Fertilization effects on biomass and nutritional quality of <em>zooplancton</em> in feeding of fry in the Furnas (MG, Brazil) hydrobiology and pisciculture station

Abstract

This investigation aimed to evaluate the effects of organic matter (pig manure) and nutrients on the structure of zooplankton in plankton tanks at Furnas hatchery station. Zooplankton is commonly used as food resource of Hoplias lacerdae fry. Total lipid levels, density and biomass of zooplankton were analyzed in two plankton tanks twice a week during 60 days. Water temperature, pH, dissolved oxygen and electric conductivity were also monitored daily. Chlorophyll-a, total phosphorus, ammonium and total kejdahl nitrogen (TKN) were measured on a weekly basis. In the first thirty days, nutrient fertilization was performed every five days. In the last thirty days, there was no external input of nutrient or organic material. The interruption of external nutrient load caused a clear and conspicuous modification in most physical and chemical variables as well as in the zooplankton. There was an abrupt and conspicuous reduction in the conductivity, ammonium, TKN, total phosphorus and chlorophyll. On the other hand, dissolved oxygen and pH increased. Biomass of total zooplankton also showed a brief peak following the interruption of external nutrient flux. During this period, the cladocera Moina and the rotifer Brachionus calyciflorus became the dominant zooplanktons. At the end of the study, the ciclopods became once more the dominant organisms, although there was a higher proportion of immature stages. Levels of total lipids of zooplankton remained at lower levels (6-10%) throughout the covered period, suggesting that zooplankton was eating food particles of lower nutritional quality. Nevertheless, the study demonstrates that nutrient input is essential for maintaining zooplankton at adequate levels. However, lower lipid levels also suggest that the quality, quantity and frequency of this nutrient input showed be thoroughly revised.