Effect of fish stocking density and water flow on the quality of the water and on the succession of plankton in fishponds

Author(s)

Márcia Noélia Eler

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD)
• Defense date: 2000-04-28

Examining board members

Evaldo Luiz Gaeta Espíndola; Takako Matsumura Tundisi; Arnola Cecilia Rietzler; José Augusto Senhorini; Lúcia Helena Sipaúba Tavares

• Advisor: Evaldo Luiz Gaeta Espíndola

Abstract

An experiment was conducted at the Tropical Fish Research Center (CEPTA, Pirassununga, SP, Brazil) to analyze the influence of the density of fish stocks and the differentiated water inflow and outflow in fishponds on the limnological characteristics of the water, its structure (composition and density), biomass fluctuations, succession and diversity of the phytoplankton and the composition and density of the zooplankton (considered only by group), in order to evaluate the balance of the entry and exit of biomass in fishponds, with emphasis on the concentration of nutrients, planktonic biomass (chlorophyll) and suspended matter. The experiment also aimed to quantify the impact of fishculture on hydric resources, demonstrating its positive and negative effects on the environment. The study was carried out for 60 days, with daily samples taken initially, and at two days\' interval after the introduction of the fish. The characteristics analyzed were water temperature, water load, dissolved oxygen, pH, conductivity, nutrients, suspended matter, chlorophyll a composition, density, diversity, abundance of phytoplankton, density and abundance of zooplankton. The results obtained demonstrated the effect of the water flow and the density of fish stocks on the physical and chemical characteristics of the water. An accentuated reduction was observed in dissolved oxygen concentrations (under 2 mg/l) and in increased phosphorous, nitrogen, organic matter and chlorophyll concentrations. The increased concentration of nutrients, mainly phosphorous, caused increased levels of phytoplankton and zooplankton biomass and density. The depletion of dissolved oxygen in the water, coupled to other variables, led to fish mortality under the T2 treatment. The Chlorophyta division predominated in the phytoplankton under the different treatments and the succession observed was a Desmidiaceae&#8594Chlorococcales&#8594Cyanophyta pattern. As for zooplancton, Rotifera predominated in the inflow phase and was later substituted by Cladocera and Copepoda. The indirect interaction of the cascade effect was observed during the different treatments. The fish culture system generated potentially polluting loads, with fish feed found to be responsible for the increased nutrient waste in the effluent. Phosphorus concentrations were, on average, 36 times higher than allowed by law, characterizing the effluent from the treatments as unsuitable to be cast into class 2 rivers. Based on the findings of this study and on environmental management strategies, it is recommended that the effluent be treated before the water is allowed to flow into the Class 2 recipient, the Mogi-Guassu river. (AU)