Larval hatching systems

Larval hatching systems come in many shapes, sizes and configurations, which depend on the needs of the hatch ery. Larvae may be allowed to hatch in the tank in which they will be cultured. They may also be hatched in the broodstock holding system and then transferred to the larval culture tank. When larvae are to be hatched within the culture tank, females with brown to grey eggs are collected and placed into enclosures (nets or cages) within the tank. Once hatching has occurred, the females and enclosures are removed. Problems associated with this technique are cannibalism of larvae by females and the obstruction of water circulation by the enclosure.

A more easily controlled system utilises a separate hatching tank. Females collected from either a pond or a holding system may be placed into a hatching system where eggs are allowed to hatch. Stage I larvae are collected with either a collecting device or simply netted from the system. Females are typically stocked at high densities (0.2-0.3 females/L) in an enclosure that prevents them access to larvae once the eggs hatch. Short pieces of piping or other structures may be added to provide shelter for females. Larvae can be attracted away from the females using light to take advantage of their phototactic nature and drawn towards a water flow to move them into another collection tank. Collection is normally at night when most hatching occurs. The tank may also be covered to prevent extraneous light sources and provide for daytime collection. Each larval culture tank is then stocked with stage I larvae collected over a period of 1 to 3 days.

Daniels et al. (1992) described a typical hatching system (Fig. 4.1) consisting of a hatching tank (300 L rectangular tank), larval collector (120 L circular tank), and a mixing tank or biological filter (120 L circular tank). Forty to sixty females with brown to grey eggs are placed into the hatching tank supplied with adequate habitat structures (e.g. a piece of pipe for each individual). The interior of the hatching tank is black, except around the area where the overflow pipe is located. This area is painted with a lighter colour. Black grating (e.g. egg crating or louvre material) divides the tank into two chambers with the chamber for holding the females occupying about 80% ofthe tank volume. Water overflows from the hatching tank into the collection tank, passes through a 180 ^m mesh screen located around a centre standpipe, andthen flows into the mixingtank. Water is returned to the hatching tank from the mixing tank by airlifts.

Sandifer & Smith (1978) described a system for holding ovigerous females consisting of two 2000 L tanks (1.83 m diameter x 0.76 m deep) attached to a recirculating water system. Each tank contained artificial habitats and could accommodate up to 130 females/m2 of tank floor and provide excellent survival (~90%) for 3 week hatching periods. The system was maintained at 28 to 30°C and 12 p.p.t. using synthetic seawater. The two holding tanks were connected to a smaller larval collecting tank. The larvae were

Airlift

Broodstock tank

Airlift

Broodstock tank

Larval collector

Fig. 4.1 A hatching system consisting of a 300 L broodstock tank and two 120 L circular tanks (larval collector and biofilter). (Reproduced from Daniels et al. 1992, copyright 1992 with permission.)

allowed to hatch in the tanks and were automatically harvested by the gentle siphoning action of water from the holding tank to the collection tank where larvae were retained. Although they expressed considerable variation in production, they averaged 46 600 larvae/day.

The University Pertanian Malaysia developed a broodstock holding system (Fig. 4.2) that allows for maintenance of individual broodstock and natural water flow to remove larvae into a separate tank upon hatching (K. Ang, pers. comm. to R. Smullen 1995). It consists of a shallow raceway approximately 1m x 0.5 m (H x W) and can either be constructed as a flow-through or recirculation system. Chambers are divided by screening with a mesh size large enough to allow the larvae to pass through, but small enough (10 mm) to prevent the female moving into an adjacent compartment. The water depth should be approximately 0.5 m to allow sufficient covering ofthe adult female and provide a low flow rate. Each compartment holds an individual female and the top of the raceway is open to allow easy access to the females for feeding, maintenance and replacement following spawning. When the female spawns, the larvae are carried with the water flow through the mesh and through each compartment into the larval rearing tank. Larvae can then flow into a number of different larval rearing tanks.

