Reception of eggs


Temperature is the one most crucial environmental parameter in relation to incubating and hatching rainbow trout eggs. Rainbow trout eggs can be incubated safely within the temperature range: 4 – 11℃ (12℃) with optimal incubation temperature in the range: 6 – 8℃. Although incubation and hatching at temperatures above 11℃ may occasionally be successful, the risk of failure is significant and AquaSearch offer no guarantees on hatching results, survival etc. under these conditions.

Incubation temperature also play a crucial role in the development of the egg and larvae, including timing of key events like time to eyed egg stage, time to hatching, duration of hatching, absorption of yolk sac and timing of swim-up and first feeding. Hence development and key events are corelated to the accumulated temperature units (ATU) of the egg / larvae. ATU in degree days = Number of days x average temperature during the period.

It should however be noted that the rainbow trout development is affected not only be the ATU, but also absolute temperature. Fewer ATU’s is required at higher temperature to achieve same developmental stage.

E.g., if primary incubation temperature has been 10℃, eggs will hatch at around 330 – 340 degree days, where at primary incubation temperature 4℃, eggs will not hatch until after 350 – 370 degree days. See graph on the correlation of absolute average temperature and key events.

Water flow

Rainbow trout eggs are compromised in their oxygen uptake by the limited surface area of the egg (no gills to increase surface and actively exchange water) and the egg obviously cannot actively change location for better conditions. Hence primary function of the waterflow in hatching devises is to exchange water and secure oxygen supply in the microenvironment surrounding each egg.

Optimal waterflow will depend on the actual hatching devises, but a rule of thumb is no less than 1 liter / minute / liter of egg will support the functionality of most types hatching devises. E.g. California type hatching trays and troughs with one liter of eggs in each of 7 serial trays, should receive min. 7 liter of water / min. in the inlet, with 10 liter / min. recommended to compensate for potential minor leakages in the fit between trays and troughs


When correct water exchange in the microenvironment of each egg has been secured, sufficient oxygen content remains crucial to normal development and hatching. Min. 8 mg oxygen / liter is recommended.

If the eggs are suffocated at an early stage, they will simply die. Suffocation at a later stage will result in premature hatching, where the larvae will either die during the process of erupting the eggshell or survive weak and vulnerable. Suffocation and premature hatching will typically also cause a prolonged hatching period, with two distinct waves of hatching. The initial premature wave and the later wave at the expected ATU’s.

Use of liquid or pressurized oxygen for eggs and larvae is not recommended. The risk of gas supersaturation or gill damage is simply too high, and very few aquaculture operations will be able to justify the use of artificial oxygen over simple aeration which will secure 90 – 99 % oxygen saturation.

Carbon dioxide

Adverse carbon dioxide levels in hatcheries will generally only be an issue in recirculated systems. E.g., when water is chilled and reused to reduce power consumption by the heat pump. A minimum water exchange of e.g., 2 – 4 % of the flow, combined with some degassing, should generally be able to control this. Maximum 6 mg CO2 / liter of water is recommended.

Total gas saturation

Yolk sac larvae are particularly vulnerable to gas supersaturation. Levels down to 102 % nitrogen saturation is lethal and levels above 100% should always be avoided. Supersaturation can be caused by already saturated inlet water, leaking pumps, excessive pumping, excessive lifting heights and insufficient degassing before water enters the hatching devises etc. Submersed inlets directly from a sealed header tank and pump is almost always critical.

Water pH

pH value outside the range 6,2 – 7,2 should be avoided, toxicity of extreme pH values will however depend on other environmental factors like various metal ions and carbon dioxide.


Eggs and larvae should be kept in dark conditions until swim up stage and first feeding, after which dim light can be kept on 24 hours a day to maximize growth rate.

Stocking density

Optimal egg density depends on the hatching device, but a maximum layer of 3 – 5 egg thickness is generally recommended for hatching, although e.g., Zuger jars will allow a thicker layer of eggs to develop normally, provided waterflow is adjusted very accurately, exactly below the flow that will cause egg movement. Please notice also, that functionality of some hatching devises may be affected negatively by to low stocking density. E.g., if the mesh surface is not covered evenly with eggs, reduced counter pressure will cause water to flow where there is no eggs, suffocating eggs in areas with higher counter pressure.