Unifying waste conversion and endangered species conservation in polyculture production with Lake Constance whitefish and noble crayfish

Bioconversion is a sustainable method to recycle nutrient-rich aquaculture waste. This study explored waste bioconversion in the context of species conservation through a polyculture system involving the endangered Lake Constance whitefish ( Coregonus macrophthalmus ) and European noble crayfish ( Astacus astacus ). Whitefish served as the primary species producing solid waste (faeces and uneaten feed), while crayfish acted as an extractive recycler. Preliminary trials showed that crayfish which fed solely on whitefish faeces had a 59 % (95 % CI: 25–78 %) reduction in specific growth rate (SGR) compared to those fed on whitefish feed. However, mixing wheat with faeces in a 1:1 ratio offset the growth reduction, so wheat was supplementarily provided to crayfish in the following polyculture experiment. The present study was conducted in a cumulative treatment design: whitefish monoculture, non-separated and mesh-separated whitefish-crayfish polyculture. Growth and survival of both species were assessed, alongside dietary contributions to crayfish from whitefish solid waste and supplementary wheat using stable isotope analysis. Survival analysis revealed a negative interaction in non-separated polyculture, adversely affecting whitefish survival. Mesh separation eliminated interspecific interaction but restricted whitefish access to uneaten pellets, reducing whitefish SGR by 0.16 (95 % CI: 0.12–0.31). Meanwhile, assimilation of solid waste by crayfish increased from 33 % to 45.6 %, probably due to their consumption of uneaten pellets. In conclusion, mesh separation improved species compatibility in polyculture, with crayfish efficiently converting uneaten pellets into biomass, outperforming their use of whitefish faeces. Introducing a detritivore species with a trophic level lower than crayfish may enhance bioconversion efficiency and system sustainability even further. • Species conservation and waste bioconversion was achieved by polyculture. • Crayfish was used as an extractive species in polyculture with whitefish as the primary species. • Stable isotope analysis was used to assess waste bioconversion by crayfish. • Mesh separation prevented interspecific interactions and enhanced waste bioconversion. • Waste conversion by crayfish was more efficient with pellets left over by the fish than with faeces.