Prophylactic measures
in rainbow trout eggs, larvae and fry husbandry
AquaSearch ova 2010
To obtain the lowest possible germ levels, eyed eggs from AquaSearch ova have been disinfected with 100 ppm iodide for 15 min in accordance with O.I.E. recommendations and in addition to this with hydrogen peroxide and sodium chloride.
Should the receiving hatchery still wish to disinfect the eggs at arrival, it is recommended that the eggs are re-hydrated and acclimatised to the hatchery water temperature prior disinfection.
Ref.: http://www.aquasearch.dk/dk/farmers-toolbox/acclimatisation.html.
Ref.: http://www.aquasearch.dk/dk/farmers-toolbox/acclimatisation.html.
Furthermore it is recommended to use a commercial available disinfectant specifically for the purpose of disinfecting eyed eggs, and to secure neutral pH in disinfectant solution at all times.
Ref.: http://www.aquasearch.dk/dk/farmers-toolbox/disinfection-and-receiving.html
Ref.: http://www.aquasearch.dk/dk/farmers-toolbox/disinfection-and-receiving.html
During the period of final incubation until the time of hatching it may be relevant to treat the eggs prophylactic against fungal infections (saprolegnia parasitica) depending on the presence of this pathogen in the hatchery water. Effective prophylaxis is achieved by daily or bi-daily treatments with up to 1500 ppm formalin for 15 min. other antifungal agents may be applied including: Sodium chloride, peracetic acid, hydrogen peroxide, various cobber compounds and more.
During the period of hatching chemical treatments should be avoided. Water flow in the hatching devises should be adjusted for maximum water flow and oxygen supply to the hatching and newly hatched larvae without resulting in stressful water currents and motion of the larvae. Also supersaturated oxygen levels should be avoided during this period. Egg shells and debris should be removed manually immediately after hatching to avoid breeding ground for germs and pathogens and to avoid clogging of the water flow.
Hatching trays may be equipped with a 3 – 4 mm mesh 2 - 3 cm from the bottom of the hatching tray, allowing the newly hatched larvae to pass, but trapping the empty egg shell and other debris for this to be easily siphoned away.
California type hatching tray with 3 – 4 mm mesh 2 – 3 cm of the bottom
The yolk sac larvae should be left as undisturbed as possible, until the yolk sac has been absorbed and the larvae starts showing feeding behaviour. At this point, before first feeding the larvae can be sieved to once again remove debris and to transfer the larvae to clean trays or troughs before the weaning process is initiated. Also at this point of time, immediately prior weaning onto dry feed, it is recommended to add 0,5 – 1,0 % sodium chloride to the holding unit of the fry for up to 20 min. to reduce flavobacterium spp. and other opportunistic pathogens in the environment of the fry before first feeding and exposing the gastrointestinal channel to whatever germs might exist in the environment of the fry.
The yolk sac larvae should be left as undisturbed as possible, until the yolk sac has been absorbed and the larvae starts showing feeding behaviour. At this point, before first feeding the larvae can be sieved to once again remove debris and to transfer the larvae to clean trays or troughs before the weaning process is initiated. Also at this point of time, immediately prior weaning onto dry feed, it is recommended to add 0,5 – 1,0 % sodium chloride to the holding unit of the fry for up to 20 min. to reduce flavobacterium spp. and other opportunistic pathogens in the environment of the fry before first feeding and exposing the gastrointestinal channel to whatever germs might exist in the environment of the fry.
Prophylactic treatment with 0,5 – 1,0 % sodium chloride should be repeated weekly until min. 1 gram average size to further reduce the risk of flavobacterium infections during this vulnerable period of the fry’s life.
In addition to the above, if the hatchery has a history of bacterial gill infections, weekly prophylactic treatments with chloramine T 5g / m3 is recommended. Chloramine T treatment should be avoided while feeding.
Furthermore, if the hatchery has a history of parasitic infections, weekly treatments with 100 ppm for 30 min or alternative anti-parasitic agent is recommended.
To secure the best possible health status, homogeneous fish size and maximum growth speed the fry should be feed an excessive number of times to saturation daily. More than 10 feedings round the clock are recommended.
Depending on the destiny of the fry and the disease threats of the ongrowing environment, the fry should be kept in the hatchery environment until full development of the fish immune system at about 3 g fish size, where the fish should then be vaccinated against relevant pathogens it might encounter later in its life cycle.