Prevalence of stocked whitefish in River Kemijoki, Finland, inferred by micro X‐ray fluorescence analysis of otoliths

Abstract

Micro X‐ray fluorescence (µ‐XRF) analysis of otoliths was evaluated as a method to estimate the proportion of stocked one‐summer‐old whitefish Coregonus lavaretus L. in catches of adult fish (n = 20) ascending the River Kemijoki to spawn. Laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) analysis was applied as control. Polished otoliths were scanned with µ‐XRF to obtain strontium maps that were used to infer visually the provenance of the whitefish. Thirteen of the fish showed signs of being stocked as one‐summer‐old fingerlings. LA‐ICP‐MS was applied to determine the elemental composition in a spot outside the core of the otolith. The results were largely consistent with the visual inspection of the µ‐XRF mapped otoliths. In conclusion, µ‐XRF mapping successfully identified whitefish stocked as one‐summer‐old fingerlings. The vast majority of whitefish returning to the River Kemijoki to spawn were stocked fish.

Micro‐X‐ray fluorescence image analysis of otoliths to distinguish between wild‐born and stocked river‐spawning whitefish captured in the Baltic Sea

“…To reinforce the whitefish stocks, millions of one‐summer‐old fingerlings and tens of millions of free‐embryos are stocked annually along the Finnish coast, with a major proportion stocked into the mouths of dammed rivers in the northern Gulf of Bothnia (Jokikokko & Huhmarniemi, 1998). Stocking of one‐summer‐old fingerlings is considered to be more effective than stocking of free‐embryos, an assumption supported by Finnäs et al (2020). To obtain the whitefish used for stocking, mature female and male whitefish are caught during the spawning season to be stripped of their eggs and milt, respectively.…”

“…By contrast, River Kemijoki is the longest river in Finland (≈550 km) entering the sea ≈20 km southeast of the River Tornionjoki (Figure 1), and has a hydroelectric dam located 5 km from the river mouth, acting as a barrier to spawning grounds further upstream. To counter the strong decline in whitefish spawning in the River Kemijoki, annual supplementary stockings of fingerlings (≈3.1 millions) and a larger amount of surplus free‐embryos occur below the lowest dam (Finnäs et al, 2020; Jokikokko & Huhmarniemi, 1998). As a consequence, whitefish ascending the River Kemijoki were dominated by fish stocked as free‐embryos and fingerlings, the latter contributing 65% to whitefish in catches (Finnäs et al, 2020).…”

“…To counter the strong decline in whitefish spawning in the River Kemijoki, annual supplementary stockings of fingerlings (≈3.1 millions) and a larger amount of surplus free‐embryos occur below the lowest dam (Finnäs et al, 2020; Jokikokko & Huhmarniemi, 1998). As a consequence, whitefish ascending the River Kemijoki were dominated by fish stocked as free‐embryos and fingerlings, the latter contributing 65% to whitefish in catches (Finnäs et al, 2020). Around the Åland Islands, the southern foraging grounds for the river‐spawning ecotype, supplementary stocking of freshwater‐reared free‐embryos and fingerlings, mainly of the sea‐spawning ecotype, is also undertaken to increase the local catches (Leskelä, 2008; Linden et al, 2019).…”

“…Stocked whitefish display a low concentration of strontium (≈200–2300 μg/g) in the central part of the otolith, reflecting the low, ambient concentration found in the freshwater ponds where the fingerlings were reared before stocking (Finnäs et al, 2020; Lill et al, 2018). This can be contrasted with otoliths from sea‐spawning whitefish in the Gulf of Bothnia, which display a strontium concentration of ≈2600–4400 μg/g, reflecting the salinity‐range of ≈1–7 PSU in the north–south direction in this area (Finnäs et al, 2020; Lill et al, 2018).…”

“…Stocked whitefish display a low concentration of strontium (≈200–2300 μg/g) in the central part of the otolith, reflecting the low, ambient concentration found in the freshwater ponds where the fingerlings were reared before stocking (Finnäs et al, 2020; Lill et al, 2018). This can be contrasted with otoliths from sea‐spawning whitefish in the Gulf of Bothnia, which display a strontium concentration of ≈2600–4400 μg/g, reflecting the salinity‐range of ≈1–7 PSU in the north–south direction in this area (Finnäs et al, 2020; Lill et al, 2018). Finnäs et al (2020) use a combination of µ‐XRF mapping of otolith strontium distributions and spot analysis using LA‐ICP‐MS to estimate the proportion of whitefish stocked as one‐summer‐old fingerlings in catches of adult fish ascending the River Kemijoki to spawn.…”