An estimation of the daily food intake of milkfish (Chanos chanos Forsskal) in a semi-intensively managed commercial brackishwater pond in the Philippines.

Lückstädt, C.; Focken, U.; Coloso, R.M.; Becker, K.

Department of Aquaculture Systems and Animal Nutrition, Institute for Animal Production in the Tropics and Subtropics (480b), University of Hohenheim, 70599 Stuttgart,Germany

Most milkfish production in the Philippines is carried out in semi-intensive pond systems in small fish ponds of 1 to 5 ha. Earlier investigations reported feeding rates with 4% of the fish body mass per day as favourable for milkfish growth. However, recent investigations on the intake of supplemental feeds and natural food by the milkfish in small experimental ponds showed, that only a part of these supplemental feeds was taken in directly by the fishes (KÜHLMANN et al. 1995, 4th Asian Fisheries Forum). This study aimed to find out whether this holds true for commercial milkfish ponds as well. The daily diet intake was estimated with microscopic and gravimetric analyses of stomach contents and calculation with the MAXIMS software program.

To achieve this aim, a pond monitoring was set up in Dumangas, Iloilo, Philippines in October 1996 during the wet season. The pond was 1 ha and at least 70 cm deep. Water was changed by tidal flushing and/or additional pumping. Fishes had an initial weight of 120 g in the first week of July and were stocked with a density of 4000 pcs./ha. No manure was applied. During the first 40 days the fishes fed only on lumot, a complex of filamentous green algae (Chaetomorpha sp.). Starting in August commercial feeds (crude protein content 19.9%, crude fat 7.3%, nitrogen free extract 53.5%, crude fiber 5.4% and ash 13.9%) was given in 3 equal rations daily at 9:00 am, 1:00 pm and 4:30 pm. The daily feeding rate was 3.75% of body weight. A pre-sampling was done to get first information on the pond environment and morphometric data of the fishes. Water salinity was at 20 ppt. The daily temperature ranged from 28°C to more than 32°C. Oxygen changed from less than 1 mg/l in the early morning to greater than 9 mg/l at 4:00 pm. During pre-sampling fishes had a mean body weight of 316 ± 32 g. One week later, samples of at most 5 fishes were caught with a cast net every 5 hours over a period of 5 days, thus covering all hours of the day. A total of 82 fishes were caught during the main sampling with a mean body weight of 335 ± 82 g. Based on the feeding rate of the fishes between pre-sampling and main sampling and the calculated increase of the body mass the feed conversion ratio FCR was estimated to be 6.8 if all of the feed was consumed. This factor is high for a commercial diet. A combination of the dry matter of the stomach content of every fish, microscopic analyses of the stomach content and further investigations with the MAXIMS program showed that the actual diet intake was only 0.82% of fish body mass. Based on this the FCR of the commercial feed was 1.45. Furthermore the peaks in the feed intake were between 10:57 am and 2:48 pm and between 4:42 pm and 6:00 pm. Natural food from plankton formed only a small part of the total intake and were mainly consumed in the morning and evening hours (Copepoda, Chlorophycae). Data also showed that fishes were not ingesting feed at first feeding at 9:00 am perhaps because of low D.O. levels in the morning.

These results can be used to optimize supplemental feeding of milkfish in semi-intensive ponds and prevent wasting high quality diets. Regular application of manure may maintain the natural food in the pond and can further reduce the supplementation rate with commercial feeds. Lastly, feed should not be given at times when D.O. levels are low especially in the early morning, because this prevents a normal feed uptake.

Keywords: aquaculture, feeding behaviour, milkfish