmetics) and secondary (resulting from the degradation of larger plastics) types (Gouin et al., 2015). The identification of polymer types in microplastics is typically achieved using FTIR-ATR spectroscopy, which, due to its high accuracy, serves as an effective tool for characterizing various plastic polymers (Li et al., 2020). Given the significance of the whitefish (Rutilus frisii) in the Caspian Sea ecosystem and its role as a biological indicator, this study aims to investigate microplastic contamination in this valuable species. The results of this research could contribute to a better understanding of the impacts of microplastic pollution on the Caspian Sea ecosystem and, consequently, on human health.

The Caspian Sea, the largest enclosed lake in the world, with an area of 393,000 square kilometers and a coastline of 6,525 kilometers, is located between the continents of Asia and Europe, making it an important freshwater resource in the Eurasian region (Debus, 1995). Three stations with the highest potential for pollution were selected along the southwestern coast of the Caspian Sea for sampling: Kiashahr (impacted by the Sefid Roud River), Bandar Anzali (affected by the Anzali Wetland), and Bandar Astara (influenced by pollution inflows from Azerbaijan) (Kostianoy et al., 2005). Sampling of the Caspian Sea whitefish was conducted in March 2022 and April 2022, with 20 fish purchased from local fishermen at each station and transferred to the laboratory. In the laboratory, the biological characteristics of the fish, including their weight, total length, and standard length, were recorded. After dissection, the gastrointestinal tract was extracted and digested with a 10% potassium hydroxide solution (Rochman et al., 2015; Karami et al., 2017). Microplastics were identified and counted using a 40× light microscope (Abbasi et al., 2018). To identify the polymer type of the extracted microplastics, Fourier-transform infrared spectroscopy (FT-IR) with an attenuated total reflectance (ATR) crystal was employed (Veerasingam et al., 2021). Statistical analysis of the data was performed using SPSS software version 27, and appropriate tests (ANOVA and Kruskal-Wallis) were used for comparison of the data. Additionally, the results were presented graphically using Microsoft Excel version 2022.

In this study, 60 Caspian Sea whitefish (Rutilus frisii) were randomly sampled from three coastal stations: Kiashahr, Bandar Anzali, and Bandar Astara (20 fish from each station) to investigate microplastic contamination. The gastrointestinal content analysis revealed the presence of microplastics in all samples.
The analysis of microplastic contamination in Caspian Sea whitefish showed that all 60 fish contained microplastics. The Anzali station, with an average of 53.75 ± 35.50 microplastic particles per fish, exhibited the highest microplastic abundance. One-way analysis of variance (ANOVA) indicated that the contamination levels at Anzali station were significantly higher than those at Kiashahr and Astara stations (p< 0.05). However, no significant difference was observed between Kiashahr and Astara. The color diversity of microplastics in Caspian Sea whitefish showed that both Kiashahr and Anzali stations exhibited greater color variety compared to Astara, with blue being the predominant color. In contrast, the Astara station displayed a relative dominance of red-colored microplastics. This suggests that the distribution of microplastic colors may vary, with local factors influencing this distribution.
The microplastics extracted from the gastrointestinal tracts of the fish were classified into six size categories: 0.5–1 mm, 1–2 mm, 2–3 mm, 3–4 mm, 4–5 mm, and larger than 5 mm. In all three stations, particles in the 3–4 mm range were the most abundant. A total of 1998 microplastic pieces were extracted from the gastrointestinal tracts of the 60 fish examined. All microplastics extracted from the fish were identified as fiber type microplastics across all three stations. The microplastics extracted from the gastrointestinal tracts of the fish were analyzed using FTIR-ATR spectroscopy. Eight different polymers were identified, including polyester, polyethylene (PE), polypropylene (PP), nylon, cellophane, polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polystyrene (PS). Among these, polyethylene (PE) was the most dominant polymer, accounting for 35% of the microplastics found in the gastrointestinal tracts of fish. 

