The species sampled by the pole-and-line sampling were blue mackerel and pufferfish. The two species were caught in different depths by which we can grasped blue mackerels’echoes. The TS-TL conversion formula for blue mackerel were then been established (R2 is 0.98). We found that most of the blue mackerel migrated in Yilan Bay reached matured size and the larger blue mackerel mainly gathered around the Guishan Island. The estimated fork length is between 31.69 and 35.85 cm, mainly distributed in 33 cm, and the proportion is 51 %. Comparing estimated fork length and depth, it is found that the echoes of blue mackerel are mainly distributed between 65 to 75 meters.
From 717 adult fishes, the mean weight of mature female was 562.4 g, the sex ratio was 0.43, the mean batch fecundity was 80,064 hydrated oocytes and the spawning fraction was 0.3. The highest density of eggs were recorded in waters off Shandiao and southern area off Guishan Island and the estimated spawning area was 1.4x109 m2 . Based on the embryonic features of eggs, we assumed 18:00 was the spawning peak. The mean daily egg production obtained by the exponential decay model was 29.4 or 42.8 eggs/m2. Preliminary estimates of spawning biomass for Yilan Bay during late March 2021 were within 1,317 and 5,208 tonnes.