In the last decade, invasive suckermouth armored catfish Pterygoplichthys spp. spread among many river systems of Vietnam. Extended distribution of armored catfish might be associated with using brackish water in estuaries for fish spread from one river system to another. The first goal of our study was to assess the occurrence of armored catfish in the estuary of the Da Rang River (Phu Yen Province, Vietnam) and their distribution depending on the horizontal salinity gradient (4–25 PSU). Fish were mainly caught by stationary bottom traps in water salinity from 4 PSU to 18 PSU. The second goal of our study was to experimentally evaluate the ability of armored catfish to move and breathe in seawater (33 PSU). Fish moved in horizontal and vertical planes after transfer into seawater during the first 15 minutes. Fish moved around less by the 13^th–15^th minutes in seawater. Armored catfish moved around more in seawater than in freshwater. The exposure to seawater for 6 minutes led to deterioration of fish breathing. The results of our field and experimental studies established that armored catfish are found and able to move in brackish waters but avoid high salinity water. These facts provide support for the hypothesis of armored catfish invasion through the estuaries and coastlines.

Invasive suckermouth armored catfish Pterygoplichthys spp. successfully spread during one decade into many river systems of Vietnam. Wide and rapid invasion of armored catfish might be associated with using brackish water in estuaries to spread from one river system to another. The first goal of our study was to assess the horizontal and vertical distribution of invasive fish in freshwater (Da Rang River) and in brackish water (Da Rang River estuary) associated with circadian rhythm. In the both sampling locations, fish were mainly caught at nighttime at the bottom and near the surface using the net traps and vertical nets. In estuary, fish were caught in the net traps with distance 2.0–7.5 m from the right or left banks where water was predominantly fresh. In freshwater of the Da Rang River, fish were often caught near the left bank with gravel and stone substrate. The second goal of our study was to experimentally evaluate the circadian (12 hours of the night and 12 hours of the day) rhythm of locomotor activity (LA) of fish. Fish from different freshwater locations (Am Chua canal and Da Rang River) had a similar diurnal dynamic of LA with mostly movements (77–83% of total diurnal LA) at nighttime (18:00–6:00, GMT+7) at the end of the wet season. Armored catfish from the brackish water location (Da Rang River estuary) also were mostly active (76% of total diurnal LA) overnight. However, fish from freshwater and brackish water had difference in the timing of behavioral activity. Fish LA from estuary was significantly lower than LA of fish from freshwater locations during 18:00 to 21:00 before low tide. The results of our field and experimental studies established that armored catfish in estuary moved in horizontal and vertical planes predominantly at nighttime. Tide level regulates locomotor activity of invasive fish and could influence on the possibility of their spreading through the estuary.

Non-native suckermouth armoured catfish Pterygoplichthys spp. have spread extensively across many river systems in Vietnam. It is possible that their expanded distribution occurred through the brackish waters of estuaries and coastal zones, facilitating movement from one river system to another. It has been previously hypothesized that, for successful dispersal through brackish water, armoured catfish can tolerate changes in water salinity and are capable of avoiding high salinity levels that threaten their survival. In this study, we experimentally estimated the movements and the directions of juvenile and adult wild loricariids in fresh and brackish water. Our results showed that juveniles exhibit a circadian rhythm of locomotor activity similar to that of adults. However, juveniles display a more pronounced reaction to increasing water salinity ‒ at 5 PSU ‒ while adults respond at 15 PSU. This likely explains the absence of juveniles in natural brackish water environments and their reduced potential to spread through brackish waters compared to adults. Adult loricariids are likely capable of recognizing and avoiding high-salinity zones (>10 PSU) by increasing locomotor activity, predominantly directed toward the surface. Their ability to grasp air helps maintain positive buoyancy, allowing them to remain in the surface layer of freshwater over extended periods of time. Variability in salinity tolerance among adults (ranging from 2 to 16 hours in 15 PSU) may enable some individuals to be more successful in dispersing through estuaries and along coastlines.

The diurnal dynamics of the content of thyroid and sex steroid hormones is investigated in the blood of the resident form of Black Sea trout in summer. The maximums and minimums of concentration of...

The climbing perch Anabas testudineus is widespread in the inland waters of Vietnam and according to its ecology could have contact with floating plastic waste. Fragments of expanded polystyrene (EPS) are detected in the fresh waters of Vietnam in Khanh Hoa, Lam Dong, and Phu Yen provinces. Our study focused on estimating the probability of ingestion of EPS pellets (size 2.5–3.5 mm) by adult climbing perch. In the experiments, 3 types of treatment pellets were offered to fish: 24 feed pellets (Fps), 24 expanded polystyrene pellets (Pps), and 12 feed and 12 expanded polystyrene pellets (FPps). Fish grasping time of the first pellet was independent in all treatment types. The grasping time of the 12th pellet was insignificant in Fps (63 s) and Pps (75 s). Climbing perch grasped and ingested the 24th Fp significantly (p = 0.02) earlier (143 s), than they grasped the 24th Pp (817 s). Fish with FPp treatment grasped feed along with EPS pellets, but grasping the 12th Fp was significantly (p = 0.02) earlier (49 s) than the 12th Pp (193 s). By the end of the tests, the fish had ingested all feed pellets. We discovered that climbing perch grasped Pps and kept them in the oral cavity, but rejected them in 100% of the cases. This result provided evidence that climbing perch have an effective defense mechanism for avoiding ingestion of expanded polystyrene pellets with a size of 2.5–3.5 mm.