Collection of broodstock 411 Source

Female prawns carrying eggs attached to the abdomen are commonly termed berried or ovigerous females. In practice, the most common source of broodstock for commercial hatcheries is adult prawns obtained from grow-out ponds. It is also prevalent for farmers to obtain a number of wild broodstock from rivers and lakes every season. Phuong et al. (2006) reported that 52% of hatcheries in Vietnam used wild broodstock while 48% used farmed broodstock. Broodstock normally includes only berried females; however, under extended holding conditions, adult males may also be collected. Under tropical conditions, berried females are available year-round in farm ponds containing adult stock and, therefore, no shortage should be expected. However, even in tropical areas the percentage ofberried females may vary during an annual cycle. For example, Rao (1991) reported that in Lake Kolleru, southeastern India, berried females could be found year-round but peak activity occurred during the warmer months of August to October. New (1990) stated that peak mating activity in the wild occurs at the onset of the rainy season. Varghese et al. (1992) reported that natural spawning in the capture fisheries of Kerala, southwestern India, was associated with the onset of the rainy season. Interestingly, in Thailand, gravid females are most readily observed in estuarine waters (5-10p.p.t.)

from November to January (S. Suwannatous, pers. comm. to M. New 1998), which is the coolest time of the year and is the dry season. When breeding activity varies seasonally, some difficulty in obtaining sufficient numbers of berried females may be experienced, especially in sub-tropical and temperate areas during wintertime. In temperate climates, broodstock are typically obtained during autumn harvests and maintained indoors in special temperature-controlled environments during winter ('overwintering'). One of the advantages ofobtaining berried females from ponds, rather than open waters, is that they suffer less damage (including less loss of eggs) during harvesting and transport and are typically of a known age in single crop systems.

If berried females are not readily available from ponds, successful breeding easily takes place under controlled conditions, but it is not common practice for hatcheries to maintain broodstock. Most often the maintenance of captive stocks is restricted to experimental purposes or to those hatcheries located in areas where M. rosenbergii has been recently introduced and berried females are not yet locally available from ponds. Cavalli et al. (2001a) evaluated the reproductive performance of M. rosenbergii females held in captivity and followed over successive moults. They found females could spawn up to 5 times during 180 days compared to reports of 5 times annually for wild populations (Ling 1969; Rao 1991). They also showed that the number of eggs per female weight and hatching rates (>82%) were not significantly affected by spawning order. However, Das & Siddique (2007) followed ten individually held females (28°C, 12p.p.t. and natural photoperiod - India, June-July) and found that while fertilisation rate was almost uniform over consecutive spawnings, hatching rate gradually declined.

In the Indo-West Pacific region, where M. rosenbergii is naturally distributed, some hatcheries prefer to use berried females from natural waters based on the belief that wild females produce better quality larvae than pond reared ones. A seasonal decrease in the availability of wild broodstock associated with the onset of the rainy season should be expected. Moreover, the natural catch has been reduced drastically as a result of overfishing and the deterioration of habitat and spawning grounds (Tonguthai 1992).

Some concerns have been expressed about quality of broodstock from certain countries. Bart & Yen (2003,2007) evaluated larval performance between Thai (domesticated) and Vietnamese (wild) stocks and found no differences in first appearance ofpostlarvae (PL) (22 days), brood size and fecundity. They detected differences in days to metamorphosis (Thai, 100%, 36 days; Vietnamese, 89%, 45 days) and final survival (Thai, 33%; Vietnamese, 2%), but suggested differences might be due to inadvertent selection from domestication. Chareontawee et al. (2007) assessed the genetic diversity from five hatchery stocks and two wild populations of M. rosenbergii in Thailand where inbreeding depression was blamed for slow growth rate in farmed stocks. Through the analysis of six microsatellite DNA loci, they concluded that poor performance was not due to deterioration of genetic variability and inbreeding.