The present study demonstrated that 100% of Caspian Sea whitefish were contaminated with microplastics, indicating widespread contamination in this ecosystem. This finding is consistent with results from similar studies worldwide that have reported microplastic contamination in fishes (Arias et al., 2019; Tanaka and Takada, 2016). The primary sources of this pollution include the discharge of industrial and domestic wastewater through rivers and the release of fishing gear (Zakeri et al., 2020). Additionally, the diet of the whitefish, which includes bivalves contaminated with microplastics, plays a significant role in the transfer of this contamination to the fish (Bagheri et al., 2020). All microplastics found in the gastrointestinal tract of the fish were of the fiber type, which has also been identified as the predominant type in similar global studies (Luo et al., 2019; Hamed et al., 2023). In this study, microplastics of blue, red, pink, white, and green colors were found in the gastrointestinal tract of Caspian Sea whitefish. Blue and red colors were identified as the predominant colors. This finding is consistent with the results of Zhang et al., (2020), where blue was reported as the dominant color of microplastics. The size of the microplastics ranged from 0.5 mm to larger than 5 mm, with 3–4 mm being the predominant size. Various studies have reported different results; for instance, Tibbetts et al., (2018) reported microplastics smaller than 1 mm, and Zhao et al., (2015) reported sizes ranging from 1–2 mm. Smaller microplastics pose greater threats to marine organisms due to their larger surface area, higher chemical adsorption capacity, and greater potential for bioaccumulation (Van Cauwenberghe et al., 2015). These findings highlight the potential impacts of microplastics on the Caspian Sea ecosystem (Lu et al., 2016). In this study, polyethylene was identified as the most dominant polymer (35%) among the microplastics detected in the gastrointestinal tract of Caspian whitefish, a finding that aligns with previous research (Napper and Thompson, 2016; Li et al., 2020). The results of this study indicate that urban and industrial wastewater, fishing activities, tourism, and coastal erosion significantly contribute to the increase of microplastic contamination in the Caspian Sea. The accumulation of these particles in the whitefish can transfer contaminants to higher trophic levels in the food chain, posing a serious threat to human health. Therefore, effective plastic waste management and the implementation of stringent regulations to reduce pollution are essential.

Aquatic oligochaetes are members of a main group of macroinvertebrates and include about 1,700 species (Caramelo and Martinez-Ansemil, 2012) of 13 families with worldwide distribution (Martin et al., 2008). These species commonly inhabit within sediments of rivers, streams, lakes, marshes, ponds, springs and ground-waters (Collado and Schmelz, 2001), showing that these species have been adapted to a wide variety of habitats and environments, such as freshwater, brackish or seawater.The freshwater oligochaete genus Stylodrilus is a large and heterogeneous. Although the genus has a Holarctic distribution, approximately 28, are exclusive to the Palaearctic region (Martin et al., 2007), except Stylodrilus heringianus a peregrine species recorded in the Australasian region and assumed to have been introduced by shipping (Brinkhurst & Jamieson, 1971).From the Lumbriculidae family, only two species, L. variegatus and S. heringianus, have cosmopolitan, asexual reproduction (fragmentation) and use as live food in fish farming are the most important factors that have contributed to the distribution of these two species in the world (Pinder and Brinkhurst, 1994).Cheshmehkileh of Tonekabon and Sardabroud of Chalus Rivers are the most important, mountainous and permanent rivers running from high elevation to the sea (southern waters of Caspian Sea). These rivers are important for reproductive migration of two indigenous valuable fisheries species Salmo caspius (Caspian trout) and Rutilus frisii (Caspian Kutum) as well as other migratory fish (Khara, 2016). For these reasons, estuary of these rivers is regarded as a ‘Protected Area’ and is conserved by the Department of Environment of Iran (DOE, 1996). 26 species of aquatic oligochaetes have been reported from inland waters of Iran so far, of which only 1 species belongs to the Lumbriculidae family and from Lumbriculus genus: L. variegatus (Ahmadi et al., 2012; Basim et al., 2012).  The aim of this study was to introduce Stylodrilus heringianus from Cheshmehkileh of Tonekabon and Sardabroud of Chalus Rivers in the Southern Caspian Sea basin for the first time and investigation of its distribution pattern in the three ecosystems of river, estuary and marine combined with some environmental factors in water (temperature and salinity) and sediment (grain size and total organic matter).

