Light requirements

The topic of light requirements remains controversial. The greenwater larval rearing system, which generally fell out of favour in commercial hatcheries in the last decade of the 20th century (New 1990, 1995), requires light and the phytoplankton bloom provides shade for the larvae. However, light is consciously kept to a minimum to prevent the growth ofphytoplankton in clearwater systems. In Thailand, some hatcheries use asbestos or plastic sheeting to cover open-air tanks and light seems to be of minimal importance, whilst some others use lamps over rearing tanks, even during the day. Many backyard hatcheries completely cover their hatchery tanks with black tarpaulin to prevent potentially disease-carrying spray, caused by aeration, from being wind-transferred to other tanks (New 1995). Raju & Nair (1992) described Indian hatcheries roofed with 'Silkpaulin', which allowed adequate penetration of natural light. In Taiwan, transparent roof tiles and special lamps for hatcheries were reported by Hsieh etal. (1989).

Successful rearing has been reported at light intensities varying between 250 and 6500 lux (New 2002). In Brazilian hatcheries, a natural light intensity of 2000 to 2500 lux is supplied in indoor hatcheries through the insertion of transparent sections in their roofs. Rodrigues et al. (1998) reported that when 2500 lux of artificial light was supplied, with and without sunlight incidence, the best larval survival was achieved when sunlight was present. According to New (2002), natural light is preferable but artificial light can be used to increase the intensity on cloudy days and to extend the day length. Direct exposure to sunlight, however, has long been recognised to be detrimental (Fujimura & Okamoto 1972). Some indication of the importance of spectral quality was seen in fully enclosed hatcheries in the UK in the late 1960s and early 1970s (New 1990), especially at the near-blue UV wavelength (New 1995). The use of tungsten lamps or blue-black fluorescent tubes resulted in better larval survival to metamorphosis than the use of normal fluorescent tubes (which lack the near-blue UV wavelength found in sunlight). To this day, neither the optimum intensity nor its spectral quality have been adequately studied.

Light, especially indirect sunlight, may be extremely important for freshwater prawn larval feeding. Moller (1978) reported that larvae capture food by chance encounter and New & Singholka (1985) stressed the importance of maintaining larval food organisms or particles at densities sufficient to ensure that all larvae have available food in close proximity. However, Daniels et al. (1992) reported that prawn larvae do feed by sight, as evidenced on cloudy days when feed consumption by larvae is significantly reduced. Larvae normally respond to a decrease in light intensity (e.g. cloudy weather or evening time) by dispersing from the normal 'ring' formed around the inner perimeter of a round tank. Barros & Valenti (1997) observed that while most larvae capture food by chance, some larvae swim in the direction of the food before catching it. These authors postulated that the ability of larvae to detect inert food particles suspended in the water column (or even live food) is a polymorphic characteristic, conditioned by different gene alleles. Aquacop (1977) obtained their best production when larvae were produced in tanks with dark green walls which were located near a window. Those grown in tanks in semi-darkness did not develop beyond stage V. Thus, sufficient light appears to be necessary to ensure maximum food consumption. Indirect natural light produces the best results; direct sunlight may cause mortalities by the apparent sun burning of the exoskeleton and artificial light may have an inappropriate spectral quality. Typical direct natural light intensity on a clear day in the southern USA often exceeds 30 000 lux but indirect light has a much lower intensity. Ang (1996) recommended that a light intensity of at least 6000 lux should be provided for a recirculating hatchery by means of a transparent roof. However, satisfactory results have been obtained in an indoor hatchery in Brazil with a light intensity of only about 500 lux. Daniels et al. (1992) recommended that, for cloudy days and to extend the day length during winter months, intense artificial light should be provided.

The effect of altering photoperiod on larval performance is not clear. Lin (1997) reported that, in a trial where prawn larvae were fed on Artemia nauplii from the first zoeal stage until metamorphosis, continuous light (L:D 24:0) appeared to increase food consumption and accelerate larval development. Under continuous light, ecdysis occurred at all times but was mostly between 1200 and 1800 hours. In the other three groups (L:D 4:20; 8:16; 12:12), moulting occurred principally (93%) between 2400 and 0600 hours. In a related paper, Lin et al. (1998) reported that the other environmental conditions were 15p.p.t. salinity, 28°C and a light intensity of 1500 lux throughout the experiment. Though there were no significant differences in the carbon and nitrogen contents oflarvae reared under different photoperiods, results indicated that long periods of daylight, especially continuous light, may increase the growth rate, gross growth efficiency and assimilation efficiency of prawn larvae, thus leading to enhanced production of PL.

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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