4.1.2 Selection

Selection of females is typically based upon ripeness of eggs and clutch size (i.e. number of eggs) without regard to body size. Ripeness is indicated by egg colour. As eggs undergo development, their colour changes from bright orange to brown and finally to greyish-brown a few days before hatching. Females with brown to grey eggs are usually selected to bring into the hatchery. The normal practice is to collect them during partial or total harvests, but cast netting may also be acceptable. Some studies have provided fecundity estimates based on morphological parameters (Malecha 1983; Ang & Law 1991). Fecundity increases with female size but the number oflarvae producedbyfemales ofthe same size is quite variable. However, experienced hatchery technicians estimate the number of broodstock needed for producing the required number of larvae, normally reckoning that 500-1000 larvae will be produced from each gram of female wet weight (New & Singholka 1985; Cavalcanti et al. 1986; Phuong etal. 2006; Das &Siddique 2007). Cavalli etal. (2001a) estimated 1450 eggs per gram offemale weight with 86% hatching rate yielding 1250 larvae per gram based upon a holding females in a tank system over multiple spawns. However, they held females individually in closed systems. The total number of eggs per spawning event (NES) was estimated to range from 26 587 to 74 775 eggs for females weighing from 20 to 55.8 g. The linear relationship between NES and female somatic weight (BW; in g) was found to be

In ponds where prawns are continuously harvested year-round, the selection of females based solely on ripeness and availability does not take advantage of the biological fecundity of larger females (i.e. selection regardless of size of female) and provides no control over the breeding value (i.e. genetic worth) ofthe broodstock (Malecha 1983). This practice may select both genetically superior and inferior prawns. The alternative use of a separate brood pond or tank would allow for control of the age and size of females used for breeding.

Female size should be considered when selecting animals for reproductive purposes. In general, females become re-productively mature at a weight of between 15 and 20 g, but berried females as small as 6.5 and 8 g have been reported by Ang & Law (1991) and Ra'anan et al. (1990), respectively. When these smaller females are stunted females from ponds containing similar aged, larger 'normal' females, Paulraj et al. (2008) demonstrated that they are similar in histology to mature females by the occurrence of previtellogenic, vitellogenic and mature oocytes. However, the overall oocyte diameter and gonadal somatic index were significantly smaller. Some hatchery operators would prefer to use large females since the number of eggs produced per spawn is proportional to female size. However, though many see this as an advantage, some potentially negative impacts are associated with this procedure. Selecting broodstock as early in the production cycle as possible has been suggested to enhance the growth rate of the future progeny (Doyle et al. 1983). Taking berried females 3 months after stocking in ponds exerts an indirect selection of 28% per cycle on the weight-at-harvest, whereas the use of 6-month-old females would result in a 15% negative selection. Therefore, the selection of fast growing, younger berried females for hatchery use, rather than choosing any large female, has a positive effect on the growth potential of future broods.

The control of broodstock age is difficult where continuous stocking and harvesting of ponds is practised. New (1995) pointed out that farmers might resist such management modifications because the earliest (small) maturing females tend to produce fewer larvae and hence a greater number of berried females would be required. However, smaller females have been shown to outperform larger ones in terms of egg production over a set period of time (Cavalli et al. 2001a). Due to their higher moulting frequency, smaller females breed more often and consequently spawn more eggs. The use of younger, smaller females would also reduce the costs and time involved in rearing broodstock in ponds. Although few farms have broodstock ponds, selecting early, faster growing females and rearing them in separate ponds until they present a higher fecundity would be a simple improvement that could have a profound impact on performance. This would also allow a more efficient use of diets formulated specifically for maturing animals.

In temperate areas where broodstock are normally collected during autumn harvest (100-130 days post-stocking with 0.1-0.5 g juveniles) for overwintering indoors, both the larger males and females are typically selected from healthy populations. Here the larger females are not necessarily the fastest growing females, but rather those that have not yet spawned or recently spawned. In this case, larger females are used for overwintering because of the higher fecundity associated with size and the limited number of hatchery runs conducted. Selection of the larger females for long-term holding allows more larvae per female, especially considering the low percentage of females ready to spawn simultaneously. Selection of the larger females from a good growing pond population would provide more eggs per female and prevent selection of stunted females. The recommendations of Doyle et al. (1983) andCavalli et al. (1998; 2001a) should be considered if a hatchery will be producing numerous cycles over a spawning season. Concomitant with this selection is the selection of males of equal or larger size to ensure adequate insemination of females. While fertilisation of females by smaller blue claw (BC) males has been observed in captivity, the huge energetic effort and time required to manoeuvre the female into a reproductive position would suggest that a small-sized male would be competitively disadvantaged. On average, successful mating by males of equal or larger size takes between 1 and 4 minutes. Successful fertilisation by smaller BC males can take two to three times as long (Thomas 1998).

