11:30 AM - 11:45 AM
[ACG53-10] Quantification of fish bioturbation in tidal flats, Yatsushiro Sea, using a combination of unmanned aerial vehicle (UAV) imaging and environmental DNA analysis
Keywords:environmental DNA, bioturbation
Introdaction
Macrobenthic animals such as bivalves, crustaceans and polychaetes contribute to bioturbation in tidal flats, promoting material circulation through burrowing and their feeding activities. These processes play a crucial role in ecosystem functioning. Larger animals, including mammals and cartilaginous fishes such as walruses and stingrays, also contribute to bioturbation through their foraging activities. However, quantitative assessments of their impact remain limited. Additionally, bony fishes such as the black sea bream, which forage on tidal flats and leave distinct feeding pits, have rarely been considered bioturbators, and few studies have quantified their bioturbation rates. This study aims to assess fish-mediated bioturbation using a combination of environmental DNA (eDNA) analysis and UAV-based aerial imaging. By integrating these approaches, we aim to comprehensively evaluate the fish community composition and quantify the bioturbation activity of key species.
Materials and Methods
The study was conducted in the Yatsushiro Sea, a semi-enclosed coastal sea bordered by Kumamoto and Kagoshima prefectures, covering an area of approximately 1200 km^2. Large tidal flats extend from the northern region, near the mouth of the Kuma River. This study was conducted at the Oshima tidal flat (32.5465490 N, 130.5576281 E), located in the northern part of the Kuma River estuary. Surveys were conducted monthly from June 2023 to January 2024, with each survey spanning two consecutive days.
For fish community analysis using eDNA, three stainless steel baskets containing sponges made of natural sea sponges as DNA collection material were deployed and moored for approximately 24 hours. After retrieval, the sponges were squeezed using a syringe to remove excess water. The sponges, still inside the syringes, were then frozen for subsequent DNA extraction and fish species identification. Among the detected fish species, those leaving detectable foraging pits on the sediment surface of the tidal flat in UAV images were selected as dominant species.
For each sampling, the occurrence rate of each dominant species Pi,k(%) was calculated for each sponge (k =1,2,3) using equation (1):
Pi,k = Ni,k / &sum jNj,k *100 (1)
Here, Ni,k represents the number of sequence reads assigned to the dominant species i in sample k, and &sum jNj,k is the total number of reads for all fish species j in sample k.
For UAV-based bioturbation assessment, aerial images of the tidal flat were captured using a DJI Mavic3 drone at an altitude of approximately 100 m. These images were processed into a composite orthomosaic, and foraging pits were visually identified around fixed survey points. The bioturbation area (m^2 ) on day 1 (Ai, 1) and day 2 (Ai, 2) was measured.
The daily bioturbation rate Ri (% d-1) for each sampling (five locations) was calculated using equation(2).
Ri = (Ai,2 - Ai,1) / Si * 100 (2)
Where Si represents the analyzed tidal flat area (approximately 100m^2) at site i.
In addition, from October 2023 to January 2024, macrobenthic animals were sampled monthly using 10 cm square quadrats placed inside and outside the predator-exclusion nets (n = 5 per month) to assess predation impacts.
Results and Discussion
Bioturbation rates were higher during the active period (4.41 ± 0.72 % d^-1) from July to October compared to the dormant period (0.43±0.35 % d^-1) from November to January, correlating with water temperature fluctuations. Benthic macrofauna density and species richness were lower in areas without protective netting, suggesting significant predation pressure. eDNA analysis identified seven dominant fish species, with black sea bream exhibiting the highest occurrence rate throughout the study period. This species was active during summer, contributing to increased bioturbation, whereas its activity declined in winter, likely explaining the reduced bioturbation rate due to the cold water temperature.
Macrobenthic animals such as bivalves, crustaceans and polychaetes contribute to bioturbation in tidal flats, promoting material circulation through burrowing and their feeding activities. These processes play a crucial role in ecosystem functioning. Larger animals, including mammals and cartilaginous fishes such as walruses and stingrays, also contribute to bioturbation through their foraging activities. However, quantitative assessments of their impact remain limited. Additionally, bony fishes such as the black sea bream, which forage on tidal flats and leave distinct feeding pits, have rarely been considered bioturbators, and few studies have quantified their bioturbation rates. This study aims to assess fish-mediated bioturbation using a combination of environmental DNA (eDNA) analysis and UAV-based aerial imaging. By integrating these approaches, we aim to comprehensively evaluate the fish community composition and quantify the bioturbation activity of key species.
Materials and Methods
The study was conducted in the Yatsushiro Sea, a semi-enclosed coastal sea bordered by Kumamoto and Kagoshima prefectures, covering an area of approximately 1200 km^2. Large tidal flats extend from the northern region, near the mouth of the Kuma River. This study was conducted at the Oshima tidal flat (32.5465490 N, 130.5576281 E), located in the northern part of the Kuma River estuary. Surveys were conducted monthly from June 2023 to January 2024, with each survey spanning two consecutive days.
For fish community analysis using eDNA, three stainless steel baskets containing sponges made of natural sea sponges as DNA collection material were deployed and moored for approximately 24 hours. After retrieval, the sponges were squeezed using a syringe to remove excess water. The sponges, still inside the syringes, were then frozen for subsequent DNA extraction and fish species identification. Among the detected fish species, those leaving detectable foraging pits on the sediment surface of the tidal flat in UAV images were selected as dominant species.
For each sampling, the occurrence rate of each dominant species Pi,k(%) was calculated for each sponge (k =1,2,3) using equation (1):
Pi,k = Ni,k / &sum jNj,k *100 (1)
Here, Ni,k represents the number of sequence reads assigned to the dominant species i in sample k, and &sum jNj,k is the total number of reads for all fish species j in sample k.
For UAV-based bioturbation assessment, aerial images of the tidal flat were captured using a DJI Mavic3 drone at an altitude of approximately 100 m. These images were processed into a composite orthomosaic, and foraging pits were visually identified around fixed survey points. The bioturbation area (m^2 ) on day 1 (Ai, 1) and day 2 (Ai, 2) was measured.
The daily bioturbation rate Ri (% d-1) for each sampling (five locations) was calculated using equation(2).
Ri = (Ai,2 - Ai,1) / Si * 100 (2)
Where Si represents the analyzed tidal flat area (approximately 100m^2) at site i.
In addition, from October 2023 to January 2024, macrobenthic animals were sampled monthly using 10 cm square quadrats placed inside and outside the predator-exclusion nets (n = 5 per month) to assess predation impacts.
Results and Discussion
Bioturbation rates were higher during the active period (4.41 ± 0.72 % d^-1) from July to October compared to the dormant period (0.43±0.35 % d^-1) from November to January, correlating with water temperature fluctuations. Benthic macrofauna density and species richness were lower in areas without protective netting, suggesting significant predation pressure. eDNA analysis identified seven dominant fish species, with black sea bream exhibiting the highest occurrence rate throughout the study period. This species was active during summer, contributing to increased bioturbation, whereas its activity declined in winter, likely explaining the reduced bioturbation rate due to the cold water temperature.