Table 53 Criteria to determine the condition index for evaluating the larval quality of M rosenbergii Reproduced from after Tayamen Brown 1999 copyright 1999 with permission of Stirling University

Score*

Criteria to be checked f

Gut fullness

Gut lipid content

(state of hepatopancreas)

Pigmentation

(state of chromatophores)

Body coloration

Setation

Muscle to gut ratio

Abdominal muscle appearance (appearance of abnormal muscles) Melanisation (presence of black spots) Fouling organisms

Swimming behaviour (between stage Vfff to X) Photo positive response (between stage i to Vf)

Gut empty No lipid vacuoles

No colour pigments (fully contracted chromatophores) Grey or dark bluish on abdominal segments

Disfigured or damaged setae on rostrum, pereopods, telson, uropods

Gut appears wide, muscle narrower on Vf abdominal segment

Opaque/grainy

Appendages and parts of body affected

Major parts of body affected

Sluggish/circular motion, erratic movement

Negative response

Moderately full (30-60%) Very small vacuoles (f 0-40%)

Moderate chromatophores in one area

Moderate light orange on abdominal segments

Curled or kinked setae on rostrum, pereopods, telson, uropods

Gut appears narrow and slightly wider muscle on Vf abdominal segment

Slightly transparent

Very minor necrosis

Minor parts of body affected

Moderate movement with head upside down

Slow positive response

Full gut with faecal strands Relatively full (60-90%)

Well-dispersed chromatophores (pink or amber coloration) Tan/orange/red blend on abdominal segments

Straight and whole setae; no deformities on rostrum, pereopods, pleopods, telson, uropods

Gut appears narrow and muscle appears thick and wider on Vf abdominal segment

Clear/transparent smooth

No necrosis, absence of black spots

Body clean/absence of protozoans, ciliates, organisms Very active tail first, lateral motion/jump-like towards the side Fast positive response

* Score ratings: 0 = poor; f = fair; 2 = excellent.

rationed according to larval consumption; cleaning the bottom and water exchange (or circulation) should be regularly performed (see section 5.3.4); and adequate tank aeration needs to be maintained. Continuous aeration is essential; interruptions in supply occur during the daily feeding and cleaning operations, but should not be more than 30 minutes, to avoid larval stress. At least 5 mg/L of DO2 should be maintained.

Temperature, pH and salinity should be carefully monitored in flow-through systems, because a large quantity of new water is added to the rearing tanks daily. It is essential that the water added has the same characteristics as the water in the tank; small differences may damage larvae. Sudden changes in temperature and pH (even by as little as 1°C) are more detrimental than gradual variation (New & Singholka 1985). Therefore, adequate supplies of water at the same temperature should be available for the replacement of siphoning losses or water exchange. Most hatchery operators maintain a steady salinity (12-16p.p.t.) from hatching to metamorphosis. However, many hatcheries place their berried females directly into rearing tanks filled with slightly brackishwater and increase the salinity to the normal level after hatching. Others decrease salinity during the few days when metamorphosis starts to occur.

Recirculating systems provide relatively stable water parameters (Valenti & Mallasen 2002; Valenti et al. 2009). The temperature is kept constant (~30°C) by means of heaters. Salinity normally remains stable during the culture period and does not need to be managed with the exception ofwater replacement for evaporation and other losses. Dissolved oxygen is maintained constant and above 70% saturation level by the aeration system, which satisfies the oxygen demands of larvae and Artemia respiration and the nitrification process. Low oxygen concentrations may occur in the biofilter (Daniels etal. 1992) and should be corrected to avoid damaging the nitrification process.

Water temperature is a critical parameter that governs the survival and growth rate of prawn larvae, and thus the duration of each larval stage. The optimum temperature range is 28 to 31°C (Rodrigues etal. 1991; Valenti 1996). Ideally it shouldbekept at 30°C. Gomez Diaz (1987) showed that larvae hatched from eggs incubated at 25°C exhibited a wider range of tolerance to salinity and temperature than those incubated at 28°C. Soundarapandian etal. (1995) found that while temperatures in the range 27 to 33°C had no significant effect on survival or the time of emergence of the first newly metamorphosed PL, there was a significant effect on the total rearing period of each batch. In their experiment, the average rearing period (the criteria used to 'terminate' and harvest the batch were not identified) varied from 42.1 days in November to December (when the average water temperature was 26.9°C) to 35.2 days in July to August (when the average water temperature was 33.3°C). Survival rates were generally rather poor in these observations, ranging from 26.0 to 35.8%. The best postlarval production rate achieved was 35/L, when water temperatures averaged 31 ° C. Experience in Brazil shows that temperatures above 35°C can be lethal while in Thailand, problems occur above 33°C. In both locations, larvae do not grow well below 25°C and the time to attain metamorphosis is much longer. In tropical regions, water temperatures normally remain within the optimal range throughout the year. However, during colder months, when night-time air temperatures may fall, thermostatically-controlled water heaters are used to increase the water temperature to above 27°C. This equipment requires careful use to avoid accidents. Currently, the use of heaters in recirculating systems is normal.

