Rannveig Bjornsdottir

The Atlantic salmon (Salmo salar) aquaculture industry is growing, and with it, the need to source and optimise sustainable ingredients for aquafeeds. Black soldier fly (BSF) larvae (Hermetia illucens) have received increasing research attention since they are a good source of protein that can efficiently convert a wide range of low-value organic material into valuable resources. This study investigated the impact of three differently processed BSF meals, an untreated BSF diet (BSFC+), a dechitinated BSF diet (BSFC-) and a fermented BSF diet (BSFC+P+) at a 10% inclusion level replacing fish meal in a fish meal control (FM) and a marine and soy protein concentrate based control diet (SPC). Growth performance, gut microbiome and gut histology of salmon fry was assessed. The inclusion and processing methods of BSF showed no adverse impacts on either growth performance or gut histology. However, the gut microbiome of fish was significantly altered by both the protein source and the processing method of the BSF larvae. Fish fed BSFC+, had an increased diversity and evenness of the community compared with conventional protein sources alone, and compared with the other BSF processing methods. However, control diets had a greater presence of lactic acid bacteria and genera associated with faster growing hosts. Fish fed BSF had a high relative abundance of the genus, Exiguobacterium, a chitin-degrading bacterium and in BSFC+P+ fed fish this bacterium completely dominated the community, indicating the presence of dysbiosis. Future studies should determine, why Exiguobacterium has dominated the community for the BSFC+P+ diet, and if it provides a digestive function to the host and identify bacteria that are indicators of optimal host performance and resilience. The results confirmed that BSF is a promising fish meal replacement for salmon, and it demonstrated that BSFC+ has a potential prebiotic impact on the gut microbiome of Atlantic salmon.

To study the effect of ova disinfection, antibiotic and microbial treatments on the dominant cultivable cod rearing microbiota at pre- and posthatch stages, determining some virulence-related phenotypic traits among bacterial isolates and their relation to larval survival. Sampling of rearing systems (rearing water, ova, larvae, feeds and supplement) for analysis of cultivable microbiota took place at early stages in 2004 and 2005. Cultivation, phenotypic and genotypic (16S rRNA gene) analyses were performed. The production of putative virulence factors (PVFs), including haemolysin, siderophores and quorum-sensing signals, by bacterial isolates was investigated and related to larval survival. The study was performed during two spawning seasons, evaluating current hatchery practices (ova disinfection and antibiotic treatment of unhealthy larvae) and specific putative probiotics applied to ova and larvae or rotifers. A diversified microbiota (75 operational taxonomic units, OTUs) was observed in cod rearing systems influenced by the feeds and treatments, with prevailing γ-Proteobacteria prior to hatching towards a multiphyla microbiota posthatch. Phenotypic tests demonstrated the heterogeneity within some OTUs. Multivariate analysis of survival data in larval silos and the corresponding larval microbiota was used to divide the genotypic groups into beneficial/harmless and detrimental/opportunistic clusters. PVFs were common among the proposed detrimental/opportunistic OTUs. The results clearly demonstrate the influence of exogeneous feeding and treatments on larval gastrointestinal microbiota and the role of bacteria in larval survival. Increased understanding of the microbiota in rearing systems may contribute to successful implementation of microbial management in cod aquaculture.

Three feeding trials were conducted to estimate the minimum protein requirements for maximum growth of Atlantic cod (initial size 37–600 g). The diets in each trial were near iso-energetic and contained 47–64% crude protein (CP) in dry matter for small fish while diets for larger fish contained 36–57% CP. There was no significant difference in the final weight of small fish (90–130 g) fed different levels of CP. However, the specific growth rate (SGR) of fish fed 59% and 64% CP (0.9% day⁻¹) was significantly higher than that of fish fed either 47% or 48% CP (0.5–0.7% day⁻¹). The hepato-somatic index (HSI) increased progressively with decreased CP and the feed conversion ratio (FCR) was negatively correlated with CP of the diets. In two trials with larger fish, CP did not significantly affect mean final weight (750–900 g), SGR or FCR. As for the smaller fish, the HSI was negatively correlated with CP and in one of the experiments, the feed intake was also negatively correlated with CP. The feed efficiency tended to be reduced. The results suggest that the protein requirement for maximum growth in Atlantic cod is size dependent: for 40–107 g fish, the minimum CP is 47–52% and for larger fish (400–900 g) it is 36% or lower.

To assess the effects of bacterial treatment at the earliest stages of cod rearing on the microbial load, larval development and performance, testing three bacterial strains (Carnobacterium divergens V41, Arthrobacter sp. and Enterococcus sp.) in vivo that were previously shown to have inhibitory potential towards fish pathogens in vitro. A bacterial mixture was added eight times to the rearing water from the prehatch to the mid-larval stage (a 38-day period). Microbiological analysis of ova, larvae and rearing water was performed regularly. Larval performance and development were evaluated by survival rate, hypersalinity tolerance and physiological measurements. Different larval survival rates were observed within and between treatments, and possibly explained by variations in larval microflora and established probionts. Larvae from one silo, which had been bathed in the bacterial suspension, showed the highest survival rate (42.1%), lowest Vibrio levels, and were significantly heavier (19.3%) and more stress tolerant than control larvae (P < 0.01). This coincided with the intestinal establishment of two of the tested bacteria. Arthrobacter and Enterococcus strains added regularly to the rearing water from the postfertilized egg stage can become established in larval gastrointestinal tract. The Enterococcus strain was associated with increased larval growth, performance and microflora control, indicating its probiotic nature. Regular application of autochthonous probionts may promote larval welfare, development and stress tolerance at early stages, hence increasing production yield in intensive cod larviculture.

Aeromonas salmonicida subsp. salmonicida is a facultatively intracellular gram-negative bacterium that is the etiological agent of furunculosis, a bacterial septicemia of salmonids that causes significant economic loss to the salmon farming industry. The mechanisms by which A . salmonicida evades intracellular killing may be relevant in understanding virulence and the eventual design of appropriate treatment strategies for furunculosis. We have identified two open reading frames (ORFs) and related upstream sequences that code for two putative superoxide dismutases (SODs), sodA and sodB . The sodA gene encoded a protein of 204 amino acids with a molecular mass of approximately 23.0 kDa (SodA) that had high similarity to other prokaryotic Mn-SODs. The sodB gene encoded a protein of 194 amino acids with a molecular mass of approximately 22.3 kDa that had high similarity to other prokaryotic Fe-SODs. Two enzymes with activities consistent with both these ORFs were identified by inhibition of O ₂ ⁻ -catalyzed tetrazolium salt reduction in both gels and microtiter plate assays. The two enzymes differed in their expression patterns in in vivo- and in vitro-cultured bacteria. The regulatory sequences upstream of putative sodA were consistent with these differences. We could not identify other SOD isozymes such as sodC either functionally or through data mining. Levels of SOD were significantly higher in virulent than in avirulent strains of A . salmonicida subsp. salmonicida strain A449 when cultured in vitro and in vivo.