Experimental Trout Stocking Program in Europe

The critical need to increase the population of salmonids, especially brown trout, in our rivers was recognized a long time ago. Natural spawning of fish has dramatically decreased in recent years for many reasons. Some of the spawning habitats no longer exist, and the existing ones are either only partially effective, or there simply is no fish to spawn. However, by implementing egg boxes we can try to increase the population of brown trout and reinforce its spawning.


The history of egg boxes started in the 1920’s in Canada and the USA. On the Pacific coast, it was experimented with big boxes filled with gravel and eyed ova of salmon and trout. These boxes were called Harrison’s boxes. They were so big, that at least four men were required to move them. Another disadvantage of these boxes was their need to be filled with gravel and eyed ova, yet they produced almost the same amount of adult fish as would have been achieved through natural reproduction, or by releasing fry into the rivers. In 1949, with regards to the principles of Harrison’s system, Dr. Richard Vibert proposed a sophisticated method of ova incubation in a small, biodegradable box, designed for the hatched fry to escape.


This box is called Vibert box®. There are various types of the Vibert box®, probably the most common one being Whitlock – Vibert box®, developed by Dave Whitlock and fly fishers from Oklahoma. Other types of these boxes occur in France (Alevi box®), Italy (scatole Vibert,) Germany (Schachtel Vibert box), Canada (Jordan-Scotty incubator®), or Finland.


All these boxes are made to incubate salmonids eyed ova. In Slovakia, it is mainly brown trout. While some of them are made to only harbour the ova and the hatched fry escapes, others are more chamber-like and serve as a shelter for the hatched fry as well.


Depending on the box type, it can store from 500 to 800 ova. The boxes are placed into flowing streams, ideally into tributaries that join larger streams or rivers. Because of the natural aquatic environment that these fish are born into, they develop natural habits and behaviour of wild fish. These fish recognize the chemistry of waters, and return to spawn in their mother streams.


The objective of installing these boxes is to replenish the population, as well as to reinforce the spawning of brown trout and other salmonids in rivers. This restoring process takes several years. If the boxes are placed into a suitable stream and are properly taken care of, we can expect approximately 80% of the ova to hatch. In natural spawning, this number is significantly lower. The downside of these boxes is their maintenance in the winter – but what wouldn’t we do to save the brown trout?Furthermore, it is important to pay attention to which „genetic material“ do the ova come from. It would be ideal to work with ova of wild trout, which naturally spawn in small, forgotten tributaries.


Before we start to work with the boxes, it is of great importance to choose the right river tributary. It is unsuitable to install the boxes directly into bigger rivers. Look for flowing streams with sand – gravel – rocky bottom without migration barriers; such streams that trout seeks for spawning. Gather information from your older colleagues, and ask whether there has been any spawning in this stream in previous years. It is also a good sign if there still is some trout present in the stream.


Beware of sedimentary streams which flow through agriculturally used areas, since these can easily get cloudy. Same counts for streams with steep earthen watersides. Also, beware of streams prone to being polluted, as well as ones that freeze to the bottom in the winter, or dry out in the summer. But even a stream in the mountains may not be suitable in case of logging activity in the surrounding area. Trout seeks shelters. Meanders, cascades, undermined watersides, thresholds, rocks, fallen trees – the presence of all these in the stream increases the likelihood of hatched fry staying here. These shelters also protect the fish from predators. In case there are no such shelters in your stream, you can always build cascades, thresholds, to make it more attractive for the fish. At the same time building these helps to retain water, especially in the summer when the flow is lower.


After we have chosen the right stream, we need to find suitable spots for installing the boxes. We should look for deeper water, for the reason of a weak flow during the winter months. It does not have to be too deep, knee-level is just fine. An example of a good spot is just below a cascade. Be careful not to place the case with boxes directly underneath the waterfall, rather a bit further down. It is important to make sure that the flow of the water will not wash and carry away the stones from the case, which is most likely to happen in the spring, when the flow is stronger. It is therefore important to attach the case with boxes to something so that it doesn’t get washed away! We recommend building cascades in the spring, because then the falling water will create a deep-enough pit at the bottom throughout the year. If you build the cascade in the autumn, you will be surprised how the strong spring flow changes the structure of the bottom together with your case.


Most of the breeders have eyed ova of brown trout available in December/January. However, to be sure, it is a good idea to make a reservation earlier in the season. It is a great advantage if you can secure ova from your own reliable sources, since obtaining them can be a challenge at times.


It is recommended to place the boxes into bigger, stiff plastic cases (such as the ones from fruit). Always remember to tie, or otherwise attach the boxes onto the case with a strong and long twine, so that the boxes won’t get carried away in case of a strong flow. The twine should be long enough so that we do not have to take the whole case out of the water while we manipulate with the boxes.


There are various methods for installing the boxes. Commonly, they are placed in plastic cases together with rocks. This is also the way we do it, since it enables us to take the boxes out and check on the ova once in a while. It is also possible to place the boxes separately in between rocks on the stream bottom. We are also experimenting with new ways of placing them.


