Our sea-trout stocks can still be improved – if we do it right!

While the populations of sea-trout (sea-run brown trout, salmo trutta) are declining, or even disappearing, in many countries, Denmark has been experiencing a steady increase in populations during the last 20 years. In this article, we present the Danish trout management that has caused this positive trend. Denmark is a rather flat country with very high degree of intensive agriculture. We have several hundred small rivers and streams supporting sea-trout populations. Approximately 25 of the larger streams have a regular recreational fishery for sea-trout. Coastal fishing with lures and flies has become very popular and most sea-trout are caught here. The Danish rivers have been heavily modified (channelized, dredging, gravel extraction, damming), mainly by farmers, but importantly also by the more than 400 fish farms that once used river water in the production of rainbow trout. Today, there are much fever fish farms and they are increasingly changing to recirculation of the water, to the great benefit of the fish in the rivers.  The water quality is generally not a problem for the trout, but the lack of elevation is critical, so in a river that only drops 20 meters from source to outlet, even small dams or weirs can be detrimental even if there is good passage, due to the loss of spawning habitat. In the following we briefly introduce the research, management and practical measures, which have greatly benefitted our natural stocks of sea-trout.  

The natural production and stocking of trout in the Danish streams has increased since the 1950ies. Consequently, the production of trout smolts has gradually increased. We believe that in-stream restoration and stocking has led to growing populations of sea-trout throughout the last decades.

As in-stream habitat improvements became widespread, the natural production increased and subsequently the need for stocking is now decreasing. This development has really kicked off the last decade and several rivers have been taken out of the stocking program, but still 6 million DKK are used annually for fish stocking in our rivers.

The improvement of physical and chemical conditions in the streams are led by municipalities and state agencies, spending a two-digit million DKK annually on river restoration projects.

The management is focused on:

• Habitat improvement
• Stocking of juveniles in various life stages
• Regulation of the (recreational) fishing

The research conducted within these fields contributes to a science based management. A uniquely close cooperation between science, advisory agents and legislation secures a swift implementation of the latest knowledge into management. In Denmark there is a long tradition for research into sea-trout biology and scientist and advisors disseminate the latest results on the website www.fiskepleje.dk.

The website has gained considerable popularity and a monthly newsletter is distributed (electronically) amongst the thousands of users, who are interested in; or directly working (as volunteers) to reestablish large, healthy runs of sea-trout. This direct communication to stakeholders is crucial in bringing the latest knowledge and best practice to the people working on projects to benefit the trout populations.



The natural trout stocks in Denmark are generally improving. Every year, DTU Aqua surveys numerous Danish river-systems to monitor the development of the trout populations. Results from these annual electro-fishing surveys document this improvement and clearly show that rivers (or river stretches) where restoration measures have been actively carried out carry much higher trout densities. These measures include reestablishment of connectivity (removal of migration barriers) and restoration of suited spawning and rearing habitat. Research demonstrated that more and more streams achieve a full natural production of juvenile trout and thus the need for stocking ceases.


Stronger trout populations require active measures

The trout is a good indicator of environmental quality and a healthy natural production of juveniles is a sign of good ecological quality of the river. Thus, both anglers and environmentalist in general are very pleased with the improvement of the natural trout populations.


More and larger spawning areas

Good ecological quality does not only require good water quality in a stream, but also varied habitats, including spawning substrate (gravel), pebbles, rocks, woody debris, fallen tree trunks, roots and branches. Restoration measures like deposition of spawning gravel and larger rocks are measures in the process of achieving the aims set forth in the EU Waterframework directive. This is of direct benefit for the population of trout and will cause increased smolt production and larger sea-trout runs. 
In Denmark we have a tradition for involving organized (in clubs) anglers in the management measures, like rearing and stocking of fish as well as in-stream habitat improvement for trout and other fish species. An evaluation of projects, carried out by anglers, showed that deposition of spawning substrate on average cause a 4-fold increase in abundance of trout juveniles. Such significant improvements can be achieved by simple measures, locally enhancing the conditions for the fish as well as engaging the local community.

Free passage means more trout 

In accordance with the Waterframework Directive, the authorities must implement measures to ensure that streams fulfill the requirements for at least good ecological quality. The “Good” status dictates good connectivity, i.e. relatively free of obstacles to migration, so that the fauna can move around in the system. This particular requirement about good connectivity is very important for the sea-trout, being so dependent on the ability to move freely between spawning areas and the sea.

