Evaluation of biodiversity in estuaries using environmental DNA metabarcoding

Biodiversity is an important parameter for the evaluation of the extant environmental conditions. Here, we used environmental DNA (eDNA) metabarcoding to investigate fish biodiversity in five different estuaries in Japan. Water samples for eDNA were collected from river mouths and adjacent coastal areas of two estuaries with high degrees of development (the Tama and Miya Rivers) and three estuaries with relatively low degrees of development (the Aka, Takatsu, and Sendai Rivers). A total of 182 fish species across 67 families were detected. Among them, 11 species occurred in all the rivers studied. Rare fishes including endangered species were successfully detected in rich natural rivers. Biodiversity was the highest in the Sendai River and lowest in the Tama River, reflecting the degree of human development along each river. Even though nutrient concentration was low in both the Aka and Sendai Rivers, the latter exhibited greater diversity, including many tropical or subtropical species, owing to its more southern location. Species composition detected by eDNA varied among rivers, reflecting the distribution and migration of fishes. Our results are in accordance with the ecology of each fish species and environmental conditions of each river, suggesting the potential of eDNA for non-invasive assessment of aquatic biodiversity.

Effects of Capture by Trammel Net on Colorado River Native Fishes

Trammel nets are commonly used to sample rare fishes; however, little research has assessed delayed mortality associated with this capture technique. We conducted laboratory experiments to evaluate the effects of capture by trammel net on bonytail Gila elegans, razorback sucker Xyrauchen texanus, and roundtail chub Gila robusta, at 15, 20, and 25°C. Fish (139–288 mm total length) were entangled in a trammel net for 2 h or captured by seine net and then monitored for mortality for at least 14 d. Blood samples were collected immediately after capture, and plasma cortisol levels were quantified as an index of capture-related stress. The cortisol response varied by species, but mean cortisol levels were higher for fish captured by trammel netting (295.9 ng/mL) relative to fish captured by seine netting (215.8 ng/mL). Only one fish (of 550) died during capture and handling, but 42% of the trammel-netted fish and 11% of the seine-netted fish died within 14 d after capture. In general, mortality after capture by trammel net increased with increased water temperature and at 25°C was 88% for bonytail, 94% for razorback sucker, and 25% for roundtail chub. Delayed mortality of wild-caught fish captured by trammel net has the potential to be high, at least under some circumstances. We suggest that sampling frequency, timing of sampling (relative to reproductive cycles), and water temperature all be considered carefully when using trammel nets to sample diminished populations of imperiled native fishes.