Browsing by Subject "Milkfish"

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    Seasonal variation in growth, quantitative and qualitative food consumption of milkfish, Chanos chanos (Forsskål 1775), and Nile tilapia, Oreochromis niloticus (L. 1758), in Laguna de Bay, Philippines
    (2002) Richter, Hartmut; Becker, Klaus
    Laguna de Bay, the largest lake in the Philippines, lies directly southeast of the capital Manila. The lake has a mean depth of only 2.8m and a muddy bottom which is stirred during windy weather, causing turbid conditions (Secchi depth <30cm). In the dry season, the water level drops below that of the nearby sea, leading to a backflow of saline water which clears the water until the return of the monsoon winds (Secchi depth >100cm) and leads to algal blooms. Since the beginning of the 1970s, the lake has been used for culturing milkfish, Chanos chanos (Forsskål), in large netpens (max. 2000ha) and Nile tilapia, Oreochromis niloticus (L.), in smaller cages (max. 200m2). Initially, it was possible during the algal blooms to grow fish from fingerling (ca. 10g) to marketable size (ca. 200g) in three months, making two harvests a year possible. Aquaculture quickly spread until in the middle of the eighties, over a third of the lake was covered with cages. At the same time the growth of the fish declined, which was attributed to the excessive use of primary production. Since then, despite a reduction in aquaculture coverage to the generally recommended level of 10% of the lake, fish growth has never reached the levels of the early days of culture. In the present work, seasonal variation in growth, feeding spectrum and daily ration of these two species was to be investigated in relation to water quality in order to find out more about the interaction between aquaculture and the lake. Between May 1995 and August 1997, milkfish and tilapia were sampled on several occasions at commercial operations over the 24-hour cycle. Since milkfish, unlike tilapia, do not receive supplemental feed, the proportion of their growth derived only from natural food could be determined from the cultured fish. Tilapia were kept in cages without feed specially for this purpose and measured and weighed twice a month between March and November 1997. At the same time, selective water quality parameters (particulate organic and inorganic matter, Chlorophyll-a, zooplankton) were analysed weekly. The growth of unfed fish was significantly faster between saltwater intrusion (mid-May) and the return of the monsoons (late July) than at other times of the year. On the other hand, food consumption was only slightly higher in tilapia when the water was clear than at other times and in milkfish hardly differed over the year. Both species mainly fed on amorphous organic detritus; significant levels of phytoplankton were only found in the stomachs at times of algal bloom. In supplemented tilapia, the daily ration still consisted of 35-75% natural food, suggesting that pelleted feed was used inefficiently. The main factor limiting fish growth therefore seemed to be food quality since detritus has often been shown to be poor quality food. The weekly water samples collected in 1997 demonstrated that the total level of phytoplankton was not necessarily higher at times of rapid fish growth. The main difference was related to algal size, since at times of turbid water, small diatoms dominated but these were replaced by larger blue-green algae after saltwater intrusion. In conjunction with the feeding method of these fish, the strongly seasonal growth of the fish could now be explained. Phytoplanktivorous fish such as milkfish and tilapia can only select their food on the basis of size. Since the organic detritus in the lake consisted of particles smaller than 15µm, it was possible for the fish to selectively filter larger blue-green algae but not smaller diatoms. This could also explain why fish growth rates did not recover after a reduction in aquaculture in the mid-eighties, since the relationship between the level of algae and that of detritus seems to be more important than total algal biomass. In order to increase production to those levels found in the early seventies, the level of detritus in the lake would have to be reduced, for which its origin would have to be investigated first.