Potential impact of population increases of non-native tilapia on fish catch and plankton structure: a case study of Tangxi Reservoir in southern China.
Several non-native tilapia species, including Oreochromis mossambicus (Peters, 1852), O. niloticus (Linnaeus, 1757) and their hybrids or strains, were introduced into China as major cultured species during the 1950s to 1980s. These are now among the dominant fish species in many tropical and some subtropical reservoirs. Eutrophication is assumed to accelerate their population growth. In the present study, we reported a sharp decline in the catch of stocked bighead carp (Hypophthalmichthys nobilis (Richardson, 1845)) with an extreme increase of tilapia catch in a large reservoir in southern China. In 2006, the catch of tilapia was 150 tons, which exceeded the catch of stocked bighead carp, and reached 500 tons in 2015. Long-term meteorological data over the past two decades did not show any significant change, especially in minimum air and water temperatures, which can be factors limiting tilapia growth. In contrast, water quality conditions, including total nitrogen, total phosphorus (TP), chlorophyll a, chemical oxygen demand, and trophic status, have significantly increased in the reservoir. The phytoplankton community has been mainly composed of small and filamentous species, while the total zooplankton biomass has decreased, with the community now mainly comprised of small rotifers and copepod species. The high ratio of chlorophyll a:TP indicates a weakening top-down effect of zooplankton on phytoplankton. The annual tilapia catch was significantly associated with trophic status of the reservoir, but not with meteorological and hydrological factors. We propose an ecological mechanism for increased population densities of the non-native tilapia under eutrophication, in which an increase in tilapia population size has been accompanied by a reduction in their body size, producing a stronger negative impact on zooplankton and water quality, which contributes to the turnover between bighead carp and tilapia catch. The proposed mechanism provides a useful framework for understanding the process and management of tilapia populations in tropical reservoirs.