In two experimental trials; florfenicol pharmacokinetics following a single dose oral administration at 15 mg kg⁻¹ fish body weight and biosafety through extended medicated feeding were studied in the rainbow trout, Oncorhynchus mykiss. The pharmacokinetic trial was conducted for 5 days, whereas the biosafety experiment lasted for a 30-day safety margin followed by a 20-day residual period analysis at 3, 5 and 10 times greater than the therapeutic dose 10 mg kg⁻¹ biomass day⁻¹. C_max µg kg⁻¹ calculated for florfenicol were found to be 5,360 in intestine, 2,890 in gill, 2,250 in kidney, 973 in liver and 273 in plasma, obtained at T_max of 16 h. Intestine had utmost area under the concentration–time curve _{(tissue/plasma)} of 13.83 h μg kg⁻¹ and a prolonged half life (t_1/2ß) of 28.62 h. The highest apparent metabolic rate value in the kidney (0.327) showed a high level of biotransformation of florfenicol to its metabolite florfenicol amine. The apparent distribution rate of florfenicol amine in muscle, in comparison to the parent drug florfenicol, indicated elimination of the medication mostly in the form of florfenicol amine with t_1/2 of 16.75 h. The biosafety of florfenicol orally administered to rainbow trout recorded effective feed consumption, physiological responses, drug tolerance and significantly low drug concentrations in muscle of rainbow trout, thus its usage at 10 mg kg⁻¹ fish body weight is recommended. In the study, the rapid absorption, greater bioavailability, enhanced dispersion, slower elimination and biosafety of the drug form a significant basis for the florfenicol and its metabolite florfenicol amine as a useful antibacterial agent in aquaculture.

Mystus prabini, new species, is described from the Sinkin and the Dibang River in the Lower Dibang Valley District of Arunachal Pradesh, India. The new species differs from all South-Asian congeners except M. bleekeri, M. cavasius, M. zeylanicus, M. falcarius, M. seengtee, M. cineraceus, M. ngasep, M. rufescens and M. ankutta in having a long adipose fin that reaches anteriorly (vs. distinctly does not reach) the base of the last dorsal-fin ray. The new species can be distinguished from the named nine species in having (vs. lacking) a narrow black mid-lateral stripe extending from the anterior region of tympanic spot to the rounded black spot at the caudal-fin base. The analysis of mitochondrial cytochrome c oxidase subunit I (COI) gene sequence shows that the K2P value between Mystus prabini and all other Mystus species ranges from 8.6-22.1%. Mystus prabini is closest genetically to M. bleekeri and M. albolineatus, from which species it has a genetic distance of 8.6% and 13.9%, respectively. The genetic distance (K2P) between the new species and M. dibrugarensis is 21.1%.

In the present experiment, the attempt has been made to study the biosafety, toxicity, residue depletion and drug tolerance of graded doses of emamectin benzoate (EB) in juveniles of golden mahseer, Tor putitora as a model candidate fish for sport fishery and conservation in temperate waters through an extended medicated feeding. The graded doses of EB viz., 1× (50 μg/kg fish/day), 2 × (100 μg/kg fish/day), 5 × (250 μg/kg fish/day) and 10 × (500 μg/kg fish/day) were administered to golden mahseer juveniles through medicated diet for 21 days at water temperature of 18.6°C. The higher doses of EB did not cause any mortality during and 30 days after the end of medication period, but considerable variations in feeding and behavior were observed. Severe histological alterations observed after EB-diets (5 × and 10×) were vacuolation, pyknotic nuclei, melanomacrophage centre and necrosis in liver; Bowman’s capsule dilation, degenerated renal tubules in kidney; myofibril disintegration, muscle oedema, splitting of muscle fibres, migration of inflammatory cells in muscle; and abundant goblet cells, dilated lamina propria and disarrangement of mucosa in intestine tissues. The residual concentrations of EB metabolites Emamectin B_1a and B_1b were analyzed using muscle extracts and were found to be peaked during medication period followed by gradual depletion in post-medication period. The outcome of this study showed that the Emamectin B_1a residual concentration in fish muscle in 1×, 2×, 5×, and 10× EB treatment groups were 1.41 ± 0.49, 1.2 ± 0.7, 9.7 ± 3.3, and 37.4 ± 8.2 μg/kg at 30 days of post-medication period, respectively, which falls under the maximum residue limits (MRLs) of 100 μg/kg. The results support the biosafety of EB at recommended dose of 50 μg/kg fish/day for 7 days. As residue of EB is recorded falling within the MRL, no withdrawal period is recommended for golden mahseer.