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). Presented at AQUA 2012 – Prague, 01-05 OCTOBER 2012 In finfish larviculture, the tank wall colour (TWC) was already reported to have significant effect on the rearing effectiveness. This was related with the behavioral response of particular species to different TWC (e.g., Martin-Robichaud and Peterson 1998, Cobcroft and Battaglene 2009) and the visibility (contrast) of the food in the tank with different TWC (e.g., Martin-Robichaud and Peterson 1998). Martin-Robichaud and Peterson (1998) also proved that TWC affected swim bladder effectiveness (SBIE) in striped bass, Morone saxatilis (Walbaum). In Eurasian perch, Perca fluviatilis L., two reports were published to dat as considering the effect of TWC on larvae rearing effectiveness: Tamazouzt et al. (2000) - reported, that the black TWC affected significantly lower survival (3%) as compared to the white TWC (13% survival rate) up to the 15 DPH Jentoft et al. (2006) - reported that black TWC had positive effect on the growth rate during first 5 weeks of rearing. Although, both of the reports indicated contrary conclusions, none of those studies investigated the effect of TWC on the SBIE. The aim of the present study was to determine the effect of white and black TWC on the SBIE in Eurasian perch. ) The histological analysis demonstrated the existence of small differences in GV location in individual stages as compared to the description of those stages provided by Żarski et al. (2011). This concerns mainly stage III, where Żarski et al. (2011) reported a GV location above half of the oocyte diameter, whereas the data presented here indicates that GV in this stage is located near half of the diameter or even before it (Fig. 2c). The GV location reported in this study is more similar to the description of individual maturation stages modified for the pikeperch, Sander lucioperca (L.) (Żarski et al., 2012a). This can be caused by the fact that oocytes of the perch have a thicker outer layer (zona radiata externa) as compared to pikeperch (Sulistyo et al. 1998; Żarski et al. 2011), which makes cytoplasm clarification less effective and determination of the GV position less precise. Wild perch females (n=60) were caught in the middle of winter and placed in 1000 L tanks with controllable photo-thermal conditions (4°C; 6L:18D). After 20 day of acclimation and temperature increment to 10°C hormonal stimulation was applied (hCG, 500 IU kg -1 ) and photoperiod was extended (10L:14D). The next day, 7 fish were euthanized (20 min exposition in MS-222, 200 mg L -1 ) and ovary samples were taken for histological analysis (representing maturation stage I; according to Żarski et al. 2011). The remaining fish were kept until they reached subsequent particular stages (II-VI) what was controlled by the categorization of cathethered oocyte samples (Fig. 1). In this way for each stage (I-VI) samples of ovaries (from 7 females each) were obtained. In stage I, GV was situated in the centre of the oocyte (diameter of oocyte 0.76 mm). Inside the oocyte a low number of small oil droplets could be seen. In stage II, GV was still located, in most cases, in the centre of the oocyte and larger oil droplets could be observed. In two subsequent stages, the differences concerned oil droplet size and GV location and formation of a large oil droplet was noticeable, around which several smaller droplets could be seen. In stage V, GV was clearly located near the edge of the oocyte and one large oil droplet was clearly visible (Fig. 1e). In stage VI, no GV was found and only one large oil droplet was always certified (Fig. 1f). The obtained results proves that the image of oocytes in the Eurasian perch, sampled by a catheter and then clarified, does not show the location of GV in the oocyte precisely. This confirms the important role of oil droplet morphology in the assessment of the maturation stage of the Eurasian perch females for the purposes of controlled spawning as described by Żarski et al.(2011). References: Sulistyo, I., Rinchard, J., Fontaine, P., Gardeur, J.N., Capdeville, B., Kestemont, P., 1998. Reproductive cycle and plasma levels of sex steroids in female Eurasian perch Perca fluviatilis. Aquat. Living Resour. 11, 101–110. Żarski, D., Bokor, Z., Kotrik, L., Urbanyi, B., Horvath, A., Targońska, K., Krejszeff S., Palińska, K., Kucharczyk, D., 2011. A new classification of a pre-ovulatory oocyte maturation stage suitable for the synchronization of ovulation in controlled reproduction of Eurasian perch, Perca fluviatilis L. Rep. Biol. 11, 194–209. Żarski, D., Kucharczyk, D., Targońska, K., Palińska, K., Kupren, K., Fontaine, P., Kestemont, P. 2012a. A new classification of pre-ovulatory oocyte maturation stages in pikeperch, Sander lucioperca (L.), and its application during artificial reproduction. Aquacult. Res. 43, 713–721. Żarski, D., Krejszeff, S., Horvath, A., Bokor A., Palińska K., Szentes, K., Łuczyńska, J., Targońska K., Kupren, K., Urbanyi, B., Kucharczyk, D. 2012b. Dynamics of composition and morphology in oocytes of Eurasian perch, Perca fluviatilis L., during induced spawning. Aquaculture (in press).