BC males are collected since they are the mature, re-productively active males to which mature females are attracted following postmoult. The larger BC males are usually the newest recruits into the BC population following the 'leapfrog' theory proposed by Ra'anan & Cohen (1985), as detailed in Chapter 16. Daniels (1993) demonstrated a decrease in the testicular index as the BC male ages (i.e. time post BC metamorphosis). Since the testes of BC males almost exclusively contain mature sperm (Sagi et al. 1988) and may not produce new sperm, older males might provide less viable sperm for mating. This would suggest that the reproductive capacity of the BC male decreases over time and, therefore, newer BC males would have a higher sexual capacity.

Daniels (1993) also showed that BC males normally have an intermoult cycle of 3 to 4 months. Upon moulting, they shed their claws and go through a rapid moulting and regeneration of their claws until, after 3 to 5 moults, they become BC males again. It is assumed that these males would be similar to orange claw (OC) males and exhibit high somatic and low reproductive growth capacities. Therefore, selection of new BC males would allow for 3 to 4 months of sexual activity. To ensure the continual presence of reproductively active BC males, OC males or strong OC males (i.e. those with relatively long, spiny claws referred to as 'pre-transforming strong OC males' by Sagi & Ra'anan 1988) must also be stocked. In the 'leapfrog' theory, these OC males would become BC males as they surpass the size of the existing BC males or as the older BC males die or moult out of the BC phase.

Some criteria shouldbe carefully observed when selecting prawns for hatchery use. Prawns should be active, well pigmented andhave no lack of appendages or any sort ofbodily injury or damage. Prawns covered with algae or showing signs of not having moulted recently may indicate that either culture conditions were poor or the animals are not healthy. Until recently no information associating brood-stock as a vector of diseases to larvae was available. However, Sudhakaran et al. (2007) demonstrated the vertical transmission of nodavirus (MrNV) and extra small virus (XSV) by inoculating these viruses (intramuscular injection and oral route) into M. rosenbergii brooders resulting in normal egg development but 100% mortality of larvae by PL stage. RT PCR (reverse transcriptase-polymerase chain reaction) was positive for these viruses in ovaries, fertilised eggs and all larval states. Since both viruses infect M. rosenbergii by vertical and horizontal transmission (Sahul Hameed et al. 2004), they recommended that broodstock be screened using samples from pleopods before spawning. Vijayan et al. (2007) have also reported horizontal transmission of white muscle disease (WMD) among PL but did not identify the causative agent. At a minimum, prawns should be carefully examined with regard to apparent disease infections. Prawns should also be thoroughly checked for the presence of external parasites. Berried females might be a possible source for various epibionts that may affect larval performance. Protozoa such as Zoothamnium, Vorticella and Epistylis, which are routinely found on the carapace, gill lamellae and egg masses of M. rosenbergii (Brock 1993), may eventually lead to larval mortality through the impairment of swimming, feeding and moulting.

4.1.3 Handling and transportation

For short distances, buckets may be used for transporting broodstock, but double polyethylene bags placed within insulated containers are necessary for longer journeys. It is advisable to displace all the air from the polyethylene bags and replace it with oxygen in a similar manner to that used for the transportation of PL. Once inflated, the bag should be tightly twisted, doubled over and tied off to prevent loss of oxygen and water during transit. Ice is often used to maintain low temperatures since a reduction in temperature lowers metabolic rates, increases survival, and reduces activity, oxygen consumption and nitrogenous excretion (Chen & Kou 1996). The optimal temperature range for transport seems to be around 19 to 20°C. A stable water temperature during transport is also essential for success. Schmitt & Uglow (1996) reported that sudden changes in temperature increased nitrogen efflux rates of M. rosen-bergii, clearly indicating a stressful condition. At 19 to 20°C, 12 to 15 g of prawns per litre of water can be shipped safely for 42 hours (Smith & Wannamaker 1983). The use of brackishwater (up to 8 p.p.t.) instead of freshwater did not result in any significant improvement in survival. Coyle et al. (2006) also found that the addition of salt (up to 6 p.p.t.) to the transport water had no significant effect on survival but did result in a significant decrease in both pH and unionised ammonia nitrogen. Dissolved oxygen concentrations appear to be more closely related to survival rates than any other water quality parameter.

