Reintroduction of salmon in the Rur river in the Netherlands

For years, volunteers of VBC Roerdal and Arbeitsgemeinschaft Lachs und Meerforelle 2020 are working on the reintroduction of salmon (Salmo salar) on the Rur river.

Young salmon are hatched from eggs, reared in a nursery, and when big enough released in the upstream parts of the catchment of the river Rur. It is not clear however, how many smolts (young salmon) actually pass the ECI power plant in Roermond (the only measuring possibility for the volunteers) once migrating from these rearing grounds towards the Meuse, and hereby to sea. At the ECI power plant smolt migration is possible via three routes, the smolt trap, the eel pipe and the fish passage. A fourth migration path, circumventing the ECI, is via the Hambeek. Furthermore, during periods in the smolt migration individuals suffering from fungal infections are observed. It is not clear what causes these infections. However, increasing numbers of exotic crusteceans, like spiny cheeked crayfish (Orconectes limosus) and Chinese mitten crabs (Eriocheir chinensis) are observed as well and might be a source of infection.

In this study two research questions were addressed. The first question was whether the infections observed every year on several of the migrating smolts originated from exotic crustacean (i.e., spiny cheeked crayfish and Chinese mitten crabs) that occur in increasing numbers in the Rur river. The second question was how many of the annually migrating smolts were actually caught in the smolt trap.

To answer the first question, several infected smolts that were caught were brought to the laboratory for analysis. Analysis of the infected areas indicated that this comprised a secondary infection of Saprolegnia parasitica. Secondary in this case, means that the infection was not the primary cause of discomfort to the fish. Apparently, another factor was the primary cause after which the second infection could manifest itself. Especially low flow conditions, warm water and poor water quality promote infections of Saprolegnia parasitica. It was not clear how long the fish were already infected and therefore it could not be established where in the Rur catchment the fish got the infection.

To answer the second question, wild smolts were caught on four separate occasions. The smolts were marked and released upstream of the ECI power plant again. On run 1 and 3, the fish were released 285 m from the power plant and could pass via the smolt trap, eel pipe and fish passage. In run 2 and 4, the fish were released 2836 m from the ECI power plant and had an additional route to reach the river Meuse, i.e., the Hambeek. Due to the continuous monitoring of the smolt trap and eel pipe, the number of missing smolts comprised an estimate of the fish using the fish passage to pass the ECI power plant. E.g., when 100 smolts were tagged and released and 20 and 10 individuals were captured in the smolt trap and eel fyke, respectively, this resulted in (100 – 20 – 10 =) 70 smolts that used the fish passage (or were taken by predators). Knowing this number, the potential use of the Hambeek as migratory route could be estimated from the Hambeek runs. In addition, combining the two types of runs gave information of the predation pressure on the stretches of the river investigated in this study.

During 2012, 1321 smolts were caught in the smolt trap. Of these smolts, 716 individuals (i.e., 54% of the total number) were marked and used in te four experimental runs. During the procedures of required to populate the runs, only five of the 716 smolts died during the process. This very small loss (0.7%) indicated that the whole procedure only had a minor impact on smolt well-being. The results of both types of runs showed that the percentage recaptured smolts in the eel fyke was very small, i.e., 4% and 1% in the ECI and Hambeek runs, respectively. The percentages recaptured smolts in the smolt trap were surprisingly equal between the two types of experimental runs, i.e., 24 and 29 per cent, respectively. This rendered the percentage unknown (i.e., using the fish passage, being predated, or in the case of the Hambeek runs, taking the alternative migration route) of the two types of runs as very similar, viz., 72 and 70 per cent. This was considered as being equal.

On average, ([24+29]/2=) 26.5 per cent of the released smolts passed via the smolt trap. Together with the monitoring data of previous years this allows an estimate of the total number of smolts starting their migration from the upstream catchment of the river Rur. Here an estimated total number of 4749, 5815, 4624, and 5081 smolts migrated downstream in 2009, 2010, 2011, and 2012, respectively.
Comparing these numbers with the total number of juvenile salmon stocked in these upstream areas in the previous year it becomes apparent that 3.5 to 6.5 per cent of the stocked juvenile fish reaches the migrating smolt stage and passes the ECI power plant on their way to the river Meuse and consequently towards sea. Compared to the survival rate of juvenile salmon in the wild this is a very good result, indicating that the initiative of the volunteers is indeed successfully enabling salmon to start their migration to sea. Finally, the overall success of the initiative of the volunteers will depend upon the successful maturing in sea and the return of these animals to suitable spawning grounds.