Οικολογία του τσιρωνιού (Rutilus rutilus L.) στη λίμνη Βόλβη

Abstract

This study refers to the ecology of roach (Rutilus rutilus L.), at the ecosystem of lake Volvi in Greece. The physicochemical water characteristics of its biotope were studied and specifically: temperature, conductivity, clarity, dissolved oxygen, pH, total hardness, sulfuric salts, orthophosphate salts, nitrogen compounds (ammonia-nitrogen, nitrates, nitrites), biological oxygen demand (B.O.D5) and the chlorophyll-a. Totally 3,384 specimens were captured using nets The age determination was based on scale<s annual ring formation and the time of those rings formation was found to occur in April. The older roach specimen was found to be eight years old. The annual growth of roach was calculated according to length as well as weight, based upon the observed and calculated values, by the backcalculation growth method. The application of this method is based on the relationship of the fork length to the scale radius. It was found out that males overtake female<s fork length only during the first year. The maximum annual growth, which was found to be 71 mm, was observed during the first year of their lives. The final length (L@), that roach can attain in lake Volvi was calculated to be 185.60 mm for males and 277.18 mm for females. The length – weight relationship it showed that roach<s growth is allometric. The condition factor showed that the female values were higher than the male ones and the reduction value index occurs from February to May for both sexes. The study of reproduction, was carried out on mature females, according Kesteven< s index. Compared with the other ecosystems, the roach mature earlier, the male in the age of one year and female one year later. The reproduction period is short, from the end of March until early April. The mean absolute fecundity was calculated to be 8,102 eggs which was higher than other ecosystems. The mean relative fecundity was 250.75 eggs/g of body weight and was independent for length, weight and the age. Analogy of sex indicates that there was advantage of the male fish on the female at the small length classes and ages. The fast matureness of male at the small length classes probably covers the male generative of population or the other hand the greater female increase the production of eggs. The total mortality was calculated with two methods, the catch curve (male 1,231, female 1,019 and both sexes 1,202), and linearized catch curve based on length composition data (male 1,140, female 0,921 and both sexes 1,094). The natural mortality was calculated with two independently methods, Pauly< method (male 0,451, female 0,306 and both sexes 0,241) and methods dependent on the gonadosomatic index (0,293). The roach in lake Volvi according to present conditions, don’t face any population problem until 3-4 years of age The year production of roach was equal to the 2/3 biomass, the female were more productive in comparison to the male. From produced biomass , the 39% was obtained fishing by fisheries and the other lost in natural mortality. Yield per recruit gave 23.093 g /roach according to fishing condition. This means that the fishing management gives as the best yield