EFFLUENT-GROWN ULVA AS A FUNCTIONAL INGREDIENT FOR FARMED ABALONE: IMPACTS ON GROWTH, PHYSIOLOGY AND MICROBIOME

Introduction

Integrated Multi- Trophic Aquaculture (IMTA) represents a sustainable production method  that can reduce the environmental impacts of aquaculture ,  facilitate diversification and increase production. S everal large-scale  commercial  abalone farm s in South Africa  practice IMTA, growing  Haliotis midae in land-based raceway tanks ,  interconnected with Ulva in adjacent paddle raceways using abalone effluent. Ulva serves as a biofilter, allowing water from the Ulva systems to be re-circulated back to abalone tanks,  and Ulva  is used as supplementary feed for abalone .  Dietary Ulva supplementation has conveyed benefits to a variety of cultured animals , enhancing feed consumption, growth, product quality and health ,  with  some of the improvements to  abalone  culture believed to be  linked to the carbohydrate fraction of Ulva (Naidoo et al., 2006; Mulvaney et al., 2013; Kemp et al.  2015; Bansemer et al., 2016). The aim was to  investigate the effects of (1) Ulva as a partial or complete replacement of formulated feed, and (2) the  inclusion of Ulva,  or specific components of Ulva, on  feed consumption,  growth, physiology and  the  gut  microbiome  of c ultured H. midae  to gain a better understanding of the functional potential of Ulva as an aquafeed ingredient.

 Materials and Methods

To test the effects  of Ulva as a partial or complete replacement of a local formulated feed Abfeed®S34 (diet AB )  on total  feed consumption, abalone (67.30±5.49g; n=10 per basket) were  fed  for 28 days  with AB at a rate of 1.27% BW.day-1 or with graded levels of AB (75, 50, 25 & 0 % of AB); with the balance of the feed constituting of  fresh IMTA grown Ulva (FU). A  separate one-year on farm growth trial  was conducted  under farm conditions to assess the extent to which AB can be replaced by FU. Abalone (51.10±3.09g; n=200 per basket ) were fed AB at a rate of 0.27% BW.day-1 or graded levels of AB (100, 80, 70, 60 & 40% of AB ) supplemented with Ulva ( 0.21% BW.day-1,  dry weight approximation). All treatments were offered in triplicate and growth and condition were assessed on days 112, 223 and 366.

A controlled laboratory trial was conducted t o test  the effects of specific  components of Ulva  on  abalone ( 20-30g) growth, physiology and gut microbiome. I sonitrogenous diets consisting of dried Ulva (10% w/w; AB10U ), Ulvan (1% w/w; AB1U ), and glucuronic acid (0.1% w/w; AB0.1U) were formulated. Feed conversion ratio (FCR), specific growth rate (SGR), tissue glycogen, and gut microbiome was assessed around day 0, 105 and 215  and compared to abalone fed  diets  AB,  FU, or a combination  of the latter  (ABFU).  The bacterial microbiome was characterised by sequencing the V3-4 hyper-varia ble region of the 16S rRNA gene. NGS was performed on an Illumina MiSeq sequencing platform, sequence data assessed using QIIME2 (Boylen et al., 2019), reads mapped against the SILVA 16S rRNA database (Quast et al., 2013) and summarized taxonomic abundance at different hierarchical levels were assessed.

Results and Discussion

Incorporation of small amounts  of  fresh  IMTA grown Ulva (25% ; diet 75A25U) were shown to  significantly improve total feed consumption by  ca. 90% , compared with  abalone fed  diets AB and FU (

 1). N o significant differences in SGR ,  CF  and tissue glycogen content  were recorded between treatments in the 1 year growth trial , suggesting as much as  60% of a formulated feed  can be  substituted with  FU  without negatively affecting growth and condition of abalone.

 The controlled laboratory trial, showed that abalone maintained on AB FU  and  AB0.1U  had the highest SGR,  significantly higher than abalone fed AB, AB10U, and FU (Fig. 2). Ab alone maintained on FU alone grew at a rate not statistically different to  abalone  maintained on AB , suggesting farmed abalone can be maintained on a diet consisting only of FU during the grow-out phase of production . Abalone fed diets supplemented with 0.1% glucuronic acid not only  had improved SGR, but significantly lower  FCR than abalone fed AB ; suggesting glucuronic acid may be one of the components within Ulva contributing towards growth. NMDS analysis of microbiome data revealed  that abalone fed FU diets, and its components, produced significant associations in their intestinal bacterial communities, suggesting specific bacterial species are selected for and are associated with the digestive tract of abalone fed components of Ulva. Using DESeq2, s everal differentially abundant OTUs were  identified across dietary treatments , with  various bacterial genera, including  members of the genus Vibrio found to be less abundant in the gut of abalone fed FU supplemented  diets  compared to AB alone.  This study  has  demonstrated that dietary supplementation with IMTA- grown Ulva can reduce an abalone producer’s reliance on fishmeal-based dry formulated feeds and have several other health benefits.

References

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