Feeding ecology of silverperch, Terapon plumbeus Kner, and the impact of fish-pens in Laguna de Bay, Philippines

Aquaculture is an important factor in the fishery of Laguna de Bay in the P hilippines; fish-pens and net-cages covered approximate to 10% of the lake surface in the late 1990s. The present study was carried out to assess the possible influences of aquaculture on a wild fish species, silverperch, Ter apon plumbeus Kner, with a special emphasis on the feeding ecology of this fish. For the purposes of the investigation, 24-h samples were taken at 2-m onth intervals close to a fish-pen as well as in open water over a one-year period to acquire more information on this species. Significant difference s in standard length and total weight were found between stations and sampl ing months. In open water, a mean standard length of 53.6 mm and a mean tot al weight of 4.2 g were found, whereas close to the fish-pen, the correspon ding values were 57.6 mm and 5.4 g, respectively. The maximum mean standard length was attained around December 1996 and February 1997 (59.5 mm in ope n water; 66.1 mm close to the fish-pen), and the minimum was found in June 1996 (49.1 mm in open water; 46.2 mm close to the fishpen). Noticeable diff erences were found in the food spectrum between the two sampling stations. Zooplankton, the major food source at both stations, was more important in the stomach content of fish in open water. The same was true for insects (i .e. chironomid larvae), although these did not make up such a large fractio n of the diet. On the other hand, close to the fish-pen, aufwuchs-algae, ph ytoplankton and fish were more important. Generally, benthic organisms were consumed more frequently close to the fish-pen. Zooplankton was more impor tant in the diet of smaller fish. In all size groups, the importance of zoo plankton decreased during the rainy season.