Ecological effects of introduced rainbow trout (Oncorhynchus mykiss) in pristine Ecuadorian high Andean lakes

High altitude waters in the Ecuadorian Andes are devoid of native fish, but rainbow trout (Oncorhynchus mykiss) have been introduced widely. We surveyed 14 small and shallow, high altitude (3800-4300 m a.s.l.) lakes in Ecuador, seven lakes without fish, and seven with introduced rainbow trout. The main purpose of this study was to explore the effects of introduced rainbow trout on biomass, abundance and composition of pelagic (phyto- and zooplankton) and benthic (algae and macroinvertebrate) communities in lakes where fish arc naturally absent. As expected based on studies from temperate zone high alpine lakes, there were considerably (although non-significant) lower mean total zooplankton biomass (13 versus 105 mu g DW L-1) and higher mean phytoplankton biomass (2.26 versus 1.56 mu g Chl-alpha L-1) in lakes with than in lakes without fish, and significant differences in the taxonomic composition of the zoo- and phytoplankton community were observed between the two groups of lakes. While the genus Daphnia dominated in almost all of the fishless lakes (88 compared to 4 mu g DW L-1, respectively), other genera of smaller-sized cladocera (or calanoid copepods) dominated in the fish lakes. Fish caught in gill nets ranged from 8.6 to 24.5 cm (fork length) and the weight was 6.0 to 199.5 g WW. Analyses of stomach content showed that all size classes had consumed considerable proportions of macrophyte material and filamentous algae. Of the consumed animal prey, benthic macroinvertebrates were by far the main food source, even for the smallest size classes (82-100 % by volume for individual fish, overall mean 99 %). In contrast, zooplankton contributed only 0-18% of consumed animal prey (overall mean 1%). In conclusion, our study showed that the introduction of an alien predator such as the rainbow trout to naturally fishless equatorial high Andean lakes had effects on the pelagic part of lake food webs, mainly on the cladoceran community.