Environmental requirements

Environmental requirements have not been fully defined for prawn broodstock. More recent studies provide additional guidance for their maintenance and insurance of good embryo development and hatching.

It can be assumed that temperature plays a major role in the timing and intensity of spawning, while salinity is more important for larval production. Adult prawns are tolerant of a wide temperature range (18-34° C), but temperatures ranging from 27 to 32°C are believed to be optimal. In wild populations of M. rosenbergii from Lake Kolleru (India), peak spawning occurred when the water temperatures were between 29.0 and 30.5°C (Rao 1991).

Tidwell et al. (2005) recommended holding temperatures of 26 to 28°C based on an unpublished study they conducted. When stocking berried females and OC claw males collected from temperate ponds at a ratio of 4:1 and maintaining at constant temperatures of 20, 26 or 32°C for 70 days, they found significantly higher survival at 26°C (91%) compared to those at 32°C (73%) and 20°C (24%). The percentage of berried females was significantly lower at 20°C (5.1%) compared to those at 26°C (48.1%) and 32°C (58%). The percentage of males reaching BC status was also lower at 20°C (7%) compared to those at 26°C (53%) and 32°C (68%). Colder temperatures reduce the number of eggs, increase the time for egg development and appear to promote fungal growth on eggs (Daniels et al. 1992). Females may drop eggs if temperatures are too cold (<25°C). Chavez Justo et al. (1991) found that the growth rates and frequency of reproductive moults were enhanced by the elevation of the water temperature to 32°C rather than 24 or 28°C, even though the highest condition factor was obtained for females maintained at 28°C. Manush et al. (2006) evaluated the influence of temperature (25, 29, 33 or 36°C) on developmental rates, morpho-metrics and survival of embryos grown in vivo on newly spawned M. rosenbergii brooders acclimated at 1°C/day from 30°C. Embryonic development was directly rated to temperature (y = 40.075x + 348.75; R2 = 0.993; where y = time from early morula hatch and x = temperature, °C). At 48 h, abnormal embryonic development was evident at 36°C and complete mortality occurred at 192 hours. Hatching occurred within 240 hours (10 days) at 33°C, 289 hours at 29°C and 313 hours at 25°C, which is earlier than reported by Ogasawara (1984). He reported hatching at 25 days at 26°C, 20 days at 28 to 28.5°C and 17 days at 32°C. Manush et al. (2006) also proposed that the increase in larval length observed at higher incubation temperatures may lead to development of dominant prawns ('shooters').

Chavez Justo et al. (1991) also suggested that a photope-riod of 12L:12D and a temperature of 32°C increased the frequency ofreproductive moults, compared to a photope-riod of 15L:9D. This is surprising since photoperiod typically does not play a major role in reproduction in tropical aquatic species.

Another important factor to broodstock holding is salinity and mineral composition. Typically, broodstock are held in freshwater or brackishwater (15p.p.t.) at 27 to 32°C. Hatchability is higher when berried females are kept in brackishwater (New 1990, 1995). However, Souza et al. (1997) found no difference in hatching rates when berried females were maintained in salinities ranging from 0 to 20p.p.t. According to New & Singholka (1985), some hatcheries allow eggs to hatch in freshwater for simplicity, while others place the females in brackishwater (5 p.p.t.). Yet others place the females directly into larval rearing tanks containing 12 p.p.t. brackishwater. Immediate transfer from zero salinity into 12 p.p.t. does not appear to cause osmotic shock in berried females. Hatcheries in Thailand find that keeping berried females in 3 to 5 p.p.t. water stimulates improved egg development (S. Suwannatous, pers. comm. to M. New 1998).

Damrongphol et al. (2001) evaluated the effect of various medium compositions compared to the control (20% artificial seawater) on survival and hatching rates on M. rosenbergii embryos and survival of newly hatched larvae cultured in vitro at 28 to 29°C. They found that ionic composition ofthe medium relative to age ofthe embryos is an important consideration. Embryos cultured starting at 0.5 day post-oviposition were more sensitive to variations in medium composition than those started at 10.5 days. For 0.5 day embryos, removal of sodium chloride (NaCl) and potassium chloride (KCl) reduced survival rates and none survived to hatching. Doubling or tripling the amounts of NaCl and KCl also lowered survival at all developmental stages. No significant effect on survival rates occurred with variation of calcium chloride. Variation in magnesium chloride plus magnesium sulphate also reduced survival significantly. For 10.5 days embryos, survival rates were less affected. Removal of NaCl drastically reduced survival but doubling or tripling it only showed significant reduction at final stage (18.5 days). Removal or addition of KCl only reduced survival at 18.5 days. Changes in calcium chloride or magnesium chloride plus magnesium sulphate had no significant effect on embryo survival. Hatching of embryos required NaCl and calcium chloride, but not KCl or magnesium chloride plus magnesium sulphate. Variation in NaCl, KCl or calcium chloride, but not magnesium chloride plus magnesium sulphate, significantly affected survival of newly hatched larvae.

Similarly, Samuel et al. (1997) studied the effects of salinity using these same minerals on in vitro embryonic development of M. malcolmsonii compared to embryos carried by brooding female in the laboratory (salinity 0.5 p.p.t., 28 ± 2°C, photoperiod 12:12). Different salinities (0.5-10 p.p.t.) were prepared by adding required amounts of NaCl. Days to hatching was reduced to 11 in those held in 0.5 p.p.t. compared to that ofthe control (14 d), but survival was reduced from 95.2 to 66.7%. No survival to hatching occurred for the other salinities and number of days in each developmental stage increased linearly as salinity increased. Considering the fact that they used 12-hour post spawning embryos and the effects of doubling or tripling NaCl on 0.5-day embryos shown by Damrongphol et al. (2001), the results of increasing salinity might be related to the actual minerals used and not salinity.

Law et al. (2002) evaluated the effects of hydrogen ion (pH) on M. rosenbergii egg (yellow stage, in vitro) hatchabil-ity in brackishwater (12 p.p.t., 30°C and pH between 5 and 10). The results indicated that the eggs are extremely sensitive to hydrogen ion concentration in brackishwater. The highest hatching occurred at pH 7.0 (92.22%) but drop to 13.33% at pH 7.5 and 5.00% at pH 6.5. No hatching occurred at pH 5, 8, 9 and 10. The authors did not mention if the eggs were acclimated to these pH values from the starting pH 7.0 of the holding water, which may have affected the results.

As with other aquatic species, excellent water quality should be maintained throughout the holding period. The tolerance of prawn broodstock to normally toxic compounds has not been completely defined, but based upon existing knowledge for other life stages, some general guidelines can be provided. It can be assumed that the levels of ammonia (especially unionised), nitrite and nitrate should be minimised, with unionised ammonia and nitrite kept at negligible levels. Information concerning the effects of heavy metals on reproduction, either lethal or sub-lethal, is also not available, but extrapolations from the available literature (Chapter 13) suggest a need to be extremely cautious. For M. rosenbergii larvae, the levels ofcadmium, copper and zinc should not exceed 0.033,0.176 and 0.268 mg/L, respectively (Liao & Hsieh 1990). The median lethal concentration for 96 hours exposure (96 h LC50) to lead nitrate (Pb(NO3)2) was estimated at 1.0 mg/L (Piyatiratitivorakul et al. 1994). The amount of organic materials, especially suspended solids, should be minimised to prevent the proliferation of bacteria, reduce biological oxygen demand, and prevent stress to the broodstock and eggs. Tanks should be kept clean by siphoning excess food and waste as often as needed.

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