Population Ecology of a Pelagic Fish, Xenomelaniris Venezuelae (Atherinidae), in Lake Valencia, Venezuela
Authors: Unger Philip A. , Lewis William M. 
Published: May 2006

Trophic bottleneck, which is a common phenomenon in the pelagic food chains of large tropical lakes, was studied in Lake Valencia, a large and eutrophic lake located in Venezuela. Mechanisms of trophic bottleneck in lake Valencia were demonstrated through two years of population studies of Xenomelaniris venezuelae, a small planktivorous fish that accounts for most of the zooplankton consumption in open water. Major components of variation in abundance of Xenomelaniris included small—scale spatial (0.1—1.0 km, 70%), large—scale spatial (1—10 km, 23%), and temporal (months, seasons, 7%). The population showed a repeated annual cycle based primarily on a dominant cohort recruited during January and February. Spawning in the littoral zone continued throughout the year, but larvae survived only when rotifer densities exceeded 100 individuals/L, which occurred predictably during late seasonal mixing, but only irregularly thereafter. Steady mortality (15%/mo) of spawning fish occurred through predation. Catastrophic, predictable mortality of all fish occurred through anoxia during early seasonal mixing (57%/mo for two months). Xenomelaniris showed a strong ontogenetic shift in diet from rotifers during early development to copepods and Chaoborus during later development. Daily rations ranged from 28% of dry body mass in the smallest fish to 10% in the largest. Although Xenomelaniris was the only planktivore and was capable of consuming all major zooplankton taxa, it consumed only 3.6% of zooplankton production because it was never sufficiently abundant to exploit its food resource fully. Total growth of adult Xenomelaniris (somatic plus reproductive) conformed to the same equation as somatic growth of juveniles; growth potential was diverted increasingly to reproduction as the fish aged. Young adult fish showed 66% assimilation efficiency and 11% growth efficiency. Respiration losses decreased during development as the 0.73 power of body mass. The annual ratio of production to biomass was high (4.0), reflecting food sufficiency, a long growing season, and continuous loss of older fish to spawning mortality. Annual fresh—mass production of Xenomelaniris (31 kg/ha) is a small proportion of primary production (<0.03%) and of zooplankton production (3.6%); energy budgets show that actual Xenomelaniris production is only one third of potential production given the observed zooplankton production and the measured growth efficiency of Xenomelaniris. The explanation of this trophic bottleneck is mortality, which prevents the population from reaching carrying capacity and thus blocks energy flow between zooplankton and pelagic fish. Although there were three significant kinds of mortality, larval mortality–apparently through starvation–was the explanation of the trophic bottleneck because it restricted the reproductive output of Xenomelaniris, which could otherwise have brought the population to carrying capacity. The trophic bottleneck may be of significance beyond Lake Valencia, in view of the low diversity and abundance of pelagic planktivores in tropical lakes.