Local studies have documented a very strong and immediate positive effect of barrier removal on trout populations. The best results are seen when a barrier is completely removed, thus giving the possibility to reestablish the original slope and substratum of the given river stretch. This provides not only free up- and downstream passage but also spawning possibilities in the area.

Some obstacles prevent fish access to large well suited spawning and rearing areas. It is often possible to enhance downstream habitat by restoration measures and deposition of substrate, but experience clearly proves that the final removal of the obstacle, gives immediate access to thousands of square meters of good habitat. Such areas cannot be created, even by intensive downstream restoration.

Currently numerous Danish dams and weirs are being removed/breached, and this will most certainly cause further increase in sea-trout runs the coming years and even some highly degraded stretches, have been restored and now provide good habitat for trout and other fish species.


Reestablishing natural condition gives the best value for the money invested 

Sea-trout are fully dependent on the possibility to migrate between spawning areas in the upper streams and the feeding grounds along the coast or in the open sea. Research has clearly shown that even small obstacles have very negative effects on the stocks. Several studies have documented significant loss of downstream migrating smolts when passing dams or weirs. This loss is mainly caused by predation on halted/delayed fish and can be of 80 % even at small obstacles.


Small streams matter

In Europe, the general focus has been on larger rivers as being important for the production of sea-trout. In DK, there are sea-trout stocks in almost all small streams, provided there is passage and some natural spawning habitat. As this type of small streams is dominant in DK, they are essential for a large and healthy sea-trout population.

Despite the great improvement of the stocks, lots of potential remains in our rivers. Fortunately, we do have the knowledge required for creating even larger runs – it is now merely a question of political will (and courage) to implement the necessary measures to restore natural conditions for the trout.

National waterplans dictate the restoration of 1/3 of app 3000 km streams that are currently running in underground pipes throughout the country. Such a measure will contribute significantly to a better environment in general and provide habitat for fish and invertebrates.

Also in the small streams removal of obstacles has a huge positive effect, when the whole structure is removed and the original slope and flow conditions are restored.


Deposition of spawning substrate cause great increase in the number of trout

Despite all the positive measures, of which the removal of barriers have been the most important, there is still a long way to go before all our sea-trout and salmon stocks are at a “natural high level” without stocking. If we continue the work and use the experience gained by research, we will still see more and more rivers with large, healthy runs, even without stocking. One current and future threat for the stream fauna is the sediment load due to run-off from the intensive farmed fields, surrounding most streams in DK. The solution to this widespread problem is the compulsory establishment of a buffer zone along both sides of the streams, preventing sand and soil to directly wash into the streams.

DTU Aqua annually survey app 1000 locations around the whole country. The main purpose is to monitor the density of wild trout juveniles. The survey program has been carried out for the last 30 years and is quite unique in the world of trout monitoring. The large database contains a wealth of information and when we conclude that the trout stocks are increasing, it is mainly based on results from this survey. When discussing the need for a continued work for improving connectivity and habitats, it is crucial to have such solid scientific documentation of the positive effects of the measures.



Every year in August-September, DTU Aqua monitor the natural production of trout in a number of river systems, including very small streams. This is done in close cooperation with local angling associations. In this way, every stream will be monitored every 7th year, enabling evaluations of the development in each station, stretch, stream, river system, local area or nationwide. The results are published in reports that are distributed among managers and stakeholders. The reports show the actual need for stocking at each site and recommend measures that can improve the conditions if needed, including creation of spawning and rearing habitat. The reports are available on www.fiskepleje.dk



Our future sea trout stocks will be wild

Current Danish and international research has proved the superiority of wild fish compared with hatchery on almost all fields. Thus, wild fish are much better to reestablish natural stocks than offspring from hatchery fish. Thus, since 2006, only offspring from wild fish has been used for stocking. Furthermore, juveniles must come from broodstock caught in the same (or nearby) river.

Fisheries regulation is one of the 3 main areas in the management, and is concurrently being adapted to the need in each stream and the adjacent coastal area (estuary, fjord). Relevant is legislation (rules) regarding fishing season (closing periods), gear limitations, legal size (40 cm for sea-trout), bag-limits and protected areas around outlet of all streams (on the coast). These closed areas are maintained year-round to protect sea-trout spawners entering or leaving the stream. Another set of rules, regulate the type, amount, location and season for the use of different types of nets, aiming at avoiding by-catch of sea-trout smolts or adults.


Af Finn Sivebæk og Niels Jepsen DTU Aqua. Institut for Akvatiske Ressourcer.  

17 APRIL 2024