Most important principles in installing the boxes:

  • Use a heavy plastic case to put the boxes in, so that it does not get carried away
  • Eliminate the amount of daylight accessing the ova, to lower the risk of fungus infection
  • Ensure that the water flows towards the ova as much as possible
  • Place the boxes in the case so that they are not all the way at the bottom, to avoid coverage by sediments
  • Work with the ova in temperatures above 0°C (32°F)
  • Secure a slow transition from water in the carrying box to water in the stream, with regards to the water temperature


In the following we describe how we proceed in installing the boxes in our organization:
We place the ova into the special boxes, either indoors, or by the stream, depending on the air temperature. If it is 0°C and lower, we do it indoors. After we place the case with boxes in the stream, we let it stay there for 2 weeks, depending on the weather. In case the temperatures are low, and the case is underneath a layer of ice, we do not need to check them. Only if we are worried that they may freeze. When we go to check on the boxes, we bring a plastic bowl, a knife, a spoon, a siff with slightly bigger holes, a towel, and a second plastic case. We use the second plastic case to put the rocks in, as we take them out of the one under water. This way we will have them prepared again at the end, so we won’t need to search for them in the snow or water.


We use the knife to open the boxes and then transfer their content into the bowl. Using the spoon (or a different tool), we remove dead, damaged or moldy ova, strain them through the siff to filter any dirt or filth, and finally transfer them back to the box. While we work with the next boxes, the cleaned ones should be in the water. We proceed in this manner, until we have cleaned all the boxes. We strive to minimize the time the ova are out of water as much as possible. We use the towel to dry and warm up our hands. Approximately after 2 weeks we repeat this procedure, until all the ova have hatched. In case of cloudy water, it is a good idea to check and clean the ova more often. Important note: In the time of hatching, make sure to approach the case with the boxes from a different direction than up-stream, so that you avoid stepping on the hatched fry hiding at the bottom.


We work with 3 types of boxes in our fishing organization. The one we use most commonly is the original french Vibert box, which we call „the little Frenchman“. This small box can store up to 900 ova. It is biodegradable, so it only lasts one season. As mentioned earlier in this article, this box is made to only harbour the ova, while the hatched fry escapes. Manipulation with this box is simple, we just need to practice opening it with a flat tool, such as a table knife. Since it is biodegradable, there may appear some defects, such as cracks. However, it does not impair the box’s function as long as the ova stay in it and don’t get washed away. The drawback of this type of box is the fact that it only lasts one season. It is also likely to get covered by sediments in cloudy waters, if there is not a sufficient space around it, or a sufficient flow.


Another type of box we work with is Whitlock – Vibert box. We call him „the American“. It has two chambers, it can store up to 500 ova, and it is bigger in size than the french Vibert box. After hatching, the fry falls through to the other chamber, where it safely lives of the yolk sack until it loses it, and then escapes from the box. The advantage of having two chambers is also sediment filtration. This box is easy to open even with numb fingers and it lasts for several seasons. The drawbacks may be the greater size and the lower number of ova stored. Furthermore, it is not biodegradable so it does not decompose in nature in case of loss. If installed incorrectly, it is likely to get covered by sediments.


Third type of box we use is Alevi box, which we call „the big Frenchman“. It also has two chambers and can store up to 8000 ova. Unfortunately it did not live up to our expectations. The drawback of this box is the challenging maintenance of the ova, plus it is very prone to accumulate sediments inside. The price of this box is high and the size big. This season we tried to install it at a slightly higher level than the bottom of the stream, and also made sure that the water flowing through is fast enough. So far it seems to have positive outcomes, but we will see in the spring. The producer recommends putting the ova in just before hatching. If we can prevent the sediments from accumulating inside of it, this box can replace almost 9 Vibert boxes or 16 Whitlock boxes. We can use it for many seasons. In theory, it could be used for ova of grayling, since the development of these is much faster compared with trout, resulting in a lower risk of the ova getting covered by sediments.


To get a picture of how successful the boxes were, we need to extract fish from the stream in the autumn months, where the boxes were placed. It is important to note that we should not release any other artificially raised fish of any age category, so that they don’t interfere with our evaluation. We recommend keeping written record of the number, approximate age, and size of the caught fish, perhaps even take photographs of their head details. If you have the possibility to mark the fish in some way, do it. We used special colours VIE by NMT from USA and marked each caughed fish, of all age categories and size. Release the fish you catch in the main river that your tributary joins. Let’s say up to 2 km down the stream, from the confluence point. If you photographed and marked each fish, then note down the spot where you released it on the map. Why to do this? The reason of for keeping evidence is to find out whether these fish will return to their mother stream. So if in a year or two you come across the marked fish in the tributary, you will know that you are doing it the right way. And if you have photographed it, measured its’ size and noted down the spot of release, you can find out how far it traveled to come back, and how much it has grown. After a few seasons of installing the ova boxes you can try to switch to a 2-year extraction cycle.


Various negative factors influencing not only the population of brown trout should make us question whether the traditional ways of restocking fish populations is still efficient. The process of buying and releasing fish from intensive artificial breeding stations seems to be the easiest, yet inefficient. Fishing organizations should consider implementing new, innovative and sustainable ways of replenishing fish populations. Moreover, every fishing organization and its members should prioritize maintaining the mother fish, and support natural spawning. Implementing the eyed ova boxes is one of the ways. Merely the fact that fish hatched in this way live their lifes from beginning in their natural environment, should sufficient for their implementation.


Words from MO SRZ Liptovský Hrádok and translation from Lea Kurpasová.

Photos courtesy of Kurt Konrád, check him out on Instagram at @konradproduction





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