Abundance and Composition of Zooplankton (Copepoda, Cladocera, Rotifera) in Lake Taal: Potential Impact of Intensive Size-Selective Predation by the Freshwater Sardine, Sardinella tawilis (Herre 1927)

Abstract

Zooplankton are considered to be important members of the lake ecosystem. The abundance and composition of which are due to several factors, for example, the impact of intensive size-selective predation by planktivorous fishes residing in the lake. In this study, the abundance and composition of zooplankton found in the vicinity of Isla Napayun - a known fishing ground of the freshwater sardine, Sardinella tawilis (Herre 1927) was analyzed using samples taken from vertical plankton tows from the 10 and 20 m depths in 2 sampling sites from the said area. These were then compared with the stomach contents of S. tawilis taken during the same period. Results show that there were 4 genera of Copepoda, 4 genera of Cladocera and 2 genera of Rotifera found in the plankton tows that were analyzed. Of these, the nauplius larvae of the copepods were noted to be the most abundant followed by the rotifer Brachionus spp. Analysis of the stomach contents of the collected S. tawilis samples revealed a zooplankton diet that was composed of 90% Copepoda with the remaining 10% being made up of the Cladocera and Rotifera. These results indicate that small-bodied organisms dominate the lake. This is indicative of intensive size-selective predation by the S. tawilis on the zooplankton population of the area. This is further validated by the high preference of S. tawilis for Copepoda, which is the largest of the 3 groups (in terms of size) in the samples that were analyzed in spite of the fact that small-bodied zooplankton dominate the zooplankton community during the entire sampling period. The zooplankton found in this particular area of Lake Taal has been observed to be highly influenced by intensive size selective preference of planktivores such as the S. tawilis for bigger zooplankton.