Sampling of oligochaetes and environmental factors in both Sardabroud and Cheshmekile rivers, was carried out between November 2014 and September 2015 and random sampling was done at six stations with three replicates for each bimonthly sampling along each river. Sampling was done using 0.03 m2 Van Veen grab for soft sediments at the estuary area and for sampling at inner parts of the river with pebbles a 0.1 m2 and 0.2 mm-mesh size Surber sampler was applied. Biotic samples were fixed in situ using a 5% formalin solution. In the laboratory, sediments were sieved through mesh sizes of 1, 0.5 and 0.25 mm and specimens were preserved in 70% ethanol and then sorted and counted under a stereomicroscope (Nikon SMZ800, Japan) and eventually the wet weight of worms was measured using a digital balance (0.0001 g). For identification at species level, worm specimens were mounted on glass slides in Amman’s lactophenol clearing agent (Smith, 2001) and covered by a coverslip and left for several hours to a day or two, and then for observation of setae and other details, a microscope was used (Nikon E200). The main identification keys used were: Brinkhurst (1971a and b, Brinkhurst and Wetzel (1984), Brinkhurst (1986), Pinder and Brinkhurst (1994), Smith (2001), Arslan and Sahin (2003), Krieger and Stearns (2010), and Pinder (2010). In this study environmental variables, such as temperature and salinity in water were measured in situ by multimeter portable HACH-HQ40d model, and organic matter content was measured by the weight lost during ashing (Wildsmith et al., 2011), and grain size was analyzed by dry mechanical separation through a column of standard sieves of different mesh sizes, corresponding to classes described by Wentworth.

In this study, the annual average water temperature was 18.2±0.2ºC. The average water temperature showed a significant difference between the sampling months (P<0.05), so that the lowest average water temperature were recorded in March as 10.1±0.08ºC and the highest in July as 30±0.8ºC. In both studied rivers, station (S1) has the lowest annual average salinity (0 g/l), station (S2) has average salinity in moderate level (6.7±1.2 g/l), and station (S3) has the highest annual average salinity (10.6±0.05 g/l). In both studied rivers, the average water salinity between sampling stations and months showed a significantly different (P<0.05), so that in station (S2), the highest average salinity was in July (8.3±1.5 g/l) and its lowest in January (5±0.8 g/l) and in station (S3) the highest average salinity in July (11.6±0.06 g/l) and its lowest in March (9.2±0.04 g/l) was recorded (Fig. 1).The systematic account and description for the described species is as follows:Kingdom: Animalia ; Phylum: Annelida Linnaeus, 1758 ; Class: Clitellata Linnaeus, 1740 ;  Subclass: Oligochaeta Grube, 1850 ; Order: Lumbriculida Claus, 1872 ; Family: Lumbriculidae Claus, 1872; Genus: Stylodrilus Claparède, 1862 ; Species: heringianus Claparède, 1862.Figure 1: Locality of sampling stations in studied transects (Google Earth, 2014-2015).  (S1: River, S2: Estuary, S3:Marine) Worms are olive gray or dark yellow with brown spots (Fig. 2A). Prostomium is without proboscis and most specimens have no eyes (Fig. 2B). Worms's body length was 23 to 69 mm and 0.5 to 1.3 mm in diameter with 28-71 segments. Dorsal and Ventral chaetae beginning in segment II and have 2 chaetae per bundle and 8 chaetae per segments (Fig. 2C). Chaetae in the anterior segments were simple pointed (Fig. 2D) and the posterior segments, bifid with upper tooth very short and reduced (Fig. 2E, F), with 35-55 μm long. A pair of penes on ventral side of X segment were visible in mature individuals (Fig. 2G).Figure 2: Comparison of average (± standard deviation) of water temperature in sampling stations and months in studied transects, (S1: River, S2: Estuary, S3: Marine). Different letters indicate significant differences among averages (p<0.05).In total, 40 individuals of S. heringianus were examined. During the present study, this species occurred in stations 1 (river) but was absent in station 2 (estuary) and 3 (marine) in both sampling areas. Density and biomass of this species among sampling months andstations were significantly different (p<0.05), as the highest average density and biomass were observed in Cheshmehkileh (station 1) in January (78±32.6 ind m-2 and 197±70 mg m-2), and the lowest of those were observed in Sardabroud (station 1) in September (9.7±4.3 ind m-2 and 18±7 mg m-2) respectively. Spatial distribution of S. heringianus among sampling stations (S1, S2 and S3) showed significant differences (p<0.05). In freshwater stations (S1) within the river average density and biomass were 42.6±17.3 ind m-2 and 113.3±40.3 mg m-2 respectively, and no specimens were observed in the estuary and marine stations (Fig. 3). In other words, no specimens were observed in the estuary (S2) and marine (S3) stations of both studied transects in all the sampling months (Table 1).Temporal distribution of this worm in both studied transects showed significantly different among sampling months (p<0.05), as the highest average density and biomass of this species were observed in January as 58.6±23.7 ind m-2 and 175±55 mg m-2, respectively. While, the lowest of those values were in September as 9.7±4.3 ind m-2 and 18±7 mg m-2 (Fig. 4).Figure 4: Comparison of average (± standard deviation) of sediment TOM percentage in sampling stations and months in studied transects, (S1: River, S2: Estuary, S3: Marine). Different letters indicate significant differences among averages (p<0.05).A significant correlation (p<0.05) between density and biomass of S. heringianus with environmental variables was found (Table 2). A positive correlation was found between this species and gravel, silt and clay, while its correlations with temperature, salinity, TOM and sand were negative.

