60 JUNE 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Embryos and pre-feeding larvae (Fig. 1) receive these fatty acids from their yolk. Because marine fishes are unable to manufacture essential HUFAs in sufficient quantities to meet physiological requirements (Tocher 2003), HUFAs contained in yolk ultimately originate from the maternal diet. It is well known that the fatty acid composition of fish eggs reflects that of the broodstock diet (Izquierdo et al. 2001). The period of influence of the maternal diet on egg composition differs among species because the duration of gonadal maturation and feeding patterns prior to and during spawning vary widely (Izquierdo et al. 2001, Johnson 2009). Few studies have attempted to identify the exact timeframe over which this occurs or, more especially, the rate at which egg composition changes in response to a change in maternal diet. Understanding how and when the maternal diet influences egg fatty acid composition would allow hatchery managers to adjust diets and feeding protocols to achieve high-quality eggs while making most efficient use of marine products in the boodstock diet. Recent experiments on red drum Sciaenops ocellatus demonstrate that ARA, one of the important HUFAs, is transferred from the diet to eggs very quickly, depending on the diet history (Fuiman and Faulk 2013a). This suggests that more environmentally sustainable egg production can be achieved. Study Methods We conducted 15 diet-shift experiments in which the ARA content of the broodstock diet was changed from one level (ARApre, in mg ARA wk-1 fish-1) to another (ARApost) and then measured ARA content of eggs from spawns produced over one month after the diet shift. Each broodstock tank contained 2-5 fish with 1-3 females per tank, which were fed three times per week (Monday, Wednesday, Friday). Broodstock diets containing the desired levels of ARA were constructed by varying the proportions of several diet components, including whole marine organisms (shrimp, squid, fish), beef liver, dry commercial feed and an ARA supplement. ARApre and ARApost were calculated as the mean weekly intake of ARA averaged over 28 days before and after the diet shift, respectively. The ARA content of eggs and diet components was measured by gas chromatography. The series of measurements of ARA concentration of eggs from each experiment represented a time course for the incorporation of ARA into eggs. All time courses were approximately linear, so a simple linear regression was used to estimate the incorporation rate of ARA in eggs (IARA, in mg ARA g-1 DW d-1) following the diet change. Simple linear regression was also used to explore how the magnitude of dietary change in ARA (ΔARA = ARApre - ARApost) influenced the dynamics of ARA incorporation into eggs (IARA). Additional details of the experiments, methods and data can be found in Fuiman and Faulk (2013a, b). Results and Discussions In the 15 experiments, ΔARA ranged from -418 to +598 mg wk-1 fish-1. Observable changes in the ARA content of eggs occurred rapidly after the diet shift and followed a linear trend in all experiments (Fig. 2). Incorporation rate (IARA) was directly proportional to ΔARA (Fig. 3) and 64% of the total variance in IARA was explained by ΔARA: IARA = 0.004 + 0.00022 x ΔARA. The immediate source of HUFAs incorporated into eggs by fishes could be body stores, such as adipose tissue and liver, the current diet, or both (Pavlov et al. 2004). Changes in dietary intake of ARA by reproductively active red drum result in rapid changes (416 days) in the ARA content of eggs. Changes in fatty acid composition of gilthead seabream Sparus aurata eggs following a dietary shift in ω-3 HUFAs were also rapid, within 15 days (Harel et al. 1994). These results indicate a direct pathway for HUFAs from diet to eggs, or at least a very short residence time in maternal body stores. Red drum and gilthead seabream actively feed during gonadal maturation and spawning and this may explain the rapid transfer from diet to eggs. In contrast, accumulated body stores are likely to play a greater role in egg composition for species such as Atlantic cod that cease or substantially decrease feed intake during the spawning season. The rate of change in the fatty acid profile, particularly in muscle tissue, is of considerable interest in aquaculture when a plant-based fatty acid profile is “washed out” before harvest by shifting from a more sustainable, less costly plant-based diet to a “finishing” diet that includes more marine ingredients. Robin et al. (2003) proposed a dilution model to describe the rate of change in TOP, FIGURE 2. Selected time courses (4 of 15 experiments) illustrating the incorporation of ARA into red drum eggs following a diet shift. Each data point represents an individual spawn. Numbers within panels denote the magnitude of the diet shift (ΔARA). Data from Fuiman and Faulk (2013 a, b). BOTTOM, FIGURE 3. Relationship between the rate of incorporation of ARA into red drum eggs (IARA) and the magnitude of the diet shift (ΔARA). IARA (± S.E.) was estimated as the slope of a linear regression fitted to each time course relating ARA content of the eggs to ΔARA (Fig. 1). Data from Fuiman and Faulk (2013a, b).
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