Improving the Reproductive Performance of Domesticated Giant Tiger Shrimp, Penaeus monodon

Abstract

The Giant or Black Tiger Shrimp, Penaeus monodon, is an economically significant aquaculturespecies globally, producing 4.5 million tonnes of product annually at a value of US$ 23.5 billion(FAO, 2016a). Recent innovations in the domestication and selective breeding of P. monodon haveresulted in significant improvements in growth rate, survival and pathogen tolerance. However, thereproductive performance of domesticated stocks is inferior compared to that of wild-caughtbroodstock. Significant reductions in the number of females maturing, egg and nauplii productionand hatch rates are commonly reported for domesticated stocks relative to their wild-caughtcounterparts.The complexities surrounding reduced reproductive performance in domesticated P. monodon areunderpinned by two critical issues: 1) a poor understanding of the specific nutritional requirementsfor reproduction in the species and; 2) a lack of clarity as to the characteristics that define a ‘goodspawner’ – particularly on a biochemical and molecular level. The studies that make up this thesisemployed a multidisciplinary approach to assess nutritional, biochemical, and molecular factors thatrelate to broodstock reproductive performance. Primarily this thesis sought to: (1) investigatewhether the current constraints to reproductive performance in domesticated stocks could beovercome by including the microbial biomass derived bioactive Novacq™ (Patent #2008201886)within pelleted diets; (2) evaluate whether current broodstock maturation diets are limiting inrelation to repeated spawning and; (3) characterise key interactions between micronutrients andregulatory gene(s) and/or pathways linked to reproduction.A series of reproductive performance trials were undertake to assess the effect of incorporatingmicrobial biomass (Novacq™) within pelleted maturation diets. Preliminary farm-based trialsobserved significant increases to maturation rate, egg production and nauplii production whendomesticated broodstock were fed an experimental pelleted diet containing the Novacq™ ingredient(20% Novacq™ inclusion rate, 2.4% of total diet fed). However, in a subsequent trial conductedunder controlled experimental conditions, broodstock fed commercial-grade pelleted diets (30%Novacq™, 5.5% of total diet fed) exhibited a significant decrease in egg hatch rate. Reductions inreproductive performance under controlled experimental conditions were attributed to a decrease inthe quality of basal pellet diets, both as a function of increased Novacq™ inclusion and theircommercial-based formulation. The above studies suggest the capacity to improve reproductiveperformance in domesticated P. monodon, using biofloc and its substituents, is highly dependent onthe quality of the basal maturation diet fed.In response to the aforementioned studies, a trial was undertaken to identify potential factorslimiting reproductive performance within current broodstock maturation diets. The effect ofrepeated spawning on reproductive performance and tissue biochemistry (ovary andhepatopancreas) was assessed in a population of broodstock fed a typical high performancematuration diet. During the initial two spawning cycles broodstock demonstrated significantvariation in aspects of hepatopancreas and ovary biochemistry. Most notably, significant reductionsin hepatopancreas and ovarian arachidonic acid (ARA) content were observed, suggesting that therequirement for and/or utilisation of ARA in relation to spawning exceeds quantities provided bycurrent maturation diets. Additionally, a number of hepatopancreas fatty acids were depleted insecond spawn, and therefore represent micronutrients likely to become limiting in subsequentspawning cycles.To further understand the impact of limiting ARA on reproduction, ovarian ARA content wasquantified in a homogeneous population of domesticated broodstock. Significant individualvariation in ovarian ARA content was observed. RNA-seq analyses was undertaken to investigatethe effect of variable ARA content on global gene expression and prostaglandin (ARA derivedhormones with significant regulation over reproduction) biosynthesis. Global gene expressionanalyses identified a total of 757 genes with >2-fold expression difference in relation to ovarianARA content. Additionally, variation in ovarian ARA content had significant impact on theregulation of prostaglandin biosynthesis genes, particularly those linked to egg production (PGE2)and maturation (PGF2α).The studies contained in this thesis shed light on the influence of nutritional bioactives, whilstproviding a comprehensive framework for the development of high-performance broodstock feedformulations and optimized nutrition strategies.