Egg Development and Thermal Units

Trout and trout eggs are ectothermic, that is, they regulate their temperature based on the temperature of the environment. In this chapter we want to show you the importance of temperature throughout the development of trout.

Egg Development and Thermal UnitsTrout eggs begin development within seconds of fertilization and continue to change until they hatch. During this developmental period, there are several stages that are important markers for when eggs are ready to be handled, processed, and shipped. The rate of development is dependent on water temperature and is termed “thermal units”. Accurate recording and monitoring of temperature units is crucial to successfully producing, shipping, and rearing of trout eggs. Troutlodge uses this measurement to monitor healthy development and forecast important egg stages.Thermal UnitsA thermal unit (TU) is defined as one degree, above zero, for a 24-hour period. For example: If eggs are incubated in 10°C water from the time of fertilization, they are gaining 10 thermal units (TU) per day.Accumulated Thermal UnitsAccumulated thermal units (ATUs) also known as Degree Days (DD), are the cumulation of the thermal units (TUs) also referred to as: cumulative temperature units or degree days. For example: If eggs are incubated in 10°C water from the time of fertilization, for a total of 10 days, they have an ATU of 100. The use of colder water through development stages will not only slow down the advancement, it will delay the processing a few extra days. For example, if eggs are held in 5°C temps to 210 units, it is necessary to wait an extra 2-3 days before shocking and processing. The advancement of eggs is not linear, its slightly curved.Egg Development - Fertilization to ShippingEgg development can be roughly broken into 4 stages.

Stage 1:
0 ATUs Fertilization - Taking place when viable sperm enters the ova.

Stage 2:
1-20 ATUs Water Hardening - Completed relatively close in time to fertilization. During this period of up to 48 hours from fertilization, eggs are handled, disinfected, and set into incubators. Eggs are placed into incubators after water hardening for a minimum of 45 minutes, post fertilization. It is standard procedure to let them sit in ambient temperatures for 24 hours in the incubator before decreasing temperatures.

Stage 3:
20-200 ATUs Incubation - During this period, eggs are extremely sensitive to movement and disruption. Temperatures are manipulated to adjust ATU’s, to meet the needs of the customers and their individual orders. During this stage, eggs are treated to prevent fungus and to ensure the highest quality product.

Stage 4:
200-245 ATUs Eyed Eggs - Eyes become visible and the egg becomes harder and more robust. Once eye development is completed, eggs can be removed from the incubator and processed in stages, so as not to stress the eggs. Oxygen requirement of the eggs increases during this eyed stage. Eyed eggs are put through processes so that the infertile and dead eggs are turned white and can be easily separated from the fertile healthy ones. This is accomplished by “shocking” the eggs, enough to rupture the chorionic membrane in the infertile and dead eggs. This allows water to enter the eggs and coagulate the yolk, which turn them white. Before shocking, it is routine to test shock a small amount of eggs to see if bruising occurs, since bruising of an embryo is usually fatal. Because of all of these factors, Troutlodge relies on the expertise of our incubation staff to manage and accommodate the needs of each individual batch of eggs.

ShippingEggs are processed starting at 210 ATUs and kept in 5°C water throughout the remainder of time in the Troutlodge hatchery. Our normal ATUs for shipping are between 220-265. During transport, eggs only gain a few units. Eggs hatch around 300-310 ATUs. In general, customers can expect hatching in 5-10 days after receiving their eyed-eggs.Example: We want to share an example of how you could use Thermal Units Accumulated data to help you plan your production. First, we want to establish that this is just an example created for this guide, it is not a strict recommendation for your operation. We only seek to explain with an example how the ATU information can be applied. The following table shows recommendations for moving your fingerlings from a vertical (rack-type) incubator to the production units for their first feeding. In the table we must find the average temperature of our hatchery in the left column. We must record the days post hatch; the start day is when (80% -95%) of the eggs hatched. Reference the top row to see the ATU post hatch. Highlighted range would be time to expected ponding and first feeding.