Aquac. J., Volume 1, Issue 1 (December 2021) – 4 articles

The importance of microalgal lipids for the survival and growth of shrimp postlarvae has been recognized in a range of studies. Microalgae with fast growth rates and high levels of polyunsaturated fatty acids (PUFA) are considered vital to maximise production and minimise cost in shrimp larviculture. The lipid content and fatty acid composition of microalgae used in shrimp production varies substantially between the algal classes and species being used in Vietnam. This study aims to characterise microalgal lipid and fatty acid (FA) profiles and evaluate the most promising species under growth conditions that are most suitable for shrimp aquaculture. Here, we report that the highest lipid contents were obtained in the Haptophyta microalgae, Tisochrysis lutea and Isochrysis galbana, at 90.3 and 61.1 mg/g, respectively. In contrast, two of the most popular diatom species being used for shrimp larval cultivation in Vietnam, Thalassiosira pseudonana and T. weissflogii, displayed the lowest lipid contents at 16.1 mg/g. Other microalgal species examined showed lipid contents ranging from 28.6 to 55 mg/g. Eicosapentaenoic acid (EPA, 20:5ω3) ranged from 0.6 to 29.9% across the species, with docosahexaenoic acid (DHA, 22:6ω3) present at 0.01 to 11.1%; the two omega (ω)–3 long-chain (LC, ≥C20) LC-PUFA varied between the microalgae groups. Polar lipids were the main lipid class, ranging from 87.2 to 97.3% of total lipids, and triacylglycerol was detected in the range of 0.01 to 2.5%. Saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) increased and PUFA decreased with increasing growth temperatures. This study demonstrated the differences in the lipid contents and FA profiles across 10 microalgal species and the effect of the higher temperature growing conditions encountered in Vietnam. Full article

Replacement of fishmeal (FM) with alternative plant proteins, especially soybean meal (SBM), can cause a diarrhea-like symptom in rainbow trout (RBT), characterized by very fine fecal particles. These fines do not settle out in raceway effluent for collection and can contribute to pollution of receiving waters. In this study, two experiments were conducted. Experiment 1 examined effects of nine protein sources (sardine meal, menhaden meal, soy protein concentrates (SPC) (three types), SBM (regular and high protein), corn protein concentrate (CPC), and poultry by-product meal (PBM)) on fecal particle size distribution. Results showed that all five soy-based diets produced feces in RBT having 75.7–89.3% fines and only about 1.0% large particles, while the remaining four diets yielded feces having a balanced particle size distribution. Oligosaccharides present naturally in soy products, thought to contribute fecal fines, were not correlated to fecal particle size classes. Instead, high crude fiber content in soy-based diets was found to be responsible for unbalanced fecal particle distribution in RBT. Experiment 2 examined if improvements in formulation could reduce the negative effect of soy-based ingredients. Eight practical diets (FM, SPC, SPC + 0.3% guar gum, PBM + CPC, PBM + CPC + 20 or 30% SPC, and PBM + CPC + 20 or 30% SPC + 0.3% guar gum) were formulated to contain 40% protein and 20% lipid. Results showed that diets containing mixtures of PBM, CPC, and 20% or 30% SPC plus guar gum produced trout feces with the highest percentage of large particles and lowest of fines, while the diet containing SPC alone (56%) plus guar gum resulted in trout feces having the highest content of mid-size particles. It was concluded that crude fiber in soy protein products (SBM and SPC) caused undesirable fecal particle profiles in RBT, and the addition of guar gum could significantly alleviate this negative effect. Full article

Mussels often hybridise to form the Mytilus species complex comprised of M. edulis and M. galloprovincialis as the main species cultivated in Europe and, where their geographical distribution overlaps, the species M. trossulus. It has been suggested that M. trossulus have a weaker shell than the UK native M. edulis and hybridisation reduces farmed mussel yields and overall fitness. Here, we investigate the hypothesised link between species and shell weakness, employing multi-locus genotyping combined with measurements of six different phenotypes indicative of shell strength (shell thickness, flexural strength, Young’s modulus, Vicker’s hardness, fracture toughness, calcite and aragonite crystallographic orientation). Historic evidence from shell strength studies assumed species designation based on geographical origin, single locus DNA marker or allozyme genetic techniques that are limited in their ability to discern hybrid individuals. Single nucleotide polymorphic markers have now been developed with the ability to better distinguish between the species of the complex and their hybrids. Our study indicates that shell strength phenotypic traits are less associated with species than previously thought. The application of techniques outlined in this study challenges the historic influence of M. trossulus hybridisation on mussel yields and opens up potential for the environment to determine mussel shell fitness. Full article