Variation in the spatial density of sampled clams in seven size classes (D1–D7) divided by sieves of different mesh sizes (a–d), and comparison of the gradients of regression lines fitted to the temporal variations of standardized spatial density in the size classes (e). Lower limits of the shell length in D1–D7 were approximately 4, 7, 11, 16, 23, 27, and 30 mm, respectively.