T he life stages which are normally considered within the incubation category include green eggs, eyed eggs, hatching, and alevins (sac fry). Green Eggs is the term primarily used to describe eggs which have not yet been fertilized. The outer shell (or clarion) is fairly soft. It is usually safe to move or handle the eggs at this stage. After fertilization, the shells harden and become more impermeable, and the cells begin to divide. This is a particularly sensitive time when handling should be minimized. When an eye is visible through the shell, the eggs are called Eyed Eggs. The embryo begins to move within the shell which increases permeability of the shell to metabolites. Salmonids hatch before the embryo development is complete.
The young fish are called Alevins or Sac Fry. There is a yolk sac attached to the fish which is the source of nutrients while the fish matures. In salmonids, the gills and scales don’t develop for a couple of weeks after hatch and is frequently coincident with the consumption of the yolk sac and the beginning of external feeding. There is no standard equipment used to hold the eggs during incubation. The selections vary from species to species and operator to operator. Designs normally use upwelling water around the eggs to transport metabolites to the eggs. Usually the screens or pads which support the eggs permit the newly hatched alevins to move down into the sheltered volume underneath.
Some of the most popular technologies include:
• Vertical stacks of specially designed trays (Heath trays) where water cascades from tray to tray down the stack. The eggs rest on screens or pads and the water upwells around the eggs before overflowing into a channel on the way to discharge down to the
• Jars where water upwells around the eggs. The eggs are kept approximately suspended in the water column. • Tanks supplied with removable screens where the newly hatched fish can pass down through the screen into the lower portion of the tank.
Vertical Incubation Stacks
Combi-tanks with egg trays
The equipment must provide operators access to the eggs for removing unfertile eggs, eggs which develop fungus or deformities. Failure to do so will result in fungus quickly
spreading to all of the eggs and a general degradation in water quality. Water flow around the eggs is also necessary to transport metabolites bring oxygen to the eggs and carry away carbon
Water Quality Criteria
Control of temperature is important to survival of the eggs and alevins. There is a lower and upper threshold temperature for each species which has usually been identified by a comparison of mortality percentages at a range of temperatures. Each species also has an optimum rearing temperature for each life stage. Fish eggs and alevins tend to mature at different rates depending on water temperature. State of maturity is frequently identified by the number of days multiplied by the water temperature (in
The optimum water temperatures also tend to reflect the conditions under which the species evolved. Since water quality parameters like oxygen solubility are dependent on water temperature, this can also help establish water quality criteria for those metabolites.
Fish survival, even in the egg stage, is dependent on maintaining an adequate supply of oxygen. There is also some research that appears to show that larger eggs within a given
species are less sensitive to low dissolved oxygen concentrations(Einum et al, 2002) [although fish which produce larger eggs tend to produce fewer eggs which is why fish have not evolved to produce very large eggs]. Tolerance to low oxygen is species dependent. Eggs consume oxygen continuously. However, the consumption rate changes as the fish mature. After fertilization, when the shells harden permeability is limited and metabolite transfer is limited (including oxygen into the egg and carbon dioxide out). Green salmonid eggs will typically consume 3 to 4 nmol / egg / h. Therefore, in a Heath tray containing 10,000 eggs, we would expect the eggs to consume almost 21 mg/min.
At a water flow rate of 6 gpm (22.7 Lpm) this is a change in DO of almost 1 mg/L per tray. However, most incubation systems are designed with relatively large water surfaces which tend to take up oxygen at rates close to the consumption rate. Trays are also designed so the water cascades from tray to tray, improving oxygen transfer. As the cells in the eggs divide, oxygen consumption increases but remains low compared to later life stages. In salmonids, for about two weeks after egg hatch occurs, the gills and scales are not fully developed and metabolite transfer primarily occurs through the skin.
It is normally assumed that carbon dioxide production is directly proportional to oxygen consumption. However, since some oxygen is incorporated into the cells, a direct molar ratio has never been identified. Salmonid eggs are generally tolerant to super-saturation, although operators should be aware that bubbles forming on the eggs can cause them to float. Alevins will develop all of the symptoms of gas bubble disease (hyperinflation of the swim bladder, cranial swelling, bubbles in the blood, exophthalmus [bulging eyes], swollen gills, pneumoperitoneum [gas in the abdominal cavity] or even bubbles in the yolk sac). Gas bubble disease impacts are often greater in smaller fish in confined tanks. Many fish species will swim deeper to counter super-saturation, but in hatcheries, the fish cannot swim deeper than the bottom of the tank.
Early rearing troughs
Vertical incubation stacks installed inside portable shipping container
Silt and other fine particulates in the influent will coat the eggs, inhibiting the transfer of gases and other metabolites. As there is no feed consumed or faeces generated, there are very few solids generated within the system. However, if the shells are not removed after hatching, it can contribute to the development of fungus.
Nitro g e n
A limited amount of ammonia diffuses out of the eggs and no urea. Rather than store ammonia which is toxic inside the eggs, the fish store the waste nitrogen as urea. However, this urea is primarily converted to ammonia for release after the eggs hatch. Ammonia and urea production increases steadily throughout the alevin life stage reaching production levels 15 to 20 times higher than observed during the egg stages. Nitrite interferes with the ability of the blood to take up oxygen. Dissolved minerals such as chlorides, bicarbonates, calcium, phosphates and sulfates reduce toxicity. Eggs and alevins are typically less sensitive to nitrites than adult fish. Just like in more mature fish, chloride will protect alevins from elevated nitrite concentrations.
Article written by By KC Hosler, P.Eng and Stephen Piggott, M.Eng, P.Eng