Scaling from eDNA to biomass: controlling allometric relationships improves precision in bycatch estimation

Abstract

Environmental DNA (eDNA) has attracted interest in relation to fisheries, with its possibilities for species identification and promises for species quantification. In the context of fisheries catches, eDNA can be most useful for the estimation of bycatch proportions. The assessment of species mixtures in large catches (>1000 t) is challenging, especially when morphologically similar species are to be differentiated. We used an experimental set-up to simulate industrial sprat fishery catches, and tested two types of water (blood water and discharge water) derived from this simulated fishery for their suitability in reliable species quantification. We analysed nine mixtures of sprat and herring—the main bycatch species. Species-specific quantitative PCR was used for species identification and quantification. Species-to-species weight fractions and eDNA fractions in mixtures showed a strong correlation. Accounting for size-based differences in DNA abundance (allometrically scaled weight) reduced the estimated standard error on weight fraction prediction from 0.064 to 0.054 in blood water, and from 0.080 to 0.075 in discharge water when comparing the weight-based model with the allometrically scaled weight model, respectively. Accounting for allometric scalling in genetic analyses of fisheries process water can serve as a more precise method for the assessment of bycatch, thus in a wider sense improve the quality of fisheries-dependent data.

The Application of eDNA for Monitoring Aquatic Non-Indigenous Species: Practical and Policy Considerations

“…The estimation of the number of individuals in a sample based on eDNA metabarcoding data is an area of active research, and methods are being developed to improve the accuracy of these estimates. Recent studies have found that the relationship between eDNA particle concentration and organism abundance in nature can be more reliably measured using allometric scaling [76,84]. This method describes how the relationship between an organism's body size and various biological processes can improve the accuracy of estimating organism abundance using eDNA predictive models [76,84].…”

“…Recent studies have found that the relationship between eDNA particle concentration and organism abundance in nature can be more reliably measured using allometric scaling [76,84]. This method describes how the relationship between an organism's body size and various biological processes can improve the accuracy of estimating organism abundance using eDNA predictive models [76,84]. The authors demonstrated how estimates of allometrically scaled mass derived from eDNA samples in 'unknown' systems can be converted to biomass or density estimates with additional size structure data [76].…”

“…The authors demonstrated how estimates of allometrically scaled mass derived from eDNA samples in 'unknown' systems can be converted to biomass or density estimates with additional size structure data [76]. Others have also shown how controlling allometric relationships can enhance the accuracy of estimating bycatch (the unintentional capture of non-target species) using eDNA as a tool to improve the accuracy of estimating the biomass of different aquatic species and to better understand the impacts of fishing practices on marine ecosystems [84]. Although it has not yet been applied to detect NIS, allometric scaling shows promise as a modeling approach to enhance the reliability of eDNA concentration as an indicator of abundance.…”

eDNA based bycatch assessment in pelagic fish catches

“…Likewise, monitoring of catch composition from the water surrounding the catch in trawl nets 34 , 35 has shown correlations between species biomass and their DNA abundance. Recently, very strong quantitative relationships between fish weight and eDNA proportions were demonstrated in “production water” surrounding mixtures of pelagic fish 18 , 36 . These qPCR-based results demonstrate that sufficient precision, accuracy, and reproducibility can be attained in order to develop eDNA-based methods as a practical, validated tool for routine monitoring and control of large pelagic catches, ultimately resulting in a paradigm shift for catch monitoring.…”

“…As a representative model system for large pelagic fish catches, we use the sprat fishery in the Baltic Sea with substantial bycatch of herring to evaluate our concept. This simple catch-bycatch mixed fishery consisting of European sprat ( Sprattus sprattus , L.) and Atlantic herring ( Clupea harengus , L.), is complicated to resolve due to the high morphological similarity of the species and high heterogeneity in specimen distributions within catches 36 . The importance of assessing the herring bycatch is underpinned by the fact that the fishery is limited by the long-standing critical condition of the spawning stock biomass (SSB) of herring which has led to significant reductions in the allowed catch to help population recovery 37 .…”