11:15 AM - 11:30 AM
[ACG40-09] Quantitative evaluation of clam predation by stingrays on tidal flats at the mouth of the Midorikawa River.
Keywords:Ariake Bay, Ruditapes philippinarum, Aetobatus flagellum, Dasyatis akajei
Introduction
Stingrays have been reported to play a role as keystone species in shallow-water benthic ecosystems. However, the ecological role of stingrays in the Ariake Bay, where the feeding damage by stingrays on clam has been concern in recent years, in not yet clear. Aetobatus flagellum and Dasyatis akajei inhabit the Ariake Bay and prey on benthic organisms including short neck clam, Ruditapes philippinarum. A. flagellum, in particular, are target of extermination efforts because they prefer to consume the clam. Although the amount of predation by A. flagellum has been experimentally assessed, the evaluation should be questionable because it does not consider the migration of the stingrays. In order to balance the protection of clams and stingrays, it is necessary to accurately assess the amount of predation by stingrays.
In this study, we calculate the abundance of clams on the tidal flat at the mouth of the Midorikawa River facing the Ariake Bay, both in and outside of the feeding pit by stingrays on the tidal flat through their predation. Based on these results, we calculate the amount of predation by stingrays and compare with the rate of decline of the clam abundance, and discuss the effects of predation by stingrays on the decline of the clam population.
Materials and Methods
Field observations were conducted once a month from April to October 2022 on Midori River tidal flats.
For the quantitative survey of clam density, 5replicates of sediment samples collected from inside and outside of the feeding pits, using steel core sampler(25 cm x 25 cm). The abundance of feeding pits by stingrays was quantified by an unmanned aerial vehicle (UAV).
The amount of predation Gt (ind) was obtained by subtracting the actual number of clams from the number of clams assumed in the absence of predation.
Gt=Dot*(Aot+Ait)-(Dot*Aot+Dit*Ait) (1)
In this equation, the density of clams within the feeding pits on a given survey day t is Dit (ind m-2), the area of clams outside of the feeding pits is Dot (ind m-2), the area of feeding pits is Ait (m2 ha-1), and the density outside predation track is Aot (m2 ha-1). Predation by stingrays on the clams, Pt (% day-1), is the ratio of the number of clams actually preyed on to the total number of clams, assuming no predation.
Pt=Gt/{Dot*( Aot+ Ait)}/m/100 (2)
In this equation, m is the number of days of the feeding pits remaining, assumed to be 1 day. The decline rate of the clam population, Kt (% day-1), is the change in the number of clams outside of the feeding pits on two consecutive survey days divided by the number of days elapsed.
Kt={ln(Dot+1)-ln(Dot)}/(Dayt+1-Dayt)*100 (3)
In this study, we examined the contribution of predation on the rate of decline of the clam population based on the Pt/Kt ratio.
Results and Discussion
The number of clams outside the feeding pits decreased from 80,009 inds m-2 to 31,408 inds m-2 from May to July and from 33,334 inds m-2 to 15,846 inds m-2 from September to October. The number of clams inside and outside of the feeding pits differed significantly only in September and October.
The area of feeding pits increased from 41.8±7.4 m2 ha-1 in May to 187.6±60.9 m2 ha-1 in June and decreased to 55.2±10.7 m2 ha-1 in September, occupying at most 3 % of the tidal flats.
The Pt and Kt from September to October when there was a significant difference in clam abundance between inside and outside were 0.29 % day-1 and 1.62 % day-1, respectively. The Pt/Kt ratio indicates that predation by stingrays accounts for 17.7 % of the decline in the clam population. In this study, predation was overestimated by assuming that the number of remaining days of feeding pits was 1 day, instead of distinguishing between feeding pits and sinuous furrow. This suggests that predation by stingrays in this study area should not be a major factor in the decline of clams.
Stingrays have been reported to play a role as keystone species in shallow-water benthic ecosystems. However, the ecological role of stingrays in the Ariake Bay, where the feeding damage by stingrays on clam has been concern in recent years, in not yet clear. Aetobatus flagellum and Dasyatis akajei inhabit the Ariake Bay and prey on benthic organisms including short neck clam, Ruditapes philippinarum. A. flagellum, in particular, are target of extermination efforts because they prefer to consume the clam. Although the amount of predation by A. flagellum has been experimentally assessed, the evaluation should be questionable because it does not consider the migration of the stingrays. In order to balance the protection of clams and stingrays, it is necessary to accurately assess the amount of predation by stingrays.
In this study, we calculate the abundance of clams on the tidal flat at the mouth of the Midorikawa River facing the Ariake Bay, both in and outside of the feeding pit by stingrays on the tidal flat through their predation. Based on these results, we calculate the amount of predation by stingrays and compare with the rate of decline of the clam abundance, and discuss the effects of predation by stingrays on the decline of the clam population.
Materials and Methods
Field observations were conducted once a month from April to October 2022 on Midori River tidal flats.
For the quantitative survey of clam density, 5replicates of sediment samples collected from inside and outside of the feeding pits, using steel core sampler(25 cm x 25 cm). The abundance of feeding pits by stingrays was quantified by an unmanned aerial vehicle (UAV).
The amount of predation Gt (ind) was obtained by subtracting the actual number of clams from the number of clams assumed in the absence of predation.
Gt=Dot*(Aot+Ait)-(Dot*Aot+Dit*Ait) (1)
In this equation, the density of clams within the feeding pits on a given survey day t is Dit (ind m-2), the area of clams outside of the feeding pits is Dot (ind m-2), the area of feeding pits is Ait (m2 ha-1), and the density outside predation track is Aot (m2 ha-1). Predation by stingrays on the clams, Pt (% day-1), is the ratio of the number of clams actually preyed on to the total number of clams, assuming no predation.
Pt=Gt/{Dot*( Aot+ Ait)}/m/100 (2)
In this equation, m is the number of days of the feeding pits remaining, assumed to be 1 day. The decline rate of the clam population, Kt (% day-1), is the change in the number of clams outside of the feeding pits on two consecutive survey days divided by the number of days elapsed.
Kt={ln(Dot+1)-ln(Dot)}/(Dayt+1-Dayt)*100 (3)
In this study, we examined the contribution of predation on the rate of decline of the clam population based on the Pt/Kt ratio.
Results and Discussion
The number of clams outside the feeding pits decreased from 80,009 inds m-2 to 31,408 inds m-2 from May to July and from 33,334 inds m-2 to 15,846 inds m-2 from September to October. The number of clams inside and outside of the feeding pits differed significantly only in September and October.
The area of feeding pits increased from 41.8±7.4 m2 ha-1 in May to 187.6±60.9 m2 ha-1 in June and decreased to 55.2±10.7 m2 ha-1 in September, occupying at most 3 % of the tidal flats.
The Pt and Kt from September to October when there was a significant difference in clam abundance between inside and outside were 0.29 % day-1 and 1.62 % day-1, respectively. The Pt/Kt ratio indicates that predation by stingrays accounts for 17.7 % of the decline in the clam population. In this study, predation was overestimated by assuming that the number of remaining days of feeding pits was 1 day, instead of distinguishing between feeding pits and sinuous furrow. This suggests that predation by stingrays in this study area should not be a major factor in the decline of clams.