A review of Lauzon HL, Gudmundsdottir S, Petursdottir SK, Reynisson E, Steinarsson A, Oddgeirsson M, Bjornsdottir R, Gudmundsdottir BK. 2010. Microbiota of Atlantic cod (Gadus morhua L.) rearing systems at pre- and posthatch stages and the effect of different treatments. Journal of Applied Microbiology. 109:1775-1789.
This paper aims to understand the reasons behind the high levels of larval mortality and identify the main factors that cause this to occur.
Cod aquaculture often experiences high levels of larval mortality, however the causes of this are not well understood. There are many theories as to the causes including poor rearing conditions, poor nutritional status and pathogens. Obviously the first two of these will facilitate the effect of pathogens as the fish will be more susceptible. It has also been suggested that the combination of these effects may provide the conditions for opportunistic bacteria to become harmful to the fish. It is therefore the bacterial interactions at the mucosal surfaces of ova and larvae that will determine the success or failure of rearing success.
As pathogens are the main cause of mortality, even when not facilitated by poor rearing conditions, it is important to understand not only which bacteria and viruses are detrimental to health but also the factors which enable these to exist. Many fish pathogens uptake iron by siderphores which are considered to be a main virulence factor which can affect fish health.
The main objectives of this paper were to investigate the effects of ova disinfection, antibiotic and other microbial treatments on the main cultivable microbiota found in cod rearing systems. This was investigated at pre and post-hatch stages. The aim of this was to determine the phenotypic traits that are thought to be related to virulence, displayed by bacterial isolates and how these impacted on larval mortality.
This study used culture dependent methods, allowing for enumeration and characterisation, which would not be able using culture dependent methods. They defended the problems of cultivation, such as the presence of uncultivable bacteria, by stating that in nutrient rich rearing systems, this problem is far less important as long at the correct medium was used.
Ova were disinfected on day one of fertilization and again eleven days later (two days before hatching). Antibacterial treatments were applies via bathing, both pre and post-hatching or by feeding once a day with rotifers, treated with two bacterial strains and a probiotic yeast.
Bacterial analysis identified 75 operational taxonomic units within 5 phyla. Treatment of ova was effective in reducing the dominant bacterial species of Marinomonas sp. however other species survived the treatment to some extent. Algal concentrate and dry food consisted of gram-positive bacteria but additions of these into the rearing water had little or no effect on the microbiota composition.
The microbiota present was determined to fall into one of three groups; beneficial, harmless and detrimental and opportunistic. Of these three the highest levels virulence factors such as siderophores were found in the harmful OTU’s.
Healthy ova hatched sterile larvae which quickly became contaminated with the bacteria on the ova debris and the rearing water. After a few weeks of feeding, the dominating bacterial species present were those found in the treated rotifers and dry feed indicating feeding has the greatest importance for introducing new bacteria to the internals of larvae. Suspected high Vibrio levels in treated rotifers were responsible for high levels of mortality experienced in these conditions.
In conclusion it is clear that the feeding processes and the bacteria and viruses that are introduced via food sources are the most important for determining the microbiota present in the larval intestinal system. These pathogens are also the main cause for mortality in larva, even if rearing conditions are optimal.
1 comment:
did the studyfind that disinfecting the larvae was detrimental as this would prevent the development of the immune system of the fish for later life? also did they only disnfect the ova, what about the water because it would very quickly just become contaminated? i wonder if keeping the fish completely sterile and slowly introducing good bacteria would work? although it is probably not feasable.
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