Broadcast spawning fish produce large numbers of small eggs that hatch as small larvae (Bone et al., 1995). These larvae are in an undeveloped state, have eyes incapable of image formation, possess poor locomotory ability and are reliant on endogenous yolk reserves for their energy requirements (Bone et al., 1995; Werner, 2002). Fuiman (1988), cited in Kotrschal et al. (1990), suggest that the high mortality of broadcast spawning progeny is associated with insufficient sensory capability of the undeveloped larvae, which limits the detection of prey and predators. As most fish larvae are visual feeders, the transition from endogenous to exogenous feeding requires rapid morphological development of the sensory apparatus so that the larvae have the ability to locate and capture prey (Batty and Hoyt, 1995; Blaxter, 1986). This is a critical phase in larval development where the consequence of failing to capture sufficient prey, as a result of limited visual ability, is mortality due to starvation (Blaxter, 1986; Kotrschal, et al., 1990; Hjort, 1914; May, 1974).
19 Fish live in diverse aquatic habitats characterised by differences in a number of factors, including light intensity, spectral quality, concentration of primary productivity and turbulence induced by currents and winds (Costello, 2009; Jerlov, 1976). As larvae display visual adaptations that reflect their specific visual habitat (Guthrie and Muntz, 1993), abiotic and biotic factors can greatly affect the ability of larvae to perceive prey. The aquaculture environment allows the manipulation of abiotic and biotic factors such as light intensity, turbidity, tank colour, turbulence, prey density, prey size and larval density, consequently, the culture conditions that promote best feeding can be provided once the larval requirements are defined. The provision of optimum first-feeding parameters in culture leads to reduction in early mortality and improvement in health, growth and viability of larvae during their early life history (Downing and Litvak, 2001; Naas et al., 1992; Parra and Yufera, 2000).
Light is the environmental factor which has the greatest impact on feeding in most marine fish, with the threshold intensity for feeding being species- specific (Blaxter, 1986; Carton, 2005; Cobcroft, 2001; Downing and Litvak, 2001; Pankhurst and Hilder, 1998; Villamizar et al., 2011b). While larvae are capable of feeding within a given light intensity range, identification of the optimum light level significantly improves larval performance in terms of growth and survival. On the other hand, sub-optimal light levels have been linked with malformation incidence and poor swimbladder inflation (Blaxter, 1968; Downing and Litvak, 1999; Stuart and Drawbridge, 2011; Woolley et al., 2012a). The use of algae (green water) in larviculture has been documented to improve larval performance in a number of ways including enhanced nutrition of live prey, diffuse scattering of light reducing walling behaviour, increased prey detection by providing a greater prey contrast against the background, and water conditioning (Cobcroft and Battaglene, 2009; Miner and Stein, 1993; Naas et al., 1992; Shaw, 2006; Utne-Palm, 2002). Tank colour provides contrast of prey against background colour, but it also affects the distribution of light within the culture environment through reflection and scattering. It may also match the spectral sensitivity of the larvae (Cobcroft and Battaglene,
20 2009; Monk et al., 2008; Naas et al., 1996; Strand et al., 2007; Tamazouzt et al., 2000). Higher turbulence levels in culture have the potential to increase the rate of predator-prey interactions providing the larvae with a greater opportunity for successful feeding (Stiansen and Sundby, 2001), although turbulence levels should not exceed the ability of the larvae to detect, pursue and capture prey. Increasing prey density also has the ability to improve larval feeding by providing an increase in predatory encounter rate, although, feeding to excess has the potential to pollute the aquaculture environment and waste economic resources (Houde and Schekter, 1980; Robert et al., 2009; Shoji and Tanaka, 2004). The
relationship between prey density and feeding response is known to vary among species and developmental stage (Dou et al., 2000; Parra and Yufera, 2000; Shaw, 2006). In addition to prey density, the size of the prey must be ingestible by the larvae as they are gape-limited predators
(Puvanendran et al., 2004). Larvae are known to consume zooplankton prey items as large as their mouth gape will allow, which benefits the larvae in terms of enhanced energy intake, increasing the probability of survival (Anto et al., 2009; Graham and Sprules, 1992). Correct larval stocking density should provide the highest yield from the culture tank in terms of the number and quality of seed, while also encompassing the fastest rate of production (Hitzfelder et al., 2006; King et al., 2000). Plasticity in the aquaculture environment allows the manipulation of culture variables in order to meet species-specific requirements.
Recent studies have documented the importance of high light intensity and turbidity in the culture of S. lalandi larvae (Carton, 2005; Stuart & Drawbridge, 2011; Woolley, et al., 2012a), although information regarding the requirements of T. maccoyii larvae is much more limited (Hutchinson, 2009; Cobcroft, et al., 2012b). Many of the culture parameters for T. maccoyii and S. lalandi implemented at Clean Seas Tuna Ltd have been determined through empirical evidence (Table 5). The novel nature of T. maccoyii larviculture, in addition to high larval mortality, has restricted the identification of optimum larviculture conditions and consequently requires further elucidation.
21 Table 5. Culture parameters used at Clean Seas Tuna Ltd Arno Bay SA.
Culture parameter Thunnus maccoyii Seriola lalandi
Temperature 260C 240C
Salinity 36 ‰ 36 ‰
Dissolved oxygen > 100% > 100%
pH 8.0 7.8
Light Intensity 50 µmol s-1 m-2 * 126 to 203 µmol s-1 m-2 **
Turbidity 1 to 2 NTU 1 to 1.5 NTU
Photoperiod 14 :10 (h L: D) 14 :10 (h L: D)
Turbulence Water induced up-welling Air induced up-welling
Rotifers 10 mL-1 first feeding 5 mL-1 first feeding
Artemia 0.1 mL-1 > 11 dph 0.1 mL-1 > 10 dph Newly hatched
larvae
0.5 L-1 > 15 dph Nil
*Large variation in ambient sunlight levels (natural sunlight recorded levels >766 µmol s-1 m-2 entering the larviculture tanks). **Converted from lx using Thimijan and Heins (1982). Information obtained from Cobcroft, et al., 2012b; Cobcroft, 2013 and B. Chen pers. comm. at CST Ltd.