Although natural incubation is by far the most common practice, hatching may also be performed by artificial (in vitro) incubation (Balasundaram & Pandian 1981; Mathavan & Murugadass 1988; De Caluwe et al. 1995; Das et al. 1996; Cavalli et al. 2001b). The use of in vitro incubation is thought to increase the number of larvae released from a single egg clutch because berried females usually lose eggs as incubation proceeds. Wickins & Beard (1974) hypothesised that losses could amount to 31% of the initial egg number. Malecha (1983) observed that females maintained in ponds had a lower fecundity than those kept in the laboratory, probably because ofthe continual sloughing off of dying eggs due to epizootic infestation. Ang & Law (1991) reported a similar observation and attributed the decrease in fecundity to the continual consumption of eggs by

Water flow in broodstock holding tank

Mesh barrier - creates an enclosure for broodstock female but allows larvae to pass through into larval rearing tank

Water flow in broodstock holding tank

Mesh barrier - creates an enclosure for broodstock female but allows larvae to pass through into larval rearing tank

Fig. 4.2 Broodstock spawning system developed by University Pertanian Malaysia, which allows for maintenance of individual broodstock and natural water flow to remove larvae into larval tank.

females and the loose nature of grey eggs, which would make them more prone to physical losses. To offset losses, eggs should be manually collected from females a few days after spawning, preferably within 3 to 5 days. The advantages of applying this methodology consist not only in the potential increase in the number of larvae produced, but in relieving females from the task of incubation, which increases their reproductive output by an increased breeding frequency, as demonstrated for M. nobilii (Pandian & Balasundaram

1982) and M. rosenbergii (Damrongphol et al. 1991). On the other hand, the risk of microbial infections increases because non-viable eggs, usually lost during in vivo incubation (Malecha 1983; Ang & Law 1991) are retained during in vitro incubation. Furthermore, antimicrobial secretions, thought to be produced by the incubating females (Fisher

1983), are not present. A successful disinfecting protocol for M. rosenbergii eggs using formaldehyde, hydrogen peroxide and a commercial antibiotic and anti-fungal solution has been developed (Caceci et al. 1997).

Hatching occurs naturally under estuarine conditions and, as noted in section 4.2.4, egg hatchability is increased in brackishwater rather than in freshwater. De Caluwe et al. (1995) found that a combination of 6p.p.t. salinity with temperatures of 26 to 28°C is optimal for the incubation of M. rosenbergii eggs. However, larvae obtained from eggs incubated at 25° C showed a higher resistance to temperature fluctuations than those from eggs incubated at 28°C (Gomez Diaz 1987). Although Brock (1993) alerted that temperatures above 30°C may prompt the development of protozoa and other undesirable micro-organisms, Manush et al. (2006) suggested that 29 to 33°C would be optimal for M. rosenbergii embryonic development, as they found that increasing incubation temperature reduced the hatching duration of embryos and produced larger larvae. The light regime does not seem to affect egg hatchability (De Caluwe et al. 1995) although direct sunlight should be avoided. Cavalli et al. (2001a) used fluorescent lamps providing 750 lux at the water surface for 12 h/day and, from a total of 54 spawns, hatching rates were consistent and above 80%.

Theexposureof1-dayembryosto 10 |ig/mLofestradiol-17p for 2 days resulted in an increase in hatching rate, in the number of primordial germ cells (PGCs) and in the rate of incorporation of the PGCs into the developing go-nads (Pakdeenarong & Damrongphol 2006a). In a similar study with all-trans retinoic acid (AtRA), Pakdeenarong & Damrongphol (2006b) reported that treating embryos with 10 or 50 |g/mL of AtRA for 2 days had no effect on survival and hatching rates, but an increase in the number of PGCs and a slightly more advanced stage of the gonads were observed. The results indicated that AtRA, an active metabolite of vitamin A, affected germ cell and gonad development of embryos of the freshwater prawn.

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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