Discussion and conclusion :

This study aims was done to identify and determine the distribution and density of Stylodrilus heringianus in Cheshmehkileh and Sardabroud river and estuary, alongside Iranian coasts and has not been previously reported from Iran’s freshwater fauna and Southern Caspian Sea basin. According to the taxonomic and morphological studies of the samples and the use of valid identification keys, and also sending photos of the samples to experts to confirm the identified species, it was determined that the samples belong to S. heringianus species. This species was recorded for the first time from Iran. This paper updated a short checklist of Iranian aquatic oligochaetes to 27 species (for Lumbriculidae to 2 species and for Stylodrilus genus to 1 species). In Table 3, the identified species of Lumbriculidae from Iran until now are listed and S. heringianus was not on the list.Different letters indicate significant differences among averages (p<0.05). Capital letters indicate variation among stations (horizontal), small letters indicate variation among months (vertical).According to the results of this study, the highest average density and biomass of this species was observed in the cold months of the year (January and March) and the lowest in the warm months of the year (July and September), whereas density and biomass of this species show a significant negative correlation with water temperature, that this increase in density can be due to the asexual reproduction of Architomy and Parathomy (Smith, 2001) of this species in the cold months of the year, and its reduction in the warm months of the year (July and September) can probably be related to the feeding of fish with this worm. In the present study, S. heringianus was observed only in the river's freshwater ecosystem (salinity 0 ppt) and it was not observed in both estuarine brackish ecosystems with a salinity range of 5 to 8 g/l and marine brackish ecosystem with a salinity range of 9 to 11.5 g/l, which indicating that this species is oligohaline (does not tolerate salinity more than 1 g/l). Considering that the sediments grain size and the percentage of total organic matter were almost the same in the both ecosystems of river freshwater and estuarine brackish, it can be concluded that the limiting factor for distribution of this species in the estuarine and marine environments, maybe it is because of salinity intolerance, even to a very low level.Substrates in both rivers at Freshwater sampling stations (S1) were covered with vegetation in littoral and some central zone, but substrates in estuary and marine brackish stations (S2 and S3) vegetation cover was absent, According to the results of other researchers' studies about dependency of this species to vegetation habitats, probably, the main reason of density and biomass increase of this species in freshwater sampling stations was presence of vegetation habitats and its decrease in brackish water stations was due to absence of this habitat. Possibly, for this is reason, S. heringianus showed significant negative correlation with salinity. Among the investigated environmental factors, temperature and salinity of water and sand have the most influence on the distribution of this species. In addition to its important role in the food chain of water resources, this species is also considered as a bioindicator of water quality and is used as live feed in the aquaculture industry.

