The reproductive biology of pike perch, Stizostedion lucioperca (L.), was studied in two Bulgarian reservoirs; results were compared with data from other water bodies within its natural European range. Age at first maturity, onset of spawning, and sex ratio were determined and found to be different between the populations of the two reservoirs. Fish from the cooling reservoir of a power station matured earlier at smaller sizes because of elevated temperatures; this also resulted in an increased proportion of older males. It is suggested that age at first maturity is determined by growth rate and temperature. Temperatures alone seem not to limit the distribution or onset of spawning within the European range. The reasons for a shift in sex ratio among older fish in some populations are discussed.

The high mountain lakes in the Rila Mountains (Bulgaria) were studied in the years 2000 and 2001 considering the following groups of variables: geography, geology, morphology, hydrology, water chemistry of the pelagial, biomass and size structure of bacterio- phyto- and zooplankton, and occurrence of three fish species. Multivariate analysis (RDA) revealed that the nutrient concentrations in the lakes were significantly influenced by the soil percentage coverage of the catchment area. The explained variation in plankton components (bacterio-, phyto- and zooplankton) and plankton size structure was determined by biological as well as morphometrical and geographical variables. Changes in the Mg2+ and NO3-N concentrations were traced over the period 1993- 2001; the Ca2+ concentration, pH, nutrients, bacterio-, phyto- and zooplankton were monitored in the period 1995-2001. A nutrient decrease accompanied by an increase in size of zooplankton organisms in the period 1995-2001 led to a considerable increase of water transparency in the lakes

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The population status of bleak (Alburnus alburnut (L.) in sand-pit lake Chepintsi near Sofia by using some biological factors as growth rate, condition and fecundity was investigated. The maximum age, length and body mass of bleak were 3 years, 12.7 mm and 20 g, respectively.The first, second and third age group were 5.77%, 63.46%, 30.77% respectively. The linear and body mass growth rate were described by the equations: SL=1.3487+2.6601R (r=0.88); W=0.1499L 1.8499 (r=0.95), respectively. The condition coefficient of bleak (k) was the lower than that for other populations from different water bodies. Also the populations from different water basins were compared by the body mass at the same length. Fecundity was 7 168 eggs, the average was 1 707 -12 284 eggs. The fecundity-length and fecundity-weight relationship were described by the equations: F=387.32L+4365.7 (r = 0.46); F=1368.4W-3151.9 (r = 0.59) respectively.

The annual fluctuations in the average length of the catches of A. immaculata, from the Bulgarian sector of the Danube River were studied. Between the years of 1962 and 2010 the average length varies within 23.7 cm and 26.3 cm, while in 2011, there was a significant drop in the average fish length (20.1 cm). The fecundity of the Pontic shad declines with increasing the linear and weight temp of growth. The condition factor was calculated by the coefficient of Fulton (K f ) using the total and gutted weight. The value of K f , calculated using the gutted weight of the individuals is 1. For males and females, the values of K f are respectively 1.05 and 1.07. Condition factor was also studied for each age group separately, for males and females and combined for both sexes.