Stock assessment of chub mackerel ( Scomber japonicus) in the central East China Sea based on length data

The chub mackerel (Scomber japonicus) has been one of the important commercial pelagic species in the East China Sea since the decline of traditional demersal economic species. Although chub mackerel resources are suffering over-exploitation with the increase of fishing effort, their response to fishing and changing trends have not been analysed because of a shortage of data. In this paper we separated sample catches of chub mackerel from the central East China Sea during 2009 and 2010 into three age groups based on the age-fork length key. We then estimated the maximum sustainable yield (MSY) and built a spawner-recruitment relationship based on the fishery statistics of purse seine vessels, lighting purse seine vessels and deep-water purse seine vessels from 2006 to 2012. We predicted the biomass, spawning biomass, catches and the length compositions of catches when fishing at the current intensity (fishing mortality (F)=0.7). The results showed that the MSY of chub mackerel is about 18.8x10(4)t, and the fishing effort at MSY (E-MSY) is about 72 standard purse seine vessels (both the lighting purse seine vessels and deep-water purse seine vessels were standardized to purse seine vessels), which is equivalent to F=0.4. If F remains at the current level, the biomass of chub mackerel will remain at 45.3x10(4)t. At the same time, more than 40% in the catch will be the individuals smaller than 200mm, and only about 8% will be larger than 300mm. Therefore, there is little potential for further exploitation of the chub mackerel resources in the central East China Sea, and it is better to reduce F to less than 0.4 for sustainable utilization.