The stock status of most fisheries resources is not well-known, which is associated with limited data available for stock assessment. To overcome this gap, several data-limited stock assessment models, including the surplus production model, were applied to estimate stock status of these data-limited resources where no sufficient data is available to apply the conventional stock assessment models. To estimate MSY, catch and CPUE data for Cyprinus carpio caught from the southeastern Caspian Sea were applied. Twenty-two years data (1999-2022) were analysed by SPM to estimate MSY. Three parameters including population growth rate (r), population carrying capacity (K), and initial biomass for an exploited population (Binit) required for estimating MSY were guessed 0.075 (/yr), 6952 tones, and 3500 tones, respectively by the catch-MSY model. The estimated MSY using these guessed parameters by SPM was 130 tones. While, estimated MSY using the repeated (multiple) starting points was 1320 tones. Phase plot indicated that the current status was both below the limited and the target biomass reference points. These illustrated the stock of C. carpio in the southeatern Caspian Sea is unsustainable and overfished. Since, survival of C. carpio depends the releasing hatchery reared juveniles to the wild, specific fisheries management should be applied to prevent the undersize catch and illegal fishing.

The quality of hatchery reared juvenile fish and their releasing quality to the wild would impact the rate of stock enhancement in the aquatic resource. This study examined the impact of size and density of the hatchery-reared juveniles on the biomass, catch and profit of fishing activities of common carp in the Caspian Sea. Data used in this study was extracted through literatures review, including the number and the size of hatchery reared juveniles of common carp at the releasing time to the wild, annual catch of common carp from 1999 to 2017 in the south-east of the Caspian Sea, and population parameters of common carp estimated using length frequency data in 2019 and 2020. The results indicated that economic rent and biomass decreased in the population without stocking. While, releasing hatchery-reared juveniles at the length of 7 cm (the length equal to the 10% of ) showed that catch profit and relative biomass declined by increasing fishing effort, which was associated with overfishing. Thus, releasing one-gram hatchery-reared, which the length of released juveniles is less than 10% of population asymptotic lengthy , may not increase common carp yield and biomass. In contrast, this may only increase the cost of stock enhancement. In addition, when the size or length of released juveniles increased to the 10% of population asymptotic length, the maximum economic rent (gross revenue) was obtained in the 30% of fishing effort unit.

The purpose of this research was to determine the concentration of poly aromatic hydrocarbons (PAHs) in the surface sediments during four seasons at different transects of the southern basin of the Caspian Sea and to investigate the causes of their temporal and spatial changes, to compare with the standard limits, and to find the origin of these compounds. The samples from the surface sediments were collected using a grab (van Veen grab) from depths of 10 and 30 m in the coasts of Guilan, Mazandaran, and Golestan provinces at eight transects (Astara, Anzali, Sefidroud, Ramsar, Nowshahr, Babolsar, Amirabad, and Torkaman) during the autumn and winter seasons of 2018, spring and summer of 2019. The sediment samples were prepared in order to investigate PAHs using a standard method (extraction with organic compounds by the Soxhlet apparatus). The results showed that the mean (±SE) annual concentration of PAHs in the four seasons of autumn, winter, spring, and summer was 29.6±11.5, 2125±611, 4.2±3.0, and 5.3±3.1 µg/kg dry weight, respectively, and the maximum concentration of PAHs was recorded in the Nowshahr transect in the winter season. The results of the current research showed that 2 and 6 rings compounds had a low percentage of abundance in the sediments and the mean concentrations of PAHs and BaP in most of the samples were lower than the international threshold standards. The origin of PAHs compound in the sediment was recorded in more pyrogenic sources based on the diagnostic ratio (LMW/HMW=0.3).

