WWW.WAS.ORG • WORLD AQUACULTURE • SEPTEMBER 2015 27 With LED lighting systems, it is possible to change light intensity and light spectrum in a single light source, and hatchery managers can establish the best light intensity and spectrum for fish reared in tanks with a specific background color. This is site specific but LED lighting allows this kind of customization in environmental lighting. Environmental Light and Photoperiod Photoperiod is the duration of light exposure during a 24-hour cycle and has an important effect on the growth and survival of marine fish larvae. Larval fish often change in response to light as they develop. Survival of early-stage cod larvae raised with continuous (24-hour) light exposure was significantly greater than that of larvae reared with a 12- or 18-hour photoperiod (Puvanendran and Brown 2002). Initially larvae subjected to continuous light exposure were larger than 12- and 18-hour photoperiod but, after a certain stage of development, photoperiod did not affect growth or survival. For some species, longer photoperiod in early development stages may create more feeding opportunities, thereby increasing the likelihood that larvae begin exogenous feeding and have a greater success rate of prey capture. Young larvae are particularly fragile, with few internal resources for sustenance if they do not successfully transition to exogenous feeding. Early high success rate of prey capture readily translates to better growth, nutrition and survival. Longer photoperiods often stimulate increased motor activity and increases in muscle development and agility, although this can have a negative effect on growth. However, continuous light exposure during early larval development is not recommended for all marine fish larvae. There is a varyiable response to photoperiod length among larvae of different fish species. The first-feeding stage of rabbitfish Siganus guttatus had excellent growth under continuous light exposure (Duray and Kohno 1988). The larvae of black porgy Mylio macrocephalus had the best survival when reared with a 13-hour photoperiod (Kiyono and Hirano 1981). Growth of European seabass Dicentrarchus labrax larvae was greatest with an 18hour photoperiod (Barahona-Fernandes 1979). For black porgy and European seabass, larvae exposed to continuous light did not perform as well as larvae reared under photoperiod regimes that were more reflective of natural habitats. Additionally there seems to be an intra-species component to the effect of photoperiod on growth of marine larvae, similar to findings on intra-species response to light intensity, that must also be considered in establishing light exposure regimes for larval fish. Under the same photoperiod regime, cod larvae from the southern range of the species (Scotian shelf region) have a lower growth rate than larvae from the more northern range (Arctic areas) (Suthers and Sundby 1996). Northern cod populations have a much longer summer photoperiod in their natural habitat and, therefore, are more active for longer periods of time and had a greater prey capture success rate. LED lighting systems with programmable controllers allow aquaculturists to specify photoperiod and light intensity levels within lighting blocks. It is no longer necessary for photoperiod exposure to be a matter of simple lights-on/lights-off control. Effects of Light Spectrum Light spectrum (color) is one of the most important but most neglected components of a lighting treatment. Current research on the effects of light spectrum on performance of marine larvae is fragmented and difficult to interpret because light treatments are not well described with respect to the lighting source used and the method and type of meter used to measure the light spectrum. At present, there is no standardized method for describing light treatments. Physical measurements of light spectrum are made at different places in the rearing environment (i.e. at the air/water interface, at certain water depths or at the tank bottom) and with light measurement tools with highly varying sensitivity because of spectral filters used. Thus, reports on benefits of certain lighting conditions are not easily replicated and overall prescriptive statements are severely limited in their application. Nevertheless, examination of past research provides insights into the effect of light spectrum on the success and performance of marine larvae. In fish, the light spectrum is detected by the eye and pineal gland (Levin and McNicol 1982, Ekström and Meissl 1997). Light spectrum is important at a very early age of development. Similar to light intensity, light spectrum may be important for successful prey capture, enabling larval fish to better “see” the prey through contrast with the surrounding environment. This is particularly important during the earliest stages of larval growth, when visual acuity is still developing. Because water is a natural filter of light, the light spectrum received by larvae changes with depth. Larval position in the water column can mean different spectrum experience and different contrast abilities, even within a single rearing unit. The influence of colored light on growth rate of larval Crucian carp Carassius carassius, rotans Perccottus glenii and guppies Poecilia reticulata was evaluated by Ruchin (2004). All larvae were from a single body of water but the fish occupy different ecological niches and have different feeding behaviors and prey items. The light intensity at the water surface was the same for each species, but the color of light in individual niches was quite different. In ponds, guppies live in the upper layers, rotans live in the middle layers and carp live in the benthic layers. There are differences in response to light in different zones experienced by different species of fish. Using a control lamp, filters were used to subject the fish to different peak color wavelengths. All species performed best with exposure to blue and green wavelengths. Red light had a pronounced negative effect on growth: a 10 percent decrease in guppies, a 9 percent decrease in rotan and a 33 percent decrease in carp. Yellow light had a severe negative impact (21 percent growth reduction) on rotan larvae, but was not as significant with guppies or carp. Guppy performed best with blue light, rotan with blue and green light and carp with green light. Carp mainly feed on benthic organisms and rotan, which lives among thickets of vegetation, feed on zooplankton. Therefore, differences in growth rates of the two fish species may be related directly to the different degrees of contrast between background and prey present in the different microhabitats. The study clearly indicates the importance of light spectrum as an ecological parameter that can significantly affect larval culture performance. Unlike traditional light sources, LED lights can be built to (CONTINUED ON PAGE 28)
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