The production of rainbow trout (Oncorhynchus mykiss) is developing and it is most cultured finfish species in Turkey. It was first introduced in Turkey in the mid 1970s from Italy. In the following years, culture studies of native trout strains, mainly Black Sea trout; Salmo trutta labrax and Abant Lake trout, Salmo trutta abanticus started in late 1990. Market size trout production was performed in two main categories, inland areas (mainly concrete ponds by flow through fresh water and cages) and in sea (offshore cages). Total trout production was 61.173 tons in 2007.

The aim of this study was to investigate the patterns of antibiotic resistance of Aeromonas spp. bacteria isolated from the sea trout (Salmo trutta) from the state fish hatcheries of the Institute of Food Safety, Animal Health and Environment ‘BIOR’, Latvia. Bacteriological investigations were performed at four state fish hatcheries located in the drainage basins of the main Latvian rivers – Daugava, Venta and Gauja, during the five-year period (2012 – 2016). In fish with visible clinical signs, bacteriological samples were collected from heart, liver, spleen, kidney and ulcer surfaces. Aeromonas hydrophila and Aeromonas salmonicida were isolated from sea trouts. A total of 52 individual sea trouts were examined. Resistance to amoxicillin, ampicillin, cephalexin, colistin, doxycycline, enrofloxacin, erythromycin, florfenicol, gentamycin, kanamycin, lincomycin, neomycin, oxytetracycline, spectinomycin, streptomycin, tetracycline, trimethoprim/sulfamethoxazole was tested. The results of this study suggest a multi-drug resistance pattern among the A. hydrophila isolates. All the isolates were resistant to amoxicillin (100%), ampicillin (100%), cephalexin (100%) and erythromycin (100%). The lowest level of resistance was found against florfenicol (4.55%), gentamycin (4.55%), kanamycin (4.55%), but susceptibility was recorded to enrofloxacin, neomycin and trimethoprim/sulfamethoxazole. A. salmonicida isolates were resistant to oxytetracycline (9.38%) and tetracycline (9.38%). For other antibiotics A. salmonicida isolates were susceptible.

In recent years, the effective immunostimulating properties of β-glucans have been widely proven not only in mammals, but also in aquatic animals. The aim of this study was to determine the effect of dietary β-glucan supplements on the percentage of individual types of leukocytes and the haematocrit of the sea trout (Salmo trutta L.). Investigations were performed at the state fish farm ‘Tome’, hatchery ‘Pelči’ of the Institute of Food Safety, Animal Health and Environment ‘BIOR’, during a six month period (September 2018 – February 2019). On the basis of feeding with immunostimulant diets, a total of 15 000 sea trout were assigned into five groups. For blood collection healthy fish were randomly selected once a month, five 5 individuals from each group. Blood smears were made to determine the leukocyte differential count. Sampling was done once a month (September 2018 – January 2019) (n=125). Haematocrit was determined by the standard microhaematocrit method. Sampling was done once a month (January 2019 – February 2019) (n=50). We concluded that the dietary β-glucan supplements can improve the haematocrit level and effect the percentage of individual types of leukocytes of the sea trout, stimulation of the preparation for parr-smolt transformation.

Morphological parameters and their changes common for artificially reared one year old sea trout were examined from January to May the year 2013 to determine whether these parameters indicated the smoltification in one year old sea trout and whether these fish achieved smolts stage. Fish were reared in flow-through and recirculation systems in hatcheries based on three different rivers (Brasla, Daugava, Venta basin). Sea trout were examined in the Latvia University of Agriculture, Faculty of Veterinary Medicine, Institute of Food and Environmental Hygiene and in the Institute of Food Safety, Animal Health and Environment BIOR, Laboratory of Aquaculture and Fish Pathology. To appreciate fish growth stage condition index, hepatosomatic index, spleen index was calculated and silvering level was evaluated. The fish condition index decreased in all hatcheries and flow-through and recirculation rearing systems from January to April and increased in May. Spleen index was the most stable parameter and did not change a lot showing that fish did not have migratory stress in April and May. Silvering level increased from January to April but suddenly the increase became slower in May without reaching the top level. These results made us to consider that one year old sea trout parrs released in May 2013 probably did not become smolts before release and they had to stay in river for one additional year until reaching pronounced smolt stage.