Using eDNA to reconstruct logbook information and improve estimates of by-catch

Recent discussions during the Indian Ocean Tuna Commission (IOTC) working party on ecosystems and bycatch (WPEB) found that despite improvements in the data reporting for sharks and rays, the overall data remains low quality and highly incomplete (WPEB17, 2021). It is estimated 50 per cent of all shark catches are aggregated and species level information for landings is unavailable. The WPEB have formally acknowledged this is a long-standing issue in reporting for sharks and rays (WPEB17, 2021). Artisanal fisheries operating in the IOTC contribute to the majority of reported nominal catches of shark and ray species, however only 5 per cent of catches are reported. Low reporting in commercial and artisanal fleets can be due to a range of reasons and any efforts to increase coverage are helpful for fisheries management.
eDNA is a promising tool for cost-effective, rapid and non-invasive fisheries monitoring. In recent years, the literature demonstrates a growing interest in the use of eDNA to monitor aquatic ecosystems for species that are cryptic or no longer visibly present in the sampled environment (Evans & Lamberti 2018). Two key features of eDNA are of particular interest. First, in natural systems eDNA has a limited residency time, in the order of a few weeks (Bista et al 2017). eDNA is sensitive to physical and environmental conditions including temperature, pH, UV and microbial activity which can degrade short fragments of DNA (Harrison et al 2019). This means that samples relate to a known and relatively short period of time, and thus are informative about the status of a system, instead of integrating over a long and unknown period. Second, the concentration of sequences extracted from eDNA have been shown to be proportional to the biomass or density of a detected species (Yates et al 2019). While this field of research is still in its infancy it does mean eDNA monitoring could be used to not only to confirm the presence of a species but also provide an estimate of its abundance in closed systems.