8342937098

ahyaudinb.au

Spring 57 DO meter with thermistor, a I lanna HI 8424 pH and HI 8633 conduciivity meter, res-pectively.

RESULTS AND DISCUSSION

Mean water temperauire (°C),pHandDO (mg/1) during the studies were 29.7 ± 0.9,5.59 ± 0.54 and 6.42 ± 2.5, respectively. The water qua!ities mca-sured were within the tolerable rangę for catfish (Srisuwantach etai 1981). Furthermore, IowpH is important in reducing the formation of toxic un-ionizedammonia,acommon problem in intensively cultured catfish fish ponds (Srisuwantach et ai 1981). The fairly high conductivitv values (204.3 + 67.2 ms/cm) was probably due to ions leaching from the uneaten feeds. The effects of water quality parameters on catfish harvest is further discussed by Ali (1988).

Fifteen zooplankton taxa were indentified (Table 1). Bothcalanoidandcyclopoidcopepods were abundant, whereas copepod nauplii, an important larval fish food (Lemly and Dimmick 1982; Boonsom 1984) was also dominant. Eight rotiferspecieswere indentified with the dominant species being Branchionus quadńdentataand Lecane luna followed by Anureopsis Jissa, Platy i as patulus, and Tńchocercaopoliensis. The percentageofoccur-rences for copepod and rotifer were h igh (>50%). For cladocerans, Macrothńxspinosa was dominant and occurred 60% of the limes in the samples, whereas Moi na micrurawas present in a smali er pro-portion (40%).

The zooplankton community dynamicsin fish ponds, dominated primarily by copepods, rotifers, and cladocerans, have also been observed byothers (Geiger 1983; Ludwig 1989). Acloseantagonistic cyclebetween copepods and rotifersindicates pos-slblepredator-preyrelationship (Fig. /).Cladocerans abundance was also Iow throughout the culture period. Catfish frydid not predateon zoo-plankton before or after feeding with pelleted feeds. Stornach content analysis indicated that fry at the size stocked in this study, fed on commercia! pellets only, ignoring the zooplankton which were probably too smali to be encrgetically useful. However, invertebrate predatorssuch as copepods can influence the dynam ics of zooplankton communities in the absence of intensive predation by vertebrate predatorssuch as fish (Lane 1979).

Three major zooplankton peakswere observed (Fig. 2). The maximum peak (2,484 ind./l) occurred injuly, followed by the other peaks in mid-August (1.038 ind./l) and October (932 ind./l). Excepi for the declines in late August and early

September, the expansion ratio (nauplii: copt pod) for the copepod population indicated tha t h e grou p was expan di ng and sho wi ng good growtj throughout the growing season (Fig. 2). The en richedand fertile ponds enabled the fil ter f eeding rotifers and calanoid copepods as well as the preda tory cyclopoid copepods to become abundant.

Sampling intervals

Fig. 1: Temporal abundance (ind./l) of the rotifers anc microcrustacean zooplankton groups sampledfrom catfu (Clarias macrocephalus) ponds.

Sampling intervals

Fig. 2: Temporal abundance (indiv./l) of the rotifers aru microcrustacean zooplankton populations and tb expansion ratiofor the copepod populations sampledfro*

catfish (Clarias macrocephalus) ponds.

L se of nursery ponds for fry and Fmgerling* production have been succesfully developed for kissinggourami, Helostoma temminckii (Cheah etai 1985,),striped bass, Morone saxatilis (Geiger 1983) golden shiners, Nolemigonus crysoleucas (Ludwig 1989) and carps (\amas 1979). Immediate availa* bil i ty of zooplankton for first feeding is important

38

PERTANIKA VOL. 14 NO.I, 1991