Contribution of omnivorous tilapia to eutrophication of a shallow tropical reservoir: evidence from a fish kill.
We examined whether a large stock of tilapia (>750 kg ha-1, in littoral areas >1300 kg ha-1), mostly Oreochromis niloticus and Tilapia rendalli, could contribute to the eutrophication of a tropical reservoir in Lago Paranoá, Brasília, Brazil by enhancing P-loading. We took advantage of an extensive fish kill (>150 tonnes removed) during May-August 1997 in a hypereutrophic branch of the reservoir to compare water quality characteristics 1 year before and after this event by means of BACI statistics. We also measured P-excretion rates in laboratory trials to assess the P-loading of the reservoir by the tilapia relative to tributary inputs and loading from a sewage treatment plant. Concentrations of chlorophyll a and total P decreased significantly (from 84 to 56 µg litre-1, P=0.018, and from 100 to 66 µg litre-1, P<0.001, respectively) in the branch of the reservoir affected by the fish kill, compared with a similar but unaffected branch that served as a control. Because P-loading by both a sewage treatment plant and tributaries remained high after the incidence, the fish kill was likely to contribute to the observed water quality improvement. Removing 150 tonnes of dead tilapia corresponded to 20 days of external total phosphorus load (TP-load) to the branch, and resulted in a reduction of 5.1 kg P day-1 in internal recycling via tilapia excretion, which is equivalent to 12% of the external TP-load. Implementing professional tilapia cast-net fisheries could be an efficient biomanipulation approach to improve water quality and limit the occurrence of cyanobacteria blooms and fish kills in hypereutrophic branches of Lago Paranoá and similar tropical lakes.