Negus c l (2 resultados)

1. Growth rates, age structure, biomass and production were studied on populations of Anodonta anatina, A. minima, Unio pictorum and U. tumidus in the River Thames near Reading. 2. The dark lines on the shells were shown to be annual winter rings. This was established by regular examination of field and laboratory populations, and by recapture of marked specimens. 3. The growing season is from mid-April to mid-October; the dark lines are laid down during the 6 months in which there is no growth. 4. The age structure of populations of Anodonta anatina in 1963 and 1964 was compared. Recruitment is irregular; in 1964 young specimens were found in large numbers. Mortality is low during the sixth year of life, gradually increasing to the eighth, and becoming very high during the ninth year of life. 5. Artificial heating of the river by the effluent of the Early Power Station, causes earlier breeding and greater growth in the first year of life in A. anatina, and a longer growing season in the later years of life of Unio pictorum. 6. The winter rings were used to interpret the growth history of each specimen. Growth is related to the temperature during the growing season; Anodonta anatina and Unio pictorum grew best in the warm summer of 1959. 7. The best estimates of population density were obtained from grab and Surber samples. The average for a selected reach of the river was twenty-two mussels per square metre, equivalent to a standing crop of 1208 kg/ha (wet weight, excluding shells). Mark-recapture experiments yielded less reliable results. The figures for biomass and annual production for each species are Anodonta anatina 648.2 kg/ha, 132.2 kg/ha; Unio pictorum 382.1 kg/ha, 52.9 kg/ha; U. tumidus 156.1 kg/ha, 20.0 kg/ha; Anodonta minima 20.9 kg/ha (production not estimated). The production of glochidia of A. anatina during 1964 was 19.9 kg/ha. The ratio of production-standing crop for all species is 1: 6. 8. The standing crop of mussels excluding shells represents more than 90% of the biomass of the bottom fauna, and is twice that of the fish population. Only very small mussels can be eaten by fish, and the greater part of the population will be available for food for other organisms only after the mussels have died. 20 pp., 16 figs, br. gr. 8.…

150pp. 1st ed., clean in worn D/W.