The marinka is a polyphage that consumes both plant and animal food in different seasons of the year (among which there is small trash fish), which contributes to the high infection of the fish with helminth species, especially during the period of post-spawning fish feeding (Najmidinov et al. 2020). The extreme western border of the mountains in northern Iran; the name of the Alay-Turkestan mountains in the Fergana Valley, called the Western Tien Shan, which stretches quite far to the east and is directly interconnected with the Himalayas, Tibet and other mountainous countries, and they are a considerably wide area of the marinka - a typical representative of the Mountainous Asian complex of ichthyofauna, which is at the same time the main carrier of the parazitofauna of fish (Osmanov 1971).
In our study, Rh. gnedini, Rh denudata and Rh. hellichi turkestanica were observed in intestine of the common marinka sampled in Vergana Valley. The results of the present study were similar to Karimov (2007), who noted a significantly high degree of infection of marinka with Rabdochona denudata, Rh. gnedini and Rh. chellichi in the river Isfara, Tajikistan. Rhabdochona hellichi occurs in mountain rivers, lakes and some foothill reservoirs in the Central Asia region (Karimov 2007). At the same time, it should be noted the presence of some morphs and geographical races of the indicated helminth, the distribution area of which is much wider. Rh. hellichi turkestanica reported from fish lived in Indian water bodies, as well as the morphologically close subspecies Rh. hellichi kurdistanensis is found in the local subspecies of the barbel in Iraq (Scholz 1995; Moravec et al. 2010; Chavan et al. 2014; Bilal et al. 2017).
Moravec et al. (2010) reported that Rh. hellichi turkestanica were found in Schizothorax spp. (Cyprinidae, Cypriniformes), Rh. hospeti in Tor spp. (Cyprinidae) and Rh. mazeedi in Clupisoma garua in India. They are carried out light and electron microscopic analysis made it possible to reveal new, previously unknown morphological and taxonomic characters and made it possible to describe this species again in more detail. Rh.hellichi turkestanica (synonym R. denudata filamentosa, R. kashmirensis, R. schizothoracis) was considered as a subspecies different from another typical subspecies R. hellichi hellichi mainly in the shape of the distal end of the left spicule, in molecular characteristics and geographic distribution. The morphology of the aforementioned species of Rhabdochona hardly differs from those of the original subspecies Rhabdochona hellichi. In the present study, the body of the parasite was narrowed at both ends. The cuticle was smooth. At the caudal end, females have a ring-shaped formation, the edges of which were dotted with spines. The lips were poorly developed, barely visible. The oral capsule was funnel-shaped. Very small cervical papillae were normal in shape. Thus, the Rhabdochona species we have characterized differ from each other in a number of morphologic characters, including the size of the body and spicule, the shape of the tail and egg.
Marinka lives in all surveyed mountain rivers and the Syrdarya river and adjacent water bodies of Western Fergana. However, the parasites were observed only sampled in the mountain rivers Rezaksay, Chodaksay and small tributaries of the Syrdarya river in western Fergana in the present study. Main factors of fish infectivity with parasite in the upper reaches of the Syrdarya are the rich hydrofauna and a wide range of fish nutrition: the latter, the marinka becomes infected with parasites by eating small trash fish; at the same time, the marinka, abundantly eating mayfly larvae which act as intermediate host of this parasites, so the fish is strongly infected with Rhabdochona species (Karimov 2007), which corresponds to the results of Stein (1959), who associates this phenomenon with an increase in the content of dissolved oxygen in the water and, as a rule, a strong infestation of mayfly larvae with larvae of Rhabdochona spp. Further, considering the reasons for the parasite infectivity of the marinka, indicates the adaptation of Rhabdochona spp. to the fast flow and low temperature of the water of mountain rivers, the relatively poor composition of animal food species, due to which the marinka switches to feeding benthic, along with which swallows larvae of Rhabdochona spp. (Karimov 2007). The flow rate is the leading factor in the formation of ichthyofauna and ichthyoparasite fauna of mountain rivers (Mukhamediev 1982). In the present study, it was pointed out that some abiotic factors such as the flow rate, water temperature and high content of dissolved oxygen in it, as well as the nutritional characteristics of the common marinka can be most important factors to spread of Rh.denudata, Rh. gnedini, Rh. hellichi turkestanica in marinka from mountain rivers of the Fergana Valley.
In conclusion, the marinka population sampled from the mountain rivers Rezaksay, Chodaksay and small tributaries of the Syrdarya river in western Fergana are infected with Rh.denudata, Rh. gnedini and Rh. hellichi turkestanica. According to the Authors’ knowledge, Rh. hellichi turkestanica was recorded in Uzbekistan for the first time. The flow rate, water temperature and high content of dissolved oxygen in rivers, and the nutritional characteristics of the marinka can be responsible for infection.