Checking and staging larvae

Larvae should be checked daily to determine their health and developmental stage and to control feeding (Aquacop 1983; Daniels et al. 1992). Aquacop (1983) recommended observing the colour, swimming and feeding activity of larvae in the tank. They also suggested observing under the microscope the following features: eye surface, antennae, gut content, gill area, pereopods, pleopods, telson spines, uropod setae and general pigmentation. It is important to look for necrosis (opaqueness), external bacteria (blackened areas),and missing appendages during observation. Healthy larvae are active, display red-brown pigmentation and, in the absence ofaeration, remain on the water surface. On the other hand, when in poor health, they do not feed well, practise cannibalism, display blue pigmentation and swim irregularly. In the latter case, it is common to observe both dead larvae and waste food at the bottom of the tank. Tayamen & Brown (1999) developed a simple, objective method to score the condition of M. rosenbergii larvae. The method is based upon observation of larvae and a scoring system based on a condition index that employs a numerical scale devised by Apgar (1953) to assess health of human babies. Figure 5.8 and Table 5.3 give details of this assess ment technique. This method has been successfully used in the analysis of commercial hatcheries in Thailand, Malaysia and the Philippines to highlight problems in management (M. Tayamen, pers. comm. 1999). Using this method, larvae scoring high on this system have been demonstrated to show better growth and survival (M. Tayamen, pers. comm. 1999). However, in the experience on one of the authors of this chapter in the Aquaculture Centre, Sao Paulo State University, Brazil, poor survival was obtained in tanks that showed a high condition score during the whole culture period; therefore this assessment method should be used with caution.

Daniels et al. (1992) recommended staging larvae once daily until the first PL appear in the tank. A stereoscope with a range of magnification from 60 to 100 X is recommended for staging and checking guts. A microtitre plate may be used to help in examining larvae individually. Ling (1969) and Uno & Kwon (1969) developed criteria to stage M. rosenbergii larvae. The latter established that there were 11 larval stages, which are accepted by most authors (Malecha 1983; New & Singholka 1985; Valenti 1985, 1996; Daniels et al. 1992; New 2002). A rapid technique for moult staging has been described by Peebles (1977). Manzi et al. (1977) defined a larval stage index (LSI), which is a calculated weighted average ofstage determinations. An example of a stage index is: given 30% (0.3) of the larvae identified as stage 2, 40% (0.4) identified as stage 3 and 30% (0.3) identified as stage 4, then the average stage index is (0.3 x 2) + (0.4 x 3) + (0.3 x 4) = 3.

Daniels et al. (1992) also recommended checking larval gut fullness twice daily after most of the morning and afternoon ration of Artemia has been fed. Full guts will appear bright orange (i.e. presence of Artemia) and will be pressed against the heart and upper carapace. These authors indicated that if guts are not 100% full, additional Artemia should be fed and guts re-checked until they become full. Aquacop (1983) stated that food consumption is a good index to estimate larval health. If guts remain less than 100% full for several consecutive days, then a problem is assumed. This problem may be indicative of stress from poor water quality, disease or poor feeding. However, excess food consumption has been observed in Macrobrachium ama-zonicum larvae (C.R. Maciel & W.C. Valenti, pers. comm. 2008.). Very high feeding rates may not be desirable and, if the same over-consumption occurs in M. rosenbergii, full guts may not necessarily be indications of healthy larvae.

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