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.

The paper presents the results of the study of survival, the concentrations of thyroid hormones and ions of armored catfish of the genus Pterygoplichthys at increasing salinity of water. A stepwise (every two days) increase in water salinity from 0 to 12.5‰ leads to a significant increase in the concentration of Na⁺, K⁺, Cl^–, Ca²⁺ ions in the blood of fish, but does not affect the level of thyroid hormones (free and total triiodothyronine, thyroxine) and the calculated ratio of these hormones. The data indicate a low involvement of the thyroid gland in the osmoregulation of armored catfish. It is confirmed that some specimens of armored catfish can tolerate water salinity up to 15‰ for 2 days. The possibility of dispersal of armored catfish along estuaries to neighboring river systems is discussed.

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.

Cytological state of gonads and hormonal status was studied in juvenile (15-month-old) Black Sea trout Salmo trutta labrax of the two phenotypic forms (resident and migratory). The differences in the rates of hametogenesis in the compared forms were revealed. It is shown that the spermatogenesis in the resident form is more intensive; on the other hand, the ovogenesis is slower. The levels of thyroxin and testosterone in blood are significantly higher in the groups of fish with intensively running gametogenesis. It is found that the concentration of triiodothyronine is slightly slower in the resident form than in the migratory. The correlation between levels of thyroid and sex steroid hormones is revealed.

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.