Water pH is influenced by several processes that occur in closed systems, including nitrification and respiration by prawn larvae, Artemia and other aerobic micro-organisms. However, it has been observed that pH is almost stable during the culture period (Aquacop 1983, Mallasen & Valenti 1998a, Valenti & Mallasen 2002, Valenti et al. 2009). This is probably related to the relatively low biomass associated with larval culture and the relatively good buffering capacity of brackishwater. Generally, biofilter substrates (especially non-calcareous ones) do not buffer pH efficiently (Bower etal. 1981). Thus, periodic addition ofbuffering substances, such as sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3), may be necessary to stabilise pH. The pH affects larvae directly as well as the nitrification process, carbon dioxide concentration and ammonia/ammonium balance. Based on all of these multiple effects, Mallasen & Valenti (2005) recommended maintaining pH between 7.5 and 8.0 for M. rosenbergii hatcheries.

Inorganic nitrogen compounds (ammonia, nitrite and nitrate) are the most important water quality parameters in recirculating systems. Monitoring of these compounds indicates biofilter condition and system efficiency. Build-up of ammonia or nitrite indicates that one or both groups of nitrifying bacteria responsible for their conversion to nitrate are present in insufficient numbers, or water quality conditions are not suitable for their growth. The presence of ammonia but little nitrite usually indicates that bacteria represented by the genera Nitrosomonas and Nitrosococcus are absent, while the presence of nitrite but no ammonia indicates the absence of bacteria represented by the genera Nitrobacter, Nitrospira and Nitrococcus. Either condition can be alleviated by addition of more pre-conditioned media. Ammonia and nitrites can cause severe mortality in M. rosenbergii larvae. Sub-lethal concentrations ofthesetwo compounds can cause cessation in feeding, retardation of growth or increased susceptibility to parasites and diseases in prawn larvae (Armstrong etal. 1976, 1978).

In water, ammonia occurs in ionised (NH4) and unionised (NH3) forms. Un-ionised ammonia increases with increasing temperature and pH (Spotte 1979) and has a higher toxicity. High levels of ammonia in the water inhibit larval excretion (Regnault 1987) and respiration (Mallasen & Valenti 2005). M. rosenbergii larvae can tolerate high levels of total ammonia nitrogen (8 mg/L); toxicity depends mainly on the level of un-ionized ammonia (Mallasen & Valenti 2005). No deleterious effect was observed for levels ofNH3-N below 0.5 mg/L; this is therefore a safe boundary level. Further studies need to be performed to determine the upper un-ionized ammonia limit of tolerance for M. rosenbergii.

Mallasen & Valenti (2006) suggest that there are individual differences in tolerance to nitrite (NO2-) in M. rosenbergii larvae. Therefore, the resistance to ambient nitrite may be a polymorphic phenotype produced by different allele genes. Despite this, these authors did not find any difference in larval development submitted to nitrite concentrations of up to 2 mg/L. This experimentally determined value indicates a safe concentration for M. rosenbergii hatcheries that is much higher than the levels suggested in previous literature, which ranged from 0.1 to 1.0 mg/L (Wickins 1982: Valenti 1985; New 1990; Valenti et al 1998; Wickins & Lee 2002).

Nitrate (NO3-) is a stable compound that accumulates progressively in closed recirculation systems. Fortunately, nitrate presents very low toxicity for M. rosenbergii. Nitrate nitrogen has no effect on larvae at levels up to 180 mg/L and values as high as 600 mg/L are tolerated (Mallasen et al. 2003). Valenti & Mallasen (2002) established a mathematical equation to describe nitrate changes during consecutive cultures using the same water (Fig. 5.9). According to this equation, 180 mg/L NO3--N would not be exceeded until the 24th culture cycle, when re-using the same water.

Time of utilisation (days)

Fig. 5.9 Relationship between nitrate concentration (N) and time (T). (Source: Valenti & Mallasen 2002, copyright 2002 with permission of Universidade Estadual de Maringa.)

Time of utilisation (days)

Fig. 5.9 Relationship between nitrate concentration (N) and time (T). (Source: Valenti & Mallasen 2002, copyright 2002 with permission of Universidade Estadual de Maringa.)

Therefore, nitrate accumulation does not limit the re-use of water.

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.

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