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 Units
Trout 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.
A 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 Units
Accumulated 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 Shipping
Egg 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.
Eggs 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.
Fish health is key to maintaining sustainable production over time. This section is intended to serve as a general guide for maintaining healthy stocks.
Good fish health results from the interface between adequate biosecurity, sanitation, animal welfare, husbandry, proper housing and equipment usage, nutrition, immunization, and general disease prevention, control, and monitoring.
This section is intended to serve as a general guide for maintaining healthy stocks. Fish reared in different environments have slightly different requirements. Since maintaining good fish health depends on many factors, the details for proper procedures should be provided by a fish health professional.
This section is intended to be used only as a reference for fish health management. The end user is responsible for the use or misuse of this information and Troutlodge does not accept liability for the results of the use of this reference information.
Figure 1. Effect of vaccination on the amount of antibiotic used
Total sales, in tons of active substance, of antimicrobial veterinary medicinal products for therapeutic use in farmed fish in Norway in the period 1981 - 2014 versus fish produced with biomass (Slaughtered)
A basic understanding of the process of how diseases occur in fish populations is needed to adequately design a fish health program aimed at the avoidance of such disease conditions. In its most basic sense, fish health results from the interactions between three main factors: host animal, any pathogenic organisms present, and the environment in which they interact (Figure 2).
Even though pathogens may be present in a system, unless they encounter a susceptible animal in an environment conducive to their establishment within that animal, they may coexist without a disease occurrence. Any disturbance or imbalance in any of these three factors, however, tips the scales towards a disease condition.
How diseases develop?
Figure 2. Relationship of pathogen, host and environment.
Disease occurs when the three interact to bring the host in contact with pathogens in an environment conducive to pathogen growth.
Common Rainbow Trout Diseases affecting eggs and alevins.
There are many diseases that can affect trout at all ages. Some diseases are vertically transmitted (from broodstock fish to egg and from egg to alevin) whilst others are laterally transmitted (from fish to fish). Many diseases of trout can cause very high mortality making it uneconomic to continue farming.
[Important]: It is essential to ensure that your egg supplier can certify freedom of disease in the eggs and any certification should be provided by an independent authority.
It is not within the scope of this hatchery guide to give detailed information on the many diseases or possible treatments and only the most common diseases are covered below. Diseases can generally be broken down into 3 categories.
1. Viral diseases such as:
- IHN - Infectious Haematopoietic Necrosis
- IPN - Infectious Pancreatic Necrosis
- VHS - Viral Hemorrhagic Septicemia
2. Bacterial diseases such as:
- BKD – Bacterial Kidney Disease
- BCWD - Bacterial Cold Water Disease or RTFS Rainbow Trout Fry Syndrome
- BGD – Bacterial Gill Disease
3. Environmental Diseases such as:
- Gas Bubble Disease – Nitrogen Supersaturation
- Gill Hyperplastasia
The descriptions of the various diseases below is only a brief outline and the advice of professional veterinary or fish health managers should always be sought. Many diseases can only be properly identified in a laboratory.
IHN - Infectious Haematopoietic Necrosis
Infectious haematopoietic necrosis (IHN) is an infectious viral disease of trout. Most if not all salmonid species are susceptible to the virus, with fry and small fingerlings becoming infected very readily, and becoming more resistant as they mature.
The infection is often lethal and the mortality rate can be 100% in fry. Those fish that survive an outbreak of IHN can become carriers of the virus, providing a reservoir of infection. In addition, infected juveniles will shed IHN virus particles in the faeces, urine and external mucus.
IPN - Infectious Pancreatic Necrosis
Infectious pancreatic necrosis (IPN) is an infectious viral disease that affects numerous species of fish including trout in many different parts of the world. The causative agent, infectious pancreatic necrosis virus (IPNV), has also been isolated from shellfish. The disease is also sometimes called “pin-head” disease as the bodies of young fish become emaciated given them a pin appearance.
