Aquaculture America 2023

February 23 - 26, 2023

New Orleans, Louisiana USA


Mark Drawbridge*, Yuanzi Huo, Erica Fanning, Torre Polizzi, Leslie Booher


Hubbs-SeaWorld Research Institute, 2595 Ingraham Street, San Diego, CA


Nitrogenous compounds such as ammonia, nitrate, and dissolved organic nitrogen are the main waste components of marine fish aquaculture effluents. These compounds are also regarded as the primary nitrogen sources for macroalgae.  In order to understand how sea lettuce (Ulva lactuca) could be used to scrub dissolved nutrients from the effluent of flow-through white seabass (Atractoscion nobilis) raceways, we designed a series of 3-week experiments to assess the benefits of the fish nutrients, shading, algal density, and water exchange rates.  All Ulva cultivation trials were conducted in 175 L tanks run in triplicate, with water quality and growth measurements taken weekly.  Seawater was supplied by gravity from seabass raceways unless otherwise noted.  For the shading trial, Ulva was stocked at 0.56 kg/m2 with either 0%, 30%, and 60% shade with seawater supplied at 7.6 L/min for each tank.  The removal efficiency of total ammonia nitrogen (TAN) decreased with increasing shade levels, and the peak exceeded 80% at 0% shading. Growth rate and productivity of Ulva under 0% shading was 12.09±3.13 %/d and 13.28±4.66 g DW/m2/d, which were both significantly higher than other shading levels.  For the stocking density and flow rate trial, the stocking density of Ulva was 0.56 and 1.12 kg/m2, and seawater effluent was supplied at 0.5, 1.5, and 7.6 L/min in a 2-factor design.  In this trial, TAN removal efficiency was 100% across all treatment combinations. Growth rate of Ulva was highest (20.36±2.36%/d) at 0.56 kg/m2 under the flow rate of 7.6 L/min, and the productivity reached up to 30.89±6.53 g DW/m2/d which was not different than that of Ulva stocked with 1.12 kg/m2 at the same flow rate (p=0.47).  In a trial comparing raw, sand-filtered seawater (RAW seawater) with seawater used to grow 278 kg of white seabass (FISH seawater), TAN removal efficiency by Ulva was consistently nearly 100% when supplied with FISH seawater with a TAN concentration ranging from 0.11 to 0.18 mg/L. Growth rate and productivity of Ulva supplied with FISH seawater were 21.36±2.25 %/d and 33.83±7.29 g DW/m2/d, which was not different than that for Ulva supplied with RAW seawater.  However, the protein content of Ulva (22.24±2.49%) in FISH seawater was significant higher than that of Ulva (9.98±1.08%) cultured with RAW seawater (p<0.01), and the C/N ratio of 8.37±0.87 in Ulva tissue cultured with FISH seawater was significantly lower than that of the Ulva (15.94±2.01) when supplied with RAW seawater (p<0.01).  This indicated that Ulva was in a nitrogen-limited situation when supplied with RAW seawater. In light of the better understanding of Ulva performances achieved in our study, the optimum Ulva and white seabass combinations can be established to maximize the nutrient assimilation efficiency, biomass production, and diversity of seafood production.