This study examines the characteristic of microplastics (MPs) in sediments collected from the coastal areas of the southern Caspian Sea, Mazandaran province (Babolsar and Sari cities). Twelve sites for sediment sampling selected during 2022. MPs identification detected primarily under stereomicroscope and then ascertains and discovers their polymer composition by ATR-FTIR. The results showed that Babolsar areas, with 5 meters far from the shoreline, had the most abundance (660±165.15items/Kg). On the other hand, Sari station with 500 meters far from the shoreline contained the least MPs (220±81.33). Average number of detected MPs in all samples was 406.66±161.33. Fibers (80.33%) were the most abundant MPs, whereas films (3.28%) detected the least. In the case of size, the range of 1000-3000µm showed the most abundant MPs (39.34%) and MPs less than 2 µm the least ones (4.10%). Gray and black (50.82%) was the dominant color and yellow (3.28%) was rarely detected color (3.28%). Polymer constituent of examined MPs includes: (PET: 30%); (PS: 20%); (PA: 20%); and (PP: 15%). The findings of this study revealed that MPs occurrence in coastal sediments distribute from several sources; including tourism as well as wastewaters and packaging. These activities cause entering plastic litters along the coastal areas and their degradation accelerated t MPs distribution.

This study used a dynamic model to predict the fishing status of the Persian Sturgeon (Acipenser persicus) along the coast of the Golestan province in the Caspian Sea. The findings indicated that based on the initial population in 2017 equal to 2500 samples of fish if harvested at the current rate, the stocks of this valuable sturgeon fish will be destroyed in the next 50 years. However, when considering the harvest rate of 0.2 per year (nearly half of the current catch), the amount of fish harvested will increase so that this stock will completely regain its ability to regenerate itself.

This work aimed to compare hybrid and non-hybrid populations of Hircanian goby (Ponticola hircaniaensis) from two rivers, Kabudval and Shirabad, Golestan Province, respectively. For this purpose, 20 morphometric characteristics were measured. To reveal the morphological differences between the studied populations, the morphological characteristics after standardization were analyzed using Discriminant Function Analysis (DFA) and Non-parametric Multivariate Analysis of Variance (NPMNOVA) based on the P-value obtained from the permutation test with 1000 repetitions. The results of Kruskal-Wallis's test showed that out of 20 morphometric characteristics, 11 traits significantly differ between the two populations. Also, the results of DFA analysis based on the permutation test with 1000 repetitions showed a significant difference in morphology between the two populations of Kabudval and Shirabad rivers (P = 0.048). Since the habitat conditions of the two rivers, Kabudval and Shirabad, both forest streams, are almost identical, the morphological difference between these two populations of Hircanian goby can probably be due to the genetic difference due to hybridization.

Breeding and production of fish in sea cages is one of the most common methods of commercial production of aquatic animals in the world. It is possible to develop aquaculture in the Caspian Sea by fully respecting the biotic, abiotic, social, and economic conditions, as well as the environment, with regard to sustainable development. Breeding fish in cages is very important from an economic point of view, and determining the challenges and obstacles before investment in order to reduce the investment risk plays a fundamental role in it. The purpose of this article is to investigate the economic challenges of cage culture development on the southern shores of the Caspian Sea using a combination of SWOT and FAHP analysis and then determine strategic priorities based on the Quantitative Strategy Planning Matrix (QSPM). In order to meet the challenges and also formulate strategies, the SWOT analytical model was used, and the opinions of 30 fisheries experts were obtained through face-to-face interviews and completing questionnaires in 2020-2021. According to the final score obtained, opportunity factors got 2.89 points, and threat factors got 3.54 points. As a result, the threat points prevail over the opportunities, and it shows that the current situation of this activity requires strategies in the field of adopting solutions to reduce threats. According to the total scores of the obtained matrix, the score of external factors indicates the need to formulate a defensive strategy in order to reduce the minimum risk of investment. According to the SWOT analysis, the strategies with the highest score are "development of standards and criteria and locating suitable areas for cage fish breeding sites," "allocating low interest loans and encouraging investors in order to facilitate investment," and "sustainable development in order to prevent Changing the use of valuable agricultural land" and "attention to export, introduction, and cultivation of species with global marketability" were presented in order to solve the existing challenges and suggestions to make the fish farming industry in the Iranian regions of the Caspian Sea.

The purpose of this research was to evaluate the consumer’s behavior of farmed carp in the north of Iran (Golestan, Mazandaran and Gilan province).

After defining the assumptions and designing the analytical-conceptual model based on the theory of planned behavior, a suitable questionnaire was developed and completed by the statistical community. In order to evaluate the intensity of the relationship between the research variables and the model elements, as well as to check the acceptance or non-acceptance of the assumptions, the structural equation modeling method (SEM) was used.