IPNV is a very robust virus and can be transmitted in fresh and salt water, on equipment, and can survive in silage waste and in the gut of birds and mammals, allowing it to be transmitted in faeces. Movement of equipment from infected sites should be avoided, and mortalities and other wastes should be regarded as highly infectious.
Apart from the pin head appearance clinical observation may be characterised by a swollen abdomen and faecal casts (mucous) trailing from the vent. Internally there may be pancreatic necrosis.
Although there is currently no registered treatment for IPN work is well progressed on developing a vaccine against IPN.
VHS - Viral Hemorrhagic Septicemia
Viral haemorrhagic septicaemia (VHS) is historically a disease of farmed rainbow trout in fresh water in continental Europe. However, disease outbreaks in the marine environment have recently been recorded in other marine species. VHS causes high mortalities and seriously affects production. Typical outbreaks occur in spring and autumn at temperatures generally below 14°C resulting in an acute to chronic disease. All ages of fish can be affected although young fish are more susceptible.
A wide range of clinical signs may be observed. These include pale gills, dark body colour, fluid in the body cavity, bulging eyes and in some cases an intermittent period of erratic spiralling behaviour and rapid respiration. Haemorrhage is often evident in the eyes and skin, and also in the viscera.
The virus can be transmitted by diseased fish, by non-symptomatic carriers, and can be found in gonadal fluids of broodstock. Birds, blood-sucking parasites and equipment may also be a source of infection.
VHS is an OIE (World Organisation for Animal Health) notifiable disease. As with many viral diseases of fish, there is no specific treatment or cure for VHS
BKD – Bacterial Kidney Disease.
Bacterial kidney disease (BKD) is a chronic bacterial disease first reported in wild Atlantic salmon populations in Scotland in 1933.
Outbreaks can occur throughout the year, but generally accompany rising water temperatures in the spring. Infection can result in significant mortalities in trout and nearly all age groups of fish can be affected, although the disease is rare in very young fish. Losses are generally chronic, occurring over an extended period.
Clinical signs of BKD is variable and ranges from a complete lack of clinical signs to fish exhibiting protruding eyes, darkening of the skin and haemorrhage at the base of the fins. The gills may appear pale and anaemic and internally there may be fluid accumulation in the abdominal cavity and enlargement of the kidney.
Whilst there is currently no treatment for BKD the impact of the disease may be minimised by management strategies including good hygiene, reducing stress, quarantine of infected stocks, culling of infected stocks or total hatchery depopulation followed by disinfection.
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Vaccinating rainbow trout can be very effective in the prevention of diseases and minimizing environmental risks and challenges. In this chapter, we review different methodologies to provide vaccines to our stock
Vaccinating rainbow trout can be very effective in the prevention of diseases and minimizing environmental risks and challenges. Fish populations that are reared in hatcheries or controlled environments will generally not have any natural immunity to many of the pathogens that are found in the wild. The purpose of vaccines is to expose the animals to inactivated forms of natural diseases, while posing no danger to consumers. There are three ways of administering vaccinations:
Generally top coated on commercial feed and fed to stock. This method can be challenging as it is sometimes difficult to ensure proper dosage. If signs of illness are already showing in the population, fish may be lethargic and unwilling to consume the treated feed. The palatability of the feed may also be affected, furthering the difficulty of getting the entire population to ingest the diet and vaccine.
Commonly known as a “dip” or “bath” treatment and generally performed when the fish are smaller in size. Fish are immersed in a vaccine solution for a set period-of-time, and then placed back into fresh water immediately after. This method is useful for treating large populations quickly and effectively.
This is considered to be the most effective method of introducing vaccines. Fish are individually administered a dose of vaccine through injection. This method can be very time consuming and has an inherent risk of accidental injection to staff.
Troutlodge uses vaccines as one of the many tools to keep our trout stocks healthy. The current vaccination program includes an immersion treatment, when fish are less than 10 grams in size, and a booster injection, at approximately 18 months of age. All stock are vaccinated for Yersinia Ruckeri, the bacterial pathogen causing enteric red mouth disease (ERM), as well as Aeromonas salmonicida. These vaccine treatments are effective in raising a protective immune response against these common diseases.