This study demonstrates the potential of using environmental DNA (eDNA) from seawater samples to detect a diverse marine fish fauna. Researchers collected ½-litre seawater samples from a temperate marine ecosystem in Denmark and used next-generation DNA sequencing of PCR amplicons to identify eDNA from 15 different fish species, including both commercially important species and rare or unrecorded species. They also detected eDNA from a rare vagrant species, the European pilchard, and four bird species. National databases confirmed the occurrence of all detected species. The study compared the eDNA approach with nine conventional methods used in marine fish surveys and found that eDNA covered fish diversity better than or equal to any of the conventional methods. The eDNA approach was also shown to detect a wide range of local fish species, even in small seawater samples. Additionally, an experiment on eDNA degradation in seawater showed that even small (100-bp) eDNA fragments degrade beyond detectability within days. The study highlights the potential of eDNA for future marine biodiversity and resource monitoring, despite challenges in understanding eDNA dispersal in marine environments. The results suggest that eDNA from marine water samples is likely of local origin, making the eDNA approach a promising tool for detecting unexpected species in marine ecosystems. The study also emphasizes the need for further research to validate the eDNA approach in varying environmental conditions.This study demonstrates the potential of using environmental DNA (eDNA) from seawater samples to detect a diverse marine fish fauna. Researchers collected ½-litre seawater samples from a temperate marine ecosystem in Denmark and used next-generation DNA sequencing of PCR amplicons to identify eDNA from 15 different fish species, including both commercially important species and rare or unrecorded species. They also detected eDNA from a rare vagrant species, the European pilchard, and four bird species. National databases confirmed the occurrence of all detected species. The study compared the eDNA approach with nine conventional methods used in marine fish surveys and found that eDNA covered fish diversity better than or equal to any of the conventional methods. The eDNA approach was also shown to detect a wide range of local fish species, even in small seawater samples. Additionally, an experiment on eDNA degradation in seawater showed that even small (100-bp) eDNA fragments degrade beyond detectability within days. The study highlights the potential of eDNA for future marine biodiversity and resource monitoring, despite challenges in understanding eDNA dispersal in marine environments. The results suggest that eDNA from marine water samples is likely of local origin, making the eDNA approach a promising tool for detecting unexpected species in marine ecosystems. The study also emphasizes the need for further research to validate the eDNA approach in varying environmental conditions.