Feeding strategy

One of the most important aspects of larval culture is the feeding strategy. Larvae should neither be overfed nor underfed. Overfeeding results in accumulation of ammonia and/or nitrite and a build-up of organic material, which causes a proliferation of bacteria and fouling of the water and filtration material. Underfeeding results in poor growth, weak animals, cannibalism and an extended larval culture cycle. In the vast majority of hatcheries, larvae are fed newly hatched nauplii of Artemia and a freshly prepared ration (moist diet). However, larval nutritional requirements are poorly understood and diets are derived from practical observations; this topic is discussed in Chapter 6.

The establishment of feeding regimes is important in maximising feeding efficiency. It involves the choice of the type of feed and feeding schedules. Feeds include live organisms, moist inert diet freshly prepared (~20% dry matter) and dry inert diet (~90% dry matter). The prey size, mobility and nutrient contents are the main characteristics of live food. Rotifers, cladocera and nematodes have been evaluated for use as live feed (Lovett & Felder 1988, Alam et al. 1993a,b; Alam et al. 1995, Silva & Rodrigues 1997, Thomaz et al. 2004). However, no effective 100% substitute for Artemia nauplii has yet been found (Chapter 6). The major characteristics of inert diets are particle size, consistency, texture, colour, density and essential nutrients. Particle sizes in the 250 to 1190 |im range have been found to be ingested by M. rosenbergii larvae without preference in all larval stages (Barros & Valenti 2003a). This suggests that the common practice of providing inert diets of increasing particle size as larvae grow (Corbin et al. 1983; Valenti et al. 1998) may not be necessary. No information was found about the other inert diet characteristics and this is certainly an important avenue for new research. Feeding schedules comprise a timetable containing the type, frequency and quantity of feed supplied during the culture period. Aquacop (1983), Daniels et al. (1992) and Valenti et al. (1998) have described feeding schedules and management in closed systems. These authors diverge on several aspects, but they agree on the use of Artemia supplemented with an inert diet, and on the quantity of feed being adjusted based upon consumption.

The timing of feeding and the procedures used vary among hatcheries. Aquacop (1983) and Valenti etal. (1998) recommended feeding Artemia nauplii only once daily in the late afternoon or evening for nocturnal consumption. Carvalho &Mathias (1998) recommended feeding Artemia in the morning and afternoon but only during high-density pre-stocking of larvae. Daniels etal. (1992) proposed that the quantity of Artemia supplied should be divided during the day to maximise the nutritional value of nauplii and to avoid waste. These authors made food available to larvae in small quantities from daybreak until sunset, by which time guts were normally full. According to New (2002), from day 2to4, Artemia nauplii should be fed 5 times per day, with the last and main ration supplied in the evening. Valenti et al. (1998) and Carvalho &Mathias (1998) recommended two or three daily inert diet meals. Aquacop (1983) suggested turning off the recirculation system during feeding to avoid Artemia leaving the tank. On the other hand, Valenti et al. (1998) and Daniels et al. (1992) recommended the use of small mesh screens (90-150 |im) to avoid feed losses to the biofilter.

The larvae of M. rosenbergii (Plate 1, facing p. 254) do not feed during stage I (Ling 1969). Therefore, Aquacop (1983) recommended commencement of feeding the day after stocking, when larvae reach stage II. Despite these observations, Barros &Valenti (1997), Valenti etal. (1998) and New (2002) recommended feeding Artemia nauplii on the first day because many larvae moult to stage II during that period. New & Singholka (1985) recommended that inert feeds (egg custard diets) could be introduced on the third rearing day in flow-through prawn hatcheries in increasing weaning quantities. By the fifth larval rearing day, the prepared feeds can be spread over four to five feedings per day (in daylight), while Artemia are fed once per day in the evening. Aquacop (1983) suggested two daily meals of an egg custard diet should be provided in a recirculating system given at 0800 and 1200 hours, beginning on day 10. Daniels et al. (1992) suggested feeding a prepared feed twice daily after larvae have been fed to satiation with Artemia, starting on day 10. In Thai hatcheries, live food only is provided several times per day for the first 5 to 7 days of the larval cycle. From then onwards, feeding of live food is carried out once per day in the evening andmoist inert diet (egg-custard based) is introduced three to four times per day. In Brazilian commercial hatcheries, moist inert food is generally supplied from stage V onwards at 0700, 1000 and 1300 hours. Some Brazilian hatchery operators provide one application of inert food per day to larvae at stage IV in order to wean the larvae onto the main type of food used from stage V onwards. Soundarapandian et al. (1995) began using an inert diet from stage III onwards. In China, Artemia nauplii are fed from the third day after hatching onwards, while boiled fish flesh or egg is fed from approximately day 10 onwards (Miao Weimin, pers. comm. 1999). On the other hand, Barros & Valenti (2003a) indicated that less than 50% of M. rosenbergii larvae ingest inert diet until stage VI, while almost all ingest Artemia nauplii; a dramatic increase in the acceptance of moist inert diet occurs at stage VII; from stage IX onwards Artemia nauplii, moist and dry inert diets are ingested at the same rate. Based on these results, the authors suggest a feeding schedule in which only Artemia nauplii is supplied up to stage VI; from stages VII to VIII the ration should be supplemented with moist inert diet; and from stage IX onwards the supplemental diet may be a moist or a dry diet, depending on the cost and availability.

