Size variability and natural mortality dynamics of anchovy Engraulis japonicus eggs under high fishing pressure

Since the late 1990s, the anchovy Engraulis japonicus stock in the Yellow Sea has undergone considerable decline as a result of the enormous fishing pressure there. Reproductive characteristics have changed during the same period. Fisheries surveys were conducted in anchovy spawning grounds off south Shandong Peninsula in the Yellow Sea from 2000 to 2004, and in Laizhou Bay of the Bohai Sea, the wider Yellow Sea, and in coastal waters of the East China Sea from 2003 to 2008. Anchovy eggs from the spawning grounds south of the Shandong Peninsula (16 to 29 June 2003) had long axes ranging from 1.20 to 1.40 mm (1.28 +/- 0.04 mm), and short axes ranging from 0.60 to 0.68 mm (0.64 +/- 0.02 mm). In the coastal area of the East China Sea (7 to 14 May 2007), the eggs had long axes ranging from 1.12 to 1.40 mm (1.28 +/- 0.06 mm) and short axes ranging from 0.55 to 0.68 mm (0.63 +/- 0.03 mm). Compared with previous research results on anchovy egg size in Chinese waters and in other coastal regions of the North Pacific from the 1950s to 1970s, egg size was notably decreased both at the spawning grounds off the south Shandong Peninsula and in the coastal waters of the East China Sea. The natural mortality rate of anchovy eggs was 80.15% at the spawning grounds off south Shandong Peninsula, 77.63% in the Yellow Sea, and 82.95% in Laizhou Bay of the Bohai Sea. Based on our survey data, the natural mortality rate of anchovy eggs has been as high as 80% in the Bohai Sea, the Yellow Sea, and the East China Sea since the late 1990s. Compared with survey results for the Yellow Sea from the 1980s, the natural mortality rates of anchovy eggs have shown a significant rising trend. The decreased size and significantly increased natural mortality rate of anchovy eggs are long-term adaptive responses, via the reproductive biology of the anchovy population, to the enormous fishing pressure in the study area.