Green seaweed Ulva sp. as an alternative ingredient in plant-based practical diets for Pacific white shrimp, Litopenaeus vannamei

In our previous research, the supplementation of Ulva sp. seaweed meal in shrimp feeds as a replacement for fish meal (FM) resulted in growth depression. To understand the factors causing the growth reduction and explore the effects of the seaweed meal as a substitution for soybean meal (SBM), a series of growth trials were conducted in the present study. Shrimp (initial mean weight 0.24, 0.15, and 0.98 g in trials 1–3, respectively) were stocked at 10 shrimp per tank (n = 4) and offered diets for 5 to 6 weeks. In trial 1, FM level was fixed and SBM was replaced using incremental level of the second batch Ulva meal (UM2). Two additional diets were formulated to allow comparison of high inclusion levels of seaweed meal from three batches (UM1–3). Results confirmed reductions in performance as replacement of SBM by Ulva meal was increased. This data also demonstrated significant difference between batches of the Ulva meal with the UM2 producing the poorest results. To elucidate if digestible protein was limiting growth, in trial 2 feeds were formulated on an equal digestible protein basis. At the end of trial 2, shrimp fed with diets containing UM2 exhibited significantly reduced growth performance, survival, and lipid content of whole shrimp body as well as increased feed conversion ratio (FCR) compared to the reference diet. Although performance of shrimp was depressed in the treatments containing UM1 and UM3, this was less than that of trial 2, indicating that protein quality may be part of the problem. Given the level of protein replacement, other components of Ulva meal are likely to be causing poor performance. A third trial was performed to evaluate the potential of the fourth batch Ulva meal (UM4) containing relatively higher protein content than the first three batches. In this trial, the growth, survival, and lipid content of whole shrimp body also decreased as the level of UM4 was increased. To survey possible problems caused by high levels of minerals, the meals and select diets were analyzed for mineral content. Clearly there are shifts in mineral profiles; however, there is no obvious correlation to a mineral. Other possible reasons would include anti-nutrients present in the algae. If Ulva meals are to be used to their full potential, e.g., as a primary protein source, the anti-nutritional components will need to be identified, specific lines of plants with enhanced nutrient value need to be developed and of course processing technologies evaluated to produce a high quality commercial product.