Poster: Estimation of the daily food intake of milkfish in brackishwater ponds

An estimation of the daily food intake of milkfish (Chanos chanos Forsskål) in semi-intensively managed commercial brackishwater ponds in the Philippines

Christian Lückstädt¹, Ulfert Focken¹, Relicardo Coloso², Klaus Becker¹

¹Dept. of Animal Nutrition and Aquaculture
Institute for Animal Production
in the Tropics and Subtropics
University of Hohenheim (480)
70593 Stuttgart, GERMANY

²Southeast Asian Fisheries Development Center (SEAFDEC)
Aquaculture Department
Tigbauan Main Station
5021 Tigbauan, Iloilo, PHILIPPINES

Introduction
Most milkfish production in the Philippines is carried out semi-intensively in small fish ponds of 1 to 5 ha. Earlier investigations reported feeding rates of 4% of the fish body mass per day as favourable for milkfish growth in brackishwater ponds. 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 fish (Kühlmann et al., 1995). This study aimed to find out whether this holds true for commercial milkfish culture as well.

Materials & Methods
A commercial 1 ha pond was monitored in Dumangas, Iloilo in October 1996 during the wet season. A pre-sampling was done to get first information on the pond environment and on morphometric data of the fish. One week later, samples of up to 5 fish were caught with a cast net every 5 hours over a period of 5 days, thus covering all hours of the day. Fish were measured, weighed and the stomachs removed. The daily food intake was estimated by microscopic and gravimetric analyses of stomach contents and application of the MAXIMS software programm.

Results
During pre-sampling, fish had a mean body weight of 316g. At the main sampling one week later, a total of 82 fish with a mean body weight of 335g were caught. Based on the supplementation rate of the fish (3.75% BME/day) between both samplings 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. Analyses with the MAXIMS model showed that the actual diet intake was only 0.82% of fish body mass. Based on this the FCR of the commercial diet was 1.45. Feed intake occured between 11:00 am and 2:45 pm and between 4:45 pm and 6:00 pm. Detritus was a major fraction of gut content, but natural food from plankton formed only a small part of the total intake (Fig. 1). Data also showed that fish were not ingesting feed at the time of first feeding at 9:00 am, perhaps because of low D.O. levels (0.8 mg/l) in the morning. Suspended feed residues made up 33% of the fraction > 50 µm in the water column (Fig. 2).

Fig. 1 [5.3 kB]
Fig. 1.: Stomach contents of milkfishes during the day, Dumangas, October 1996

Fig. 2 [4.8 kB]
Fig. 2.: Plankton sample with 50 µm net, Dumangas, October 1996

Discussion & Conclusions
These results can be used to optimize supplemental feeding of milkfish in semi-intensive ponds. The FCR of 6.8 based on 100% feed consumption is rather high for a commercial diet, suggesting that a considerable part of supplemental feed given was not ingested directly. Regular application of manure may maintain the natural food in the pond and can further reduce the supplementation rate with expensive commercial feeds. Lastly, feeds should not be given at times when D.O. levels are low, especially in the early morning, because this prevents normal feed uptake by the fish.

References
Kühlmann, K., Coloso, R. M., Focken, U. & Becker, K. (1995): Contribution of natural food and supplemental feeds to the growth of milkfish (Chanos chanos Forsskål) in semi-intensively managed brackishwater ponds. Fourth Asian Fisheries Forum: p. 93.

Acknowledgement: The first author was partly funded by a scholarship from the German Academic Exchange Service (DAAD)