The blue mackerel (Scomber australasicus) is a key target species of Taiwan nearshore fisheries, and management measures have been implemented for this species following evidence of overfishing. This paper describes the first test application of the daily egg production method (DEPM) to estimate the spawning stock biomass of S. australasicus in Taiwan. Large numbers of eggs and adults were collected concurrently from Yilan Bay off northeastern Taiwan during April 2020. From a total of 845 adult fish, the mean weight of the mature female fish was 528.0 g, the sex ratio was 0.472, the mean batch fecundity was 43,733 hydrated oocytes and the spawning fraction was 0.1335. The highest density of eggs was recorded in the waters off Dong’ao and in the eastern area off Guishan Island, and the estimated spawning area was 1.71 × 109 m2. The embryonic features of the eggs indicated that the peak spawning time was from 20:00 to 21:00. The mean daily egg production obtained using the exponential decay model was 38.3 or 52.7 eggs/m2. The preliminary estimates of the spawning biomass for Yilan Bay during April 2020 were between 8,395 and 23,090 mt. The results of this study demonstrate that the DEPM is suitable for assessing the spawning stock of S. australasicus. However, further research is required to improve the accuracy and precision of the DEPM key parameters and facilitate an understanding of the spawning ecology to enhance future applications.