Per capita fish consumption of farmed carp consumers is 5.3 kg. Of this amount, 400 grams are canned, 600 grams are consumed in restaurants and outside the home, and 2.5 kg are consumed at home. Per capita consumption of their farmed carp is 1.4 kg, so half of their consumption is from other fish.In this research, all hypotheses were confirmed, In this way, the effect of perceived usefulness, perceived risk and quality on attitude, the effect of normative beliefs, moral obligation and household income on subjective norms, the effect of control power, control belief, experimental records, advertising and education on the perceived behavior control, the effect of attitude, subjective norms and perceived behavior control on the decision to buy farmed carp and the effect of the decision to buy and perceived behavior control on the final behavior were reported to be significant (p<0.05). The values of the effect coefficients showed that the most effective variable on the attitude, subjective norms and perceived behavior control, is quality (with an effect coefficient of 0.46), income (with an effect coefficient of 0.44) and experience records (with an effect coefficient of 0.37), respectively. Also, among the three main elements of the model, the attitude had the greatest effect on the decision to buy and consume farmed carp (with an effect coefficient of 0.48).

The purpose of this study was to determine the frequency of Vibrio parahaemolyticus and to identify pathogenic and non-pathogenic strains of this bacterium using a series of biochemical tests and polymerase chain reactions from the shores of the Caspian Sea (Gilan Province). The water and sediment samples of the Caspian Sea were collected from the depth of 30-50 and 10-20 cm respectively by aseptic method. Sampling date, sampling location, temperature, pH and salinity were measured. Isolation and identification of environmental isolates was done using standard methods in order to determine the genus and species using specific media such as thiosulfate citrate bile sucrose and Vibrio chrome agar. A total of 55 isolates were identified by biochemical, phenotypic and polymerase chain reaction tests. From the total of 55 cases of Vibrio parahemolyticus isolated, the frequency of tdh and trh genes was 1 case (1.8%) and 3 cases (5.4%) respectively. This study showed that Vibrio parahaemolyticus, which have pathogenic potential, should be continuously monitored as agents of contamination of coastal waters. The isolation of these species indicates a health warning for people who use raw seafood or are in direct contact with the waters of the Caspian Sea are in this area.

This work aimed to investigate the morphological variations and patterns of morphological adaptation to their habitat in five populations of Iranian goby, Ponticola iranicus (Gobiidae). For this purpose, samples were collected from Siah Darvishan, Shafarood, Rostam-Abad, Masal, and Asalem rivers, Guilan province, and then 20 morphometric characteristics were measured. The morphometric characteristics after standardization were compared using PCA, Kruskal-Wallis and CVA analyses to investigate the morphological variation between studied populations of Iranian goby. Based on the results, all characters showed significant differences, and the Siah Darvishan differed completely from the others. Also, Rostam-Abad and Shafarud, and Masal and Asalem populations overlapped with each other. The most important discriminating traits include the body depth (0.3), predorsal length (0.4), preanal length (0.5), anal-fin base length (0.44), head width (32. 0), distance before dorsal fin (0.42), length of pectoral fin (0.34), maximum body depth (0.39), caudal-fin width in anal fin (0.33) and width of upper lip (0.34). The CVA results showed the distinction between populations. Based on the results, the members of this species adapt themselves to different habitats with traits related to body depth, dorsal and anal fins, and head size.

The ability of fish adaptation to different salinities depends on osmotic, ionic and biochemical regulation, on the other hand, transferring fish to salt water causes the development of their osmotic and ionic homeostasis mechanism. The adaptation of stellate sturgeon (Acipenser stellatus) in four weight treatments including 0.5-1, 1-3, 3-5 and 5-10 g in water with four salinity levels of 0.5, 4, 8 and 12 g/l during 8 phases and each experiment was performed in 4 predicted treatments. Each experiment was carried out in a period of about one month along with feeding with live foods, preferably gammarus, Nereis worms, and chironomidae larvae at the saturation level of 6 times a day, as well as daily siphoning and water changes. Osmolarity and factors of chloride, calcium, sodium and potassium were measured in fish blood serum. In the examination of the compatibility and adaptability of this fish with the conditions of changing salinity, the results showed that with increasing salinity, the levels of potassium, sodium, chloride and blood osmolarity increased. The stellate sturgeon are able to adjust the osmotic pressure and ionic composition of their blood serum when they migrate from hyperosmotic to hypoosmotic environment or vice versa. The blood osmolarity of fish with a weight of 5-10 g was higher than other weights, and the lowest osmolarity was observed in fresh water (0.5 g/l). The results of this study showed that the amount of potassium in the blood serum decreased with the passage of time and the increase in salinity, and increased the amount of sodium and calcium in the blood serum.

