Occurrence of juvenile salmon, Salmo salar L., from natural spawning in the Słupia River (northern Poland)

Abstract

The native population of the salmon, Salmo salar L., in the S³upia River probably became extinct in the 1960s. The restoration of this species began in 1986, and is based on the population in the Daugava River. As indicated by historic data, salmon was decidedly less abundant than was sea trout, Salmo trutta L. This remains unchanged today. To date, it has been impossible to differentiate positively smolts of stocking origin from those of natural spawning. Since 2006, all salmon smolts released into the S³upia River have had their adipose fins clipped, which allows for easy identification. The aim of the current study was to present how many of the juvenile individuals caught in the S³upia River originate from natural spawning. During the 1999-2008 period, a total of 25 juveniles were caught that were classified as wild specimens. Although salmon spawning and redds were observed earlier, the catch of juvenile species is evidence of successful salmon spawning in the S³upia River. The occurrence of salmon, Salmo salar L., in the S³upia catchment was reported as early as in the 1920s (Henking 1929), and later by Chrzan (1947). The analysis of contemporary fishery catches lacks data that would distinguish decidedly between the sea trout, Salmo trutta L., and the salmon. Certain conclusions regarding abundance can only be drawn by comparing the sizes of the spawners caught. This indicates that salmon comprised just a small per- centage. Although Kaj (1954) describes the S³upia as a salmon river; however, calculations indicate again that the catch statistics regarding catches of salmon and sea trout lack distinct differentiation between the two species. Based on catches of spawners, Che³kowski (1966) concluded that the share of salmon in comparison to that of sea trout in Pomera- nian rivers was slight. The first full description of the