Evaluating ⁸⁷Sr/⁸⁶Sr isotope ratios and Sr mass fractions in otoliths of different European freshwater fish species as fishery management tool in an Alpine foreland with limited geological variability

Abstract

The focus of this study was to assess the potential of otolith microchemistry as a fishery management tool for different European freshwater fish species in an Alpine foreland with a diverse range of different water bodies but low geological variation. 87Sr/86Sr isotope and Sr/Ca ratios in water samples from 26 habitat sites in a pre-alpine catchment region around lake Chiemsee, Germany, an important region for recreational and economic fisheries, were analysed. 87Sr/86Sr isotope ratios and the Sr mass fractions in otoliths of 246 fish out of 16 species were determined using (laser ablation) inductively coupled plasma mass spectrometry ((LA)-ICP-MS). Habitats could be discriminated into three distinct strontium isotope regions (SIGs) and seven clusters with characteristic 87Sr/86Sr isotope and Sr/Ca ratios. The direct comparison of 87Sr/86Sr isotope ratios in water and otolith samples allowed to identify fish that might have been a) migrating b) transferred from other water bodies or c) stocked from fish farms. Sr/Ca ratios in water and the Sr mass fraction in otoliths were highly correlated, although significant differences between species from the same environment could be documented. Sr mass fractions in sagittae of Perca fluviatilis were about 60 % of those in sagittae of Coregonus spp and of lapilli of roach Rutilus rutilus from the same habitats. Different partition factors for water to otolith Sr/Ca mass fractions were determined for different species. Discrimination of fish otoliths by 87Sr/86Sr isotope ratios and Sr mass fractions according to habitat clusters was possible with success rates ranging from 92 % to 100 % for cyprinids, European perch Perca fluviatilis, whitefish Coregonus spp. and European grayling Thymallus thymallus, and was 74 % for salmonids. Otolith microchemistry proved to have great potential to serve as a fishery management tool at smaller spatial scales such as in the studied Alpine foreland when considering the limited variation of 87Sr/86Sr isotope and Sr/Ca ratios, the type and spatial distribution of habitats, and the species and question under investigation.

Microchemical provenancing of prey remains in cormorant pellets reveals the use of diverse foraging grounds

“…To come up with an adequate range of data using the standardization procedure, we calculated a potential maximum of 950 μg/g Sr for pike otoliths, and introduced a potential minimum of 300 μg/g Sr and a potential maximum of 1,350 μg/g for burbot. For salmonids and eel no clear relation could be documented by Zitek et al (2021) because these fish are often stocked. For these species, the existing values were standardized between 0 and 1 in direct relation to the water samples assuming that the fish and water samples had a similar span.…”

“…This may be because, depending on the taxon, the habitat cluster including Chiemsee also includes running waters T A B L E 1 Percentage of otoliths of known origin (data obtained from Zitek et al [2021]) correctly assigned to the respective habitat cluster (HC) in the study region of Chiemsee in Germany, 2012-2014, and share of otoliths collected from cormorant pellets assigned to the species-specific HC. The values in brackets are the percentage for correctly assigned otoliths of known origin when excluding otoliths of individuals showing a deviant otolithmicrochemistry because of stocking or migration.…”

“…Although intermediate habitats might introduce some ambiguity to the results of the assignment per taxon, the accuracy of the assignment tested via otoliths of known origin ranged between 95% and 56%. Besides, Zitek et al (2021) measured mismatches between 87 Sr/ 86 Sr isotope values of otoliths and their waterbodies of origin and identified respective fish as most likely to be stocked or migrated. This suggests that the differences in assignment accuracy do not directly depend on the taxon but on the potential to change habitat.…”

“…For this analysis, the water data and otolith data needed to be standardized between 0 and 1. We standardized Sr isotope data of all water samples and all otoliths (of unknown origin and from Zitek et al [2021]) together between 0 and 1 considering that minimum and maximum values of water and otolith samples were overlapping within their associated expanded uncertainties. A different approach had to be used for Sr/Ca ratios because this ratio in water is due to discrimination effects only reflected in otoliths as a certain proportion (Campana 1999) and species‐specific differences have been documented by Zitek et al (2021).…”

“…First, we determined the minimum and maximum values of the Sr/Ca ratio in all habitats. Next, based on regression formulas provided by Zitek et al (2021), we calculated the potential minimum and maximum Sr concentration values of perch, whitefish, cyprinids, pike, eel, and burbot and compared them to the values of the analyzed otoliths. To adjust the spread between water and otolith samples and to make them comparable, we used theoretical minimum and maximum Sr mass fractions expected in otoliths from these habitats for the standardization of otolith Sr mass fractions between 0 and 1 where needed.…”