| Popis: |
The growth rate of Silver carp (Hypophthalmichthys molitrix) artificially reproduced in the Deroua Fisheries Station (Morocco) since 1990 varies according to the temperature (seasons) and rearing modes (monoculture or polyculture). The condition coefficient, always greater than 1, shows that this species is well adapted to its habitat, which has offered the optimum physicochemical and biological conditions for its growth. The growth parameters (weight and length) of the silver carp in monoculture monitored from January 2013 to March 2014 show values greater than those found in polyculture (silver Carp + Grass Carp + Common Carp) or (silver Carp + Grass Carp) with a slowing of growth in Spring 2013. Its growth rate remains clearly lower than that of the grass carp and the common carp in spite of the some improvement in the growth rate observed beyond July. This is due, on the one hand, to the lack of adequate food for the silver carp, mainly phytoplankton, which shows very low densities, due the excessive development of Chara vulgaris and Ceratophyllum submersum. These macrophytes extract the mineral elements (nitrate and phosphate) necessary for phytoplankton development. Thus, we recommend that in a similar case and in a semi-arid climate, the provision of additional food to silver carp is desirable. |