Only prawns in the intermoult stage should be selected for shipment. They should also be fasted for at least 24 hours prior to transportation. Prawns usually have their rostrum cut or a plastic tube inserted on the tip of the rostrum to prevent damage to the polyethylene bags. The chelae are also usually tied with rubber bands or covered with tubing. Applying these measures, adult M. rosenbergii obtained from pond-reared sources in Thailand were air shipped to Belgium on several occasions (Cavalli etal. 1999; 2000a; 2001a,b; 2003). Although transportation time usually exceeded 36 hours, survival rates higher than 70% were commonplace. Twenty-litre double polyethylene bags were filled with one-third freshwater and two-thirds oxygen gas and placed within insulated containers. Ice was added to the insulated containers to maintain temperature around 20°C. A maximum of 20 g of prawns per litre of water was shipped. After arrival, prawns were gradually acclimatised to 28°C and a 12 hour light/12 hour dark photoperiod in a freshwater recirculation system. When open containers continually suppliedwith oxygen are used, Coyle etal. (2005) indicated that adult prawns (48 g) could be successfully live hauled (i.e. with little mortality) at a density of 100 g/L for up to 24 hours.

Alternatively, animals may also be wrapped in plastic or nylon screens and placed inside 50 mm diameter perforated PVC pipes, which are then placed into double polyethylene bags. However, Smith & Wannamaker (1983) found that prawns packed unrestricted suffered lower mortality than immobilised prawns. Similarly, the use of mesh material to increase surface area resulted in no benefit, which was later confirmed by Coyle et al. (2006). Clove oil was shown to have an anaesthetic effect on prawn PL and juveniles but concentrations of 750 mg/L and above were fatal while lower concentrations induced anaesthesia for less than 2 hours (Vartak & Singh 2006).

Daniels (unpublished) found that placement of adult prawns in hauling tanks (470 L per section of tank) at 30 to 60 g/L leads to crowding of prawns into tank corners, leaving much ofthe water column and tank bottom void of prawns. Although survival was good for 24 hours, prawns were obviously stressed. Therefore, a system to spread the prawns out or layer them within the water column using trays or shelving would probably be beneficial to survival and good egg quality.

An alternative form of transport is without water. Salin (2005,2007) evaluated cold anaesthetisation of farm-raised M. rosenbergii adults and berried females at 12 p.p.t. at three cooling rates (1.26°C/h over 8h, 2.52°C/h over 4h and 5.04°C/h over 2h) from25to 15° C, followed by transport at 15°C for durations of 6, 9, 12, 15 and 18 hours for adults and up to 24 hours for berried females and revitalisation of prawns in aerated water (using 12 p.p.t. for berried females) initially at 20°C and then raised to 29°C within 3 hours. For transport, prawns were packed between two layers of moist and chilled (2-3°C) sawdust (coarse sawdust of teak wood, Tectoma grandis). The transport durations that yielded 95% survival were computed at 16.47, 12.14 and 8.35 hours for adults and 18.49, 19.02 and 11.11 hours for berried females. No significant differences were found in the number of larvae produced per female, survival rate or production of PL/L between treatment (using only females from the moderate cooling rate/12 h transport treatment) and control indicating potential use for transport of broodstock.

As a preventive measure, in countries where its use is approved, berried females should be disinfected upon arrival at the hatchery by placing them into freshwater containing 0.2 to 0.5 mg/L of copper or 15 to 20 mg/L of formalin for 30 minutes, as recommended by New & Singholka (1985). Aeration should be provided during these treatments. However, while copper at the above levels is used to disinfect berried females in Brazil (Cavalcanti et al. 1986), current practice in Thailand is said to involve the use ofmuch higher levels of formalin (50-100 mg/L) for 1 hour prior to entry into the hatchery (S. Suwannatous, pers. comm. to M. New 1998).

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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