As the alevins consume (metabolise) the yolk to meet their energy needs their wet weight increases. This occurs because the tissue (muscles, organs, etc.) have a higher moisture content than the yolk does.
At the time of hatching, trout alevins have a large reserve of yolk remaining from the eggs which is why they are referred to as yolk-sac fry (Picture 1 & 2 in the previous chapter). In a yolk-sac fry the alevin wet weight is made up of approximately 70% yolk and 30% embryo. This yolk is denser than water causing the yolk-sac fry to live at the bottom of the incubators or early rearing pond. The membrane that surrounds the yolk is very sensitive to damage and external abrasion. Consequently yolk sac fry should be handled sensitively or not at all at this stage. As the alevins consume (metabolise) the yolk to meet their energy needs their wet weight increases. This occurs because the tissue (muscles, organs, etc.) have a higher moisture content than the yolk does. 1 gram of yolk is converted to 2 – 3 grams of tissue. The alevin weight continues to increase until just before the completion of the yolk absorption. This stage is called the Maximum Alevin Wet Weight (MAWW) and occurs at the optimal time of “ponding” the fish into the early rearing tanks and the start of feeding.
Figure 1. Maximum Alevin Wet Weight (MAWW)
It is very important to ensure that once the alevins have been placed in the early rearing ponds and at the time before and after “swim-up” that the water flows are correct. The flow should be adequate to supply all the alevins with sufficient oxygen but not too much that will cause the alevins to become exhausted from swimming against too strong a current. Ideally the alevins should be well distributed throughout the early rearing pond. If the alevins are all observed crowded at the inlet it is likely that the flow is inadequate to supply sufficient oxygen. If the alevins are all observed crowded at the outlet or pushed against the outlet screens it is likely that the flow is too strong. In both cases of inadequate or excessive flows high mortality is likely to occur. A useful tool is to use a substrate mat in the bottom of early rearing tanks. A picture on the following page illustrates a typical substrate mat. These are commercially available in a variety of styles and can also easily be made on site. The principal is that the alevin can find respite from the flow for some periods and are able to rest.The first feeding of trout is critical to ensure they get the best possible start in life. The type of feed, frequency of feeding and manner in which the feed is presented are all important. It is recommended that the alevins are fed the best diet available. Often these specialty diets may seem expensive but little feed is required and low cost diets are rarely the best ones for your fish.
It is recommended that a recognised feed supplier gives advice on the best feed suited to the local conditions of the site. Feeding charts are also available from most feed suppliers. Diets for fry and fingerling trout require a higher protein and energy content than diets for larger fish. Fry and fingerling feed should contain approximately 50 % protein and 15 % fat. Avoid the common mistake of grinding down (cheaper) feed for larger fish as this often does not contain the correct amounts or ratios of important ingredients needed by first feeding fish.
Substrate mat in bottom or early rearing pond
Once a quality feed has been selected and the amount of feed determined, the next consideration is how to feed the fish. First-feeding alevins should be fed a small amount by hand at least ten times per day until all the fish are actively feeding. The principal is less feed more frequently as the alevins need to be challenged by the feed to start feeding. After this period, an automatic feeder is most practical, with two or three additional hand feedings daily to observe the fish. As the fry grow, frequency of feeding can be gradually decreased to about five times per day. Trout can consume roughly 1% of their body weight in dry feed at each feeding, so frequency should be adjusted accordingly. Fry gain weight rapidly so they should be sample counted weekly for the first 4 to 6 weeks on feed and the daily feed ration adjusted according to their weight. Feed should be distributed over at least 2/3 of the water surface when fry are less than 5cm (2 inches).
Good feed distribution assures easy access to the feed and will help to achieve size uniformity within the population. If feed is only introduced at the inlet it is likely that only the strongest and largest fish will eat and there will not be uniform growth. Do not introduce the feed too close to the outlet screens or it may be washed out before the fish have a chance to eat it. Though the use of a published feeding chart is strongly recommended, charts are only guides and individual judgment should be exercised based on observations of the fish during feeding. Do not overfeed. Once feed settles to the bottom of the tank, small trout will ignore it. Excess feed leads to deterioration of water quality and promotes disease. Remove excess feed from the pond promptly.
Once all the alevins are actively feeding an automatic feeder can be used. Automatic feeders come in a variety of makes from a number of equipment producers. The principle in all automatic feeders for alevins is that small amounts of feed are distributed into the rearing tank constantly during the day. The most commonly used automatic feeder for alevins is the clockwork belt feeder. These are usually produced in a 12 and 24 hour option and work on the simple principle that a belt attached to a clockwork mechanism is evenly loaded with the day’s feed allocation. During the feed period the belt moves forward distributing small amounts of feed throughout the desired period. The only drawbacks of any automatic feeder is that it is advisable to have more than one in the early rearing ponds as a single feeder will distribute the feed in a single place
and often the larger and stronger fish dominate the area precluding smaller and weaker fish from obtaining adequate feed. It is also important to check frequently that the feeder is working and that the fish are receiving a regular supply of feed.
Automatic clockwork belt feeder
Clockwork belt feeder