18 DECEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG Nutrient Additions from Deep Sea Water and Finfish Effluent The surface sea water in the tropics is oligotrophic, or low in nutrients, with nitrate/nitrite levels at ~ 3ug/L. This is not adequate for commercial macroalgae production, so some form of supplemental nutrients must be provided. In tank-based trials we have tested various nutrient sources, including deep seawater (DSW), finfish effluent water in a co-culture system, and commercial fertilizer (F/2; AusAqua, Australia). For most macroalgae currently in cultivation, an addition of 5-10 percent DSW (~50ug/L nitrate/nitrite) is ideal for optimal yields. Maximum yields have been achieved for Halymenia hawaiiana of ~175g/m2/day, and for Caulerpa lentillifera of 150g/m2/day. Fish species of commercial interest grown on-site include the nenue, or brassy chub (Kyphosus vaigiensis) and the long-finned amberjack (Seriola rivoliana) (Figure 3) that provide effluent high in ammonia/ammonium conducive of high macroalgal growth. For example, Gracilaria grown in brassy chub effluent water (ammonia/ammonium concentration at 50 mg/L) achieved max SGR values of 7.7 percent at 40 percent shade (Figure 4). In a separate trial, effluent from brassy chub was used in a co-culture system with three macroalgae species; Caulerpa, Halymenia, and Ulva. All three species achieved SGRs of 5 percent, with the addition of the ammonia-rich effluent. Macroalgae at the end of the 10-week trial showed tissue nitrogen levels 2-3 times higher in effluent treatments compared to the sea surface water control. Most cultivated species are propagated vegetatively, but Ulva and Gracilaria have been manipulated to release spores/ gametes and seed them onto lines. Ulva spp. consistently undergoes sporulation/gametogenesis with nutrient loading using F/2, high concentrations of DSW, or fin-fish effluent. Various attachment methods have been tested for species in tumble culture to determine the most suitable method for a farm array (Figure 5). Production Research work has employed replicated, controlled trials in land-based tanks to optimize yields under simulated off-shore conditions. Flow rates vary among the tanks, but are typically set to 3-6 L/min with 30 min tank turnover rates. Aeration provides water mixing, keeps the macroalgae suspended, and helps with nutrient uptake. Most species can be grown in tumble culture, although Caulerpa lentillifera (sea grapes) grows best when attached to a substrate. For this species, starting biomass is stuffed into mussel socks with a 5cm diameter, and weighted down at the bottom of a tank. The C. lentillifera grows horizonal stolons and rhizoids to form attachment to the substrate, and proliferates its thalli throughout the mussel socks (Figure 6). Yields of up to 150-200g/m2/day were achieved with an SGR of 5.4 percent for Halymenia hawaiiana, at an optimum stocking density of 3-5kg/m2 in 2,700 L rectangular tanks with 10 percent FIGURE 2. Several of the cultivated species currently in production in Kona. FIGURE 3. Nenue or Brassy chub, (Kyphosus vaigiensis) (top), and Kahala or Kampachi (Seriola rivoliana), (bottom).
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