|Blue mackerel belongs to straddling fish stock and is an important commercial fish in the seas northeast of Taiwan. During its life history, blue mackerel has been exploited for several purposes: fish of body length under 17 cm for fresh bait in aquaculture, between 17 and 22 cm as bait for tuna longline fishing, and over 23 cm exported to developing countries as food or other processed products. Mackerel of body length over 30 cm as food for domestic fresh fish market and can be processed as one night salt-dried mackerel. Mackerel fisheries is therefore developing quickly, and the sustainability of its natural resource has been a focal issue for the industry. The present study, based on 13-year data of fishing information of large purse seine (Japanese type) and small purse seine (Taiwanese type), standardized the catch effort, then examined the variation of CPUE. In the meantime we collected fish specimens to study reproductive biology and assess the growth status with von Bertlanffly growth equation (VBGE). Our results showed that in recent years for the blue mackerel the theoretical maximum length and L50 have decreased, growth coefficient has increased, and CPUE has decreased. These changes clearly indicate overfishing of blue mackerel. We also found out that the mandatory fishing moratorium based on the weather and Taiwan tested the fishing moratorium in June 2013 has effectively regulated and improved the production of mackerel fisheries. After the closing of unilateral mandatory fishing moratorium enforced by Taiwan, a short term increase in fishing yield of the Taiwanese fishing vessels was evident. In the meantime fish body length was also increased. The moratorium, which was taken place in spawning season, partly protected spawners of mackerel as well as their newly hatched larvae, and resulted in later increase in recruitment of the population, consequently the increase in fishing yield. We also found short- term moratorium in March or April for 1 week was effective. These results illustrate the positive effectiveness of mandatory fishing moratorium. However, the catch effort is still too high and there is no international management mechanism to enforce the moratorium, therefore, no practical assistance to the recovery of the mackerel resources.