11:45 AM - 12:00 PM
[ACG53-11] Quantitative assessment of mortality based on production in the short-neck clams population in the Midorikawa River tidal flat: effects of predation and freshwater discharge
Keywords:Ariake Bay, Ruditapes philippinarum, Aetobatus flagellum, Mallard duck, water outflow
Introduction
Tidal flat ecosystems provide essential environmental services such as water purification and high biological productivity with bivalves play a central role in those processes. However, bivalves have been declining worldwide in recent years, and conservation is needed. Factors contributing to this decline include predation, exposure to low-salinity water, and burial in mud due to freshwater inflow following heavy rain. While previous studies have examined these factors, the impact of predation on specific clam populations remains largely unquantified. In addition, field-based assessments of mortality caused by low-salinity water exposure and burial in mud are scarce. Furthermore, these factors have been evaluated using different measurement units, which has made direct comparison with secondary production difficult. In this study, the effects of predation by stingrays and ducks, exposure to low-salinity water and burial in mud were evaluated by all mortality factors to carbon weight. To achieve this, we monitored the population dynamics of short-neck clams in the Midorikawa River tidal flat from April 2022 to August 2023 and estimated their secondary production and mortality rates.
Materials and method
Predation of stingrays was evaluated based on the density of activity pits calculated from photos taken by UAV, and the biomass of short-neck clams inside and outside of the activity pits. To evaluate the predation of ducks, we first evaluated the daily activity time of ducks on tidal flats based on behavioral observation, and then calculated the daily predation amount by dividing it by the time duck required to prey on one clam. We then calculated the amount of predation per unit area over a five-month period based on the patch density of duck and the period of their arrival, and calculated the amount of predation in each month according to the frequency of arrival. We also conducted an experiment using a net bag to evaluate the mortality caused by exposure to low-salinity water and burial in mud, and evaluated the amount of clams that died by comparing the biomass of clams before and after the experiment. Furthermore, we considered the factors that caused the deaths of the short-neck clams based on the results of the salinity and the observations of mud. We evaluated these mortality rates on a carbon weight basis, and compared them.
Results and Discussion
The secondary production of the short-neck clams exceeded 0 in many months, confirming that the population was growing, and reached a maximum of approximately 1.0 gC m-2 day-1. The maximum estimated predation rate by stingrays was 0.104 gC m-2 day-1, while ducks consumed approximately 360 individuals m-2 over five-month period with monthly predation rates reaching up to 0.03 gC m-2 day-1. The results of net bag experiment carried out during the freshwater outflow event showed that 36.1 gC m-2 of clams died mainly due to sediment burial. Further field observations indicate that sediment remained on the tidal flat for three days, leading to an estimated mortality rate of 12.1 gC m-2 day-1 due to the fresh water outflow event. When these values were summed over the year, mortality from a single three-day flood (36.1 gC m-2 day-1) was more than three times higher than the combined annual predation by stingrays (9.9 gC m-2 day-1) and ducks (1.9 gC m-2 day-1). This suggests that a single flood event can cause greater clam mortality than predators consumption over a three-year period. With the frequency of heavy rainfall events increasing in recent years, it is likely that freshwater outflows and associated sediment burial will become more frequent. To support the conservation of organisms in tidal flat areas and the tidal flat area, it is important to quantitatively assess the impact of predators and water outflow on organisms in tidal flat areas.
Tidal flat ecosystems provide essential environmental services such as water purification and high biological productivity with bivalves play a central role in those processes. However, bivalves have been declining worldwide in recent years, and conservation is needed. Factors contributing to this decline include predation, exposure to low-salinity water, and burial in mud due to freshwater inflow following heavy rain. While previous studies have examined these factors, the impact of predation on specific clam populations remains largely unquantified. In addition, field-based assessments of mortality caused by low-salinity water exposure and burial in mud are scarce. Furthermore, these factors have been evaluated using different measurement units, which has made direct comparison with secondary production difficult. In this study, the effects of predation by stingrays and ducks, exposure to low-salinity water and burial in mud were evaluated by all mortality factors to carbon weight. To achieve this, we monitored the population dynamics of short-neck clams in the Midorikawa River tidal flat from April 2022 to August 2023 and estimated their secondary production and mortality rates.
Materials and method
Predation of stingrays was evaluated based on the density of activity pits calculated from photos taken by UAV, and the biomass of short-neck clams inside and outside of the activity pits. To evaluate the predation of ducks, we first evaluated the daily activity time of ducks on tidal flats based on behavioral observation, and then calculated the daily predation amount by dividing it by the time duck required to prey on one clam. We then calculated the amount of predation per unit area over a five-month period based on the patch density of duck and the period of their arrival, and calculated the amount of predation in each month according to the frequency of arrival. We also conducted an experiment using a net bag to evaluate the mortality caused by exposure to low-salinity water and burial in mud, and evaluated the amount of clams that died by comparing the biomass of clams before and after the experiment. Furthermore, we considered the factors that caused the deaths of the short-neck clams based on the results of the salinity and the observations of mud. We evaluated these mortality rates on a carbon weight basis, and compared them.
Results and Discussion
The secondary production of the short-neck clams exceeded 0 in many months, confirming that the population was growing, and reached a maximum of approximately 1.0 gC m-2 day-1. The maximum estimated predation rate by stingrays was 0.104 gC m-2 day-1, while ducks consumed approximately 360 individuals m-2 over five-month period with monthly predation rates reaching up to 0.03 gC m-2 day-1. The results of net bag experiment carried out during the freshwater outflow event showed that 36.1 gC m-2 of clams died mainly due to sediment burial. Further field observations indicate that sediment remained on the tidal flat for three days, leading to an estimated mortality rate of 12.1 gC m-2 day-1 due to the fresh water outflow event. When these values were summed over the year, mortality from a single three-day flood (36.1 gC m-2 day-1) was more than three times higher than the combined annual predation by stingrays (9.9 gC m-2 day-1) and ducks (1.9 gC m-2 day-1). This suggests that a single flood event can cause greater clam mortality than predators consumption over a three-year period. With the frequency of heavy rainfall events increasing in recent years, it is likely that freshwater outflows and associated sediment burial will become more frequent. To support the conservation of organisms in tidal flat areas and the tidal flat area, it is important to quantitatively assess the impact of predators and water outflow on organisms in tidal flat areas.