Definition of the appropriate amount of food is important, since feeding represents a significant cost associated with postlarval production. In addition, overfeeding increases the labour costs associated with tank cleaning, screen washing, and water quality and larval health management. However, an excessively rigorous control of feed management might increase labour costs. Aquacop (1983) stated that 5 to 50 nauplii/larva should be supplied daily, according to larval development. Daniels etal. (1992) agreed but found that actual consumption may attain 10 to 100 nauplii/larva/day. Soundarapandian etal. (1995) used Artemia nauplii at 5/ml of larval rearing water from stage I to VIII and 10/ml from stage IX to PL in experiments in India. New & Singholka (1985) and Valenti et al. (1998) recommended that feeding rate should be based on tank volume, not on the number of prawn larvae present and suggested that 5 to 15 nauplii/ml/day (depending on larval stage) were required. Barros & Valenti (2003b) observed four levels of Artemia nauplii consumption during larval development. The first includes stages II, III and IV with average consumption of 40 nauplii/day. The second includes stages V and VI, with a consumption of approximately 55 nauplii/day; the third includes stages VII and VIII, with a consumption of 80 to 100 nauplii/day. The fourth includes stages IX to XI, in which the maximum consumption may exceed the load capacity of the tank and a supplementary diet is essential. Accordingly, the amount of feed supplied in commercial hatcheries generally increases progressively during the hatchery cycle.

Barros & Valenti (2003b) also demonstrated that for stages II to VIII, Artemia nauplii ingestion increases with increasing nauplii density until it stabilises at a maximum value; however, for stages IX to XI, no maximum value was found, probably because the size of the nauplii is too small to be captured by M. rosenbergii larvae and a supplementary diet is essential. The daily amount of inert diet supplied varied between 0.07 and 0.20 mg (dry weight) per larva in work by Aquacop (1983). This amount provides good guidance but may be adjusted so that there is approximately one food particle per larva (Daniels etal. 1992; Valenti et al. 1998). This can be estimated using visual observations when the feed is evenly distributed on the water surface. Daniels et al. (1992) recommended determining the appropriate amount of food to be fed by weighing and measuring it volumetrically prior to feeding and then comparing it to the amount of excess feed siphoned during tank cleaning. Little or no food should remain, otherwise feed should be reduced. Aquacop (1983) and Valenti etal. (1998)

presented empirical tables, which function as a guide to establishing a feeding programme for larvae in closed systems. However, both papers highlighted the fact that the food quantity consumed by the larvae varies considerably according to culture conditions. Close observation of the larval stage, feeding behaviour, gut content and the leftover nauplii and inert diet in the tank is necessary to determine the amount of feed supplied. Sometimes larvae take the food and then discard it; this indicates a problem, such as poor water quality, diseased prawns, or unsuitable food particle size, texture or composition.

Due to high costs and fluctuations in quality of Artemia cysts, Nair et al. (2007) tried to replace it by Cyclop-eeze®, a commercial larval feed that was claimed to contain the highest known level of astaxanthin and omega-3 polyunsaturated fatty acids. The results indicated that Cyclop-eeze®® couldeconomically replace up to 50% of the Artemia nauplii and could improve the survival and carotenoid composition ofthe M. rosenbergii larvae. On the other hand, Thomaz et al. (2004) substituted the rotifer Brachionus plicatilis for Artemia nauplii and observed that when the larvae were fed with 100% rotifers they died by the 14th day. However, they concluded that it was possible to replace 40 or 60% of Artemia nauplii.

How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

Get My Free Ebook

Post a comment