Utvärdering av ålyngelutsättningar i Mälaren och Ymsen

Restocking of imported eel elvers takes place in Sweden as one of the management measures within the national management plan for the European eel. Since it is difficult to conclude whether an eel is imported and restocked or a natural recruit, it is difficult to evaluate the effects of the restocking program. Therefore, in this study, the effects of restocking were evaluated by analysing data from specific experimental restocking events that were conducted with the purpose to follow the effects of eel restocking over time. In the two lakes Mälaren and Ymsen, experimental restockings of imported and tagged (PIT-tag or chemically marked with alizarin, barium, strontium) eels have systematically been monitored by either scientific- or commercial fishing, for over 20 years. Eels have been sampled from the fishing and dissected to collect data. The results from the monitoring show that a large proportion of the restocked eels remain in the area where they were released. Approximately 11–15% of the restocked eel, per restocking event, have been captured in the monitoring, and the majority were females. Within each lake, no differences in either condition, parasitisation of the swim bladder parasite Anguillicola crassus, or the age at the different maturation stages, were found, neither between the untagged eels (potential natural recruits) or tagged eels (imported and restocked), nor between different restocking events. The growth rate was higher for untagged eels compared to tagged eels in Lake Mälaren (however a small effect size) but still lower than for the eels in Lake Ymsen. There was also a difference in age of maturation from yellow to silver eel, between the two lakes, but the amount of eel at each restocking event (density) did not seem to have an effect. This indicates that it is the water system in where the eels were released that has the largest impact on how fast restocked eels grow and how quickly they reach the silver eel stage. The choice of which lake to use for restocking might thus be decisive for how quickly eels from restocking produce silver eels and thereby contribute to reaching the goal in the eel management plan, that 40% of a theoretical pristine biomass level shall be able to migrate towards the Sargasso Sea.