1. AKNOWLEDGMENTS This study was co-financed by the project “Innovations in finfish aquaculture with special references to reproduction” (acronym: InnovaFish), Operational Programme Sustainable Development of the Fisheries Sector and Coastal Fishing Areas 2007-2013" (OR14-61724-OR1400003/09/10/11). www.eng.InnovaFish.pl Sławomir Krejszeff 1 *, Daniel Żarski 1, Piotr Gomułka 2, Krzysztof Kupren 1, Katarzyna Palińska-Żarska 1, Joanna Nowosad 1 and Dariusz Kucharczyk 1 1. Department of Lake and River Fisheries, University of Warmia and Mazury, Olsztyn, Poland. 2. Department of Ichthyology, University of Warmia and Mazury, Olsztyn, Poland. *Corresponding author e- mail: s.krejszeff@wp.pl References: Gilderhus, P.A. 1990. Benzocaine as a fish anesthetic: efficacy and safety for spawning-phase salmon. Progressive Fish- Culturist, 52: 189-191. Gomułka, P., Antychowicz, J. 1999: Anaesthetics used in fish. Medycyna Weterynaryjna, 55: 89-93. Kamiński, R.; Myszkowski, L.; Wolnicki J. 1999. Secrets of 2-phenoxyethanol. I. Highest safe concentrations in juvenile cyprinids. Komunikaty Rybackie, 5: 11-13. Kamiński, R.; Myszkowski, L.; Wolnicki, J. 2000. Secrets of 2-phenoxyethanol. III. Safety and efficacy of anaesthesia in juvenile stages of selected cyprinids. Komunikaty Rybackie, 1: 8-10. Massee, K.C., Rust, M.B., Hardy, R.W., Stickney, R.R. 1995. The effectiveness of tricaine, quinaldine sulfate and metomidate as anesthetics for larval fish. Aquaculture, 134: 351-359. Myszkowski, L., Kamiński, R., Wolnicki, J. 2003. Response of juvenile tench Tinca tinca (L.) to the anaesthetic 2-phenoxyethanol. Journal of Applied Ichthyology, 19: 142-145. Anaesthesia is commonly used in aquaculture. It allows to conduct such manipulation like transporting, sorting, tagging, taking fish measurements, hormonal induction, eggs stripping (Gomułka and Antychowicz 1999; Myszkowski et al. 2003). In case of fish larvae, anaesthesia is the method allowing to omit mortality caused by handling during measuring of body weight and length. In vivo larvae sampling allows to increase the frequency of sampling and the number of rearing treatments and repetition. It allows also for lowering the number of specimens and reduction of the size of rearing units when experiments on larvae rearing are carried out. The need for the development of safe techniques of larvae anesthesia was triggered by above assumptions. A lot of studies focused on elaborating of sedation methods for adult and juvenile fish were published, but still data regarding anaesthesia in fish larvae are very limited (Massee et al. 1955). The aim of this study was to examine the susceptibility of ide Leuciscus idus (L.) larvae to anaesthesia. During the experimental period (from 5th to 35th day post hatch [DPH]) the efficiency of anaesthetic was tested in 10 day intervals. The tested doses of 2-phenoxyethanol were 0.2, 0.3, 0.4, 0.5, 0.6 mL L -1. The assessment of anaestesia efficiency was followed by guidelines proposed by Gilderhus (1990): the induction time of 3 min or less (criterion I), and the recovery time of 10 min or less (criterion II). The additional criterion (criterion III) was post-anesthesia survival, which was considered positively only when 100% larvae survived. Efficiency of the anaestesia was tested on 10 larvae per each concentration. For each larvae subjected to anaesthetic bath (at each solution) a time of induction (when the larvae did not exhibit reaction for ‘touch stimuli’) and time of recovery (when the larvae exhibited swimming ability) was recognized separately. Each time, a 15-min exposure of larvae for anaesthetic was applied. Obtained results indicated that the usefulness of 2-phenoxyethanol as an anaesthetic for larval ide is limited to 5 DPH. On the 5 DPH concentrations 0.4 and 0.5 mL L -1 met all three criterions. In case of older fish, it was not possible to find concentration meeting established assumptions. Similar findings was published by Kamiński et al. (1999, 2000) for 2-phenoxyethanol in ide 0.1 g of weight and Krejszeff et al. (2013) in case of efficiency of MS-22 for larval tench. However, It may be seen, that if the longer time of induction (up to 10 min between 15 and 35 DPH – see Tab. I, concentration 0.4 mL L -1 ) would be considered as the time meeting criterion I, the concentration of 0.4 mL L- 1 would be suitable for entire larval period as a proper dose (where no larvae mortality was recorded) of 2-phenoxyethanol for ide larvae. In conclusion, the guidelines used for adult fish (when criterion I and II were taken strictly) seems to be not suitable for larvae of ide during the entire larval period and should be modified. The obtained results suggest, that the criterion I should be prolonged. In the case of ide, it could be 10 min, since when the larvae where induced successfully before this duration of exposure for anaesthetic and no mortality was recorded. However, it require more detailed research to verify such hypothesis. During each day of the test, it was possible to determine the lowest concentration meeting the criterion I. It was 0.4 mL L -1 in 5th DPH and 0.5 mL L -1 in 15th, 25th and 35th DPH (Table I). The highest concentration meeting criterion II was 0.4 mL L -1 for all age groups (Table II). The concentrations of 0.5 and 0.6 mL L -1 caused larvae mortality from 15th to 35th DPH (over 80%). In the remaining concentrations no mortality was recorded. Table I. Time (s) of induction of anaesthesia (mean±SD, n=10) at tested 2-phenoxyethanol concentrations * - means the lowest concentration meeting criterion I. Age (DPH) Dose (mL L -1 ) 0.20.30.40.50.6 5> 900266±34140±16*84±659±8 15> 900 274±85134±39*74±10 25> 900 343±108137±20*91±16 35> 900 518±248119±32*63±18 Table II. Time (s) of recovery from anaesthesia (mean±SD, n=10) at tested 2-phenoxyethanol concentrations Age (DPH) Dose (mL L -1 ) 0.20.30.40.50.6 5-123±24203±21220±16270±28 15--137±20> 600 25--115±33> 600 35--100±23> 600