26 SEPTEMBER 2015 • WORLD AQUACULTURE • WWW.WAS.ORG Increasing light intensity increased growth of European seabass, but there was an upper limit where light intensity had a negative effect (BarahonaFernandes 1979). The larvae of different fish species may have different threshold levels of environmental light intensity (both high and low) and lack of knowledge of these thresholds can affect growth and survival. Timelines on Eye Structure Development Environmental lighting needs change during larval development, further complicating the choice of optimum light intensity ranges for larval fish rearing. In marine fish larvae, the structure of the eye is different at the beginning of the larval stage than at the end (Lyall 1957, Blaxter and Staines 1970, Blaxter 1975, Hairston et al. 1982, Raymond 1985). Rods are not part of the retina of newly hatched larvae. These specialized receptors, needed for vision in low light intensity environments, develop in the retina as larvae grow. First-feeding larvae, therefore, are strictly dependent on cone vision for prey identification and capture. Cone vision requires high levels of light intensity for photostimulation to occur. Consequently a specific light intensity in the environment can affect feed intake quite differently, depending on the age/stage of larvae. As larval fish body size increases, visual acuity and its reactive distance to prey also increases (Blaxter 1986). Consequently, as larvae grow larger, they are also more successful at prey capture. In contrast, cod larvae reared at low light intensities during larval development had overall lower mortality rates than larvae held at higher light intensities (Puvanendran and Brown 2002). There may be critical junctures where a light intensity that was beneficial previously is no longer applicable or not as beneficial because of developmental changes (i.e. improved visual acuity) as the eye structure develops through larval development, and even through many juvenile stages. Varying light intensity levels for different stages of larval development is easily accomplished with LED lighting systems. Prior to First Feeding Light levels for marine fish larvae are important even before the start of first feeding. Herring Clupea harengus larvae at the sac-fry stage (prior to first-feeding) have lower activity levels when exposed to high light intensities (Batty 1987). This decrease in activity affects yolk consumption rate and the time to firstfeeding, thereby affecting the proper time for introduction of prey organisms. Light intensity prior to first-feeding in larvae subsequently affects the survival and timing of first feeding. Atlantic halibut Hippoglossus hippoglossus yolk-sac fry developed abnormally in the presence of light and high mortality subsequently occurred at first feeding (Bolla and Holmefjord 1988). Therefore, the ability to change light intensity is important at the larval stage after hatch but before first feeding. Dimmable LED lighting systems allow the adjustment of light intensity at this stage to support optimum first-feeding activity (Fig. 2). Geographic Variation in Response to Light Most current, commercially-produced fish species are not wholly domesticated. Therefore, when devising an optimum lighting treatment for marine fish larvae, important consideration should be given to the specific conditions of the habitat of a particular species. Most marine fish species live in a variety of habitats/ environments during their life cycle and have evolved to adapt to the specific physical characteristics of their environment. Light intensity and photoperiod at different latitudes might have an effect on larval growth and survival (Suthers and Sundby 1996). Differences in population performance of larval cod are related to environmental light intensity (Puvanendran and Brown 1998). In that study, cod larvae from two geographically distinct locations spawned at different times during the year. The larvae of one population experienced much higher light intensities in their natural environment than the other. Cod larvae from the populations had different feeding behavior related to environmental light exposure. Application of the natural light conditions of the geographical environment of origin affected larval culture performance. Responsiveness of a species at the population level has been reported for other environmental parameters (Berg and Moen 1999, Jensen et al. 2000, Wilds and Muoneke 2001, Imsland et al. 2005, Conover et al. 2009, Burt et al. 2011) but the contribution and effect of environmental light on larval survival of different populations remains largely unexplored. The Significance of Tank Color to Larval Performance Environmental light intensity may improve capture success because it increases the contrast between prey and background (Peña et al. 2004). There is a broad variation in the color of larval rearing units and, for larval vision, the contrast of prey to background is variable depending on tank color. Studies have explored the effect of tank background color on larval performance (MartinRobichaud and Peterson 1998, Downing and Litvak 1999b, Tamazouzt et al. 2000, Bransden et al. 2005, Monk et al. 2008, Jirsa et al. 2009). Unfortunately these studies have not proven whether the contributing factor(s) to better larval growth was tank color itself, the contrasting effect provided, or a combination of environmental light intensity and light spectrum with the color characteristics of the specific prey organism and tank color. Tank color should be a consideration when establishing environmental lighting conditions. FIGURE 2. Experimental larval system at Mote Aquaculture Research Park used to evaluate ONCE Innovations LED lamps and fluorescent lamp on light spectrum and intensity effects.
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