Interventions to safeguard marble trout will be monitored through the marking/re-catching of the trout released in the three water courses being repopulated.
Marking of the 5,000 troutlet will be carried out on those that were bred until summer and are at least 6 cm long. Plantings will be made with a density of 1 fish every 10m2 (low-water area). The objective of marking/re-catching activities is to assess the colonisation and survival percentage of marble trout planted.
The assessment will be carried out during the electrofishing days which are part of the safeguarding activities that include the catching and transfer of alien species. In addition, all fish will be observed and the marked ones will be caught, measured and released again. In addition to measuring the survival between 0+ and 1+, after the first monitoring, subsequent monitorings will also enable the assessment of the population of marble trout, by defining the quantity of trout transferred with respect to those naturally present. Besides, as the population is structured into different age classes, it will also be possible to assess it according to the different classes. As for the monitoring of the hybrids, which will occur on a phenotypic basis, results are expected to occur from the third year after the planting, as we expect that the repeated removal of alien trout and planting of marble trout will have affected the presence of the various alien/hybrid/marble types. Currently there is no quantitative data available concerning the composition of the fish population, but only semi-quantitative. A percentage ratio of 90/5/5 respectively of alien/hybrid/marble trout has been hypothesised. After the first three years of planting/removal activities, we expect a ratio of 20/40/40, which will progressively reach a 10/30/60 ratio at the end of the project and a 10/20/70 ratio 6 years after the first planting.
Starting from year four (fish 3+), acclimatised marble trout will have reached sexual maturity, so it will also be possible to assess natural reproduction by counting low-water redds in sample areas. Monitoring will continue for 5 years after the end of the project.