Does harvest of the European grayling, Thymallus thymallus (Actinopterygii: Salmoniformes: Salmonidae), change over time with different intensity of fish stocking and fishing effort?

Background. The European grayling, Thymallus thymallus (Linnaeus, 1758), is a fish species of high value in recreational fishing. The monitoring of changes in grayling populations is a high priority in fisheries. Data on the harvest of recreational anglers can potentially serve as an easy and inexpensive way to monitor changes in fish populations. This study aimed to assess spatio-temporal trends in catches of grayling in a larger geographical area. Materials and methods. This study analysed harvest rates of grayling by recreational anglers on 241 fishing grounds, in the Czech Republic, within 1986-2015 (30 years). Data from individual angling logbooks were used. The data were collected by individual anglers and processed by the Czech Fishing Union (Cesky rybarsky svaz). Results. Over the period of 30 years, Czech anglers harvested a total of 9 928 grayling specimens weighing altogether 3 357 kg. Within the period surveyed, both parameters (the grayling biomass harvested and the representation of grayling in overall fish harvest) decreased to 10% of the initial values. The percentage of fishing grounds with a harvest of grayling decreased to 30% of the initial values. Harvest per effort decreased to 20% of the initial values over 11 years. There was only a weak correlation between fish stocking and fish harvest. There was a negative relation between the number of angler fishing visits with both catch (fish number) and yield (biomass) of grayling. The harvest was positively correlated with fishing effort. The mean size of harvested grayling remained constant (similar to 0.35 kg) over 30 years. Conclusion. Harvest of grayling significantly declined over the last three decades, implying that increased effort in conservation of grayling is necessary. Future studies should focus on monitoring of the remaining self-reproducing grayling populations.