More than 50 fish species exist in Guilan coast waters and 20 species of them are economic, but there is few data about their fingerlings. This study has been done in Talesh, Anzali, Kiashahr and Chaboksar shores and fish seasonally caught by beach seine with 8 mm mesh size from winter to autumn 2021, and the main purpose was determining fish abundance in Guilan province coasts, too. Results showed 32 fish species belong to 13 families in studied are and Leuciscidae family was the most diversified with 8 species. The study of fish abundance showed dominancy of Chelon saliens, Alburnus chalcoides and Atherina caspia with 42.5%, 18.1% and 11.6% in Talesh shore, A. chalcoides, C. saliens, and A. caspia with 25.1%, 22.1% and 20.8% in Anzali shore, C. saliens, Rutilus frisii and A. chalcoides with 31.8%, 17.4% and 14.8% in Kiashahr shore and C. saliens, A. chalcoides, Neogobius pallasi and chelon auratus with 29.9%, 21.5%, 14.2% and 10.5% in Kiashahr shore, of total count (number), respectively and they were predominant. In general, were abundant fish, the fingerlings to non-adults of sharp-nosed grey mullet (Ch. saliens) and Shemaya (A. chalcoides) in all stations, Caspian silverside (A. caspia) in 2 stations and fingerlings of Golden grey mullet (Ch. auratus) and Kutum (R. frisii), and fingerlings to adults of Caspian sand goby (N. pallasi) in one station, that show a more adaptation of these species to environmental conditions in 4 studied stations. The continuous monitoring of density of small size individual of fish species in southern coasts of Caspian Sea is essential for conservation of their populations.

More than 70 millions of Kutum (Rutilus frisii) fingerlings are released in the rivers of Guilan province annually for restoring, but there is few data about biological chracteristics about the fish during staying in the rivers downstream, thus this study has been done on abundance, growth, condition factor and diet of released fingerlings of kutum fish (Rutilus frisii) in Khoshkrud River downstream, and for this propose, 3 stations selected in the river downstream and sampling has been done using electroshocker monthly from Oct. 2022 until March 2023. Results showed Alburnoides samii, Capoeta razii, Rutilus frisii, Ponticola gorlap and Cobitis with 249.2, 108.7, 85.2, 32.2 and 31.4 individuals in 100 m2 catch effort had the most abundant. The fingerlings of kutum fish present in all months in the river downstream and its density varied 20.0 to 212.5 individuals per 100 m2. Studied fingerlings (n=119) had a weight of 2.10-15.10 (5.77±2.54g, average±s.d), total length of 6.62-12.20 (8.61±1.13cm) and condition factor 0.71-0.97 (0.86±0.05) and there was significant statistical difference only in condition factor between autumn and winter. The length-weight parameters was calculated for b-value 3.09 and correlation coefficient (r2) 0.98 and indicated isometric growth patterns. It was determined vacuity index 40.9% and 59.6%, the mean relative length of intestine 0.78±0.08 and 0.75±0.08 and the mean fullness intensity 76.1±97.1 and 71.3±118 in kutum fingerlings in autumn and winter seasons, respectively. There was 6 macro-invertebrates and 14 genera of algae in gut of kutum fingerlings and in the fish intestine were abundant Chironomidae from benthos and genera Synedra, Navicula, Diatoma and Melosira from algae cells in both seasons. In general, fullness intensity, condition factor and water temperature were no suitable for staying fish fingerlings in the stuied duration, but was not distinguished the reason of settle of fish in rivers and no emigration to sea water.