Spawning ecology of the anchovy Engraulis japonicus in the spawning ground of the Southern Shandong PeninsulaⅠ. Abundance and distribution characters of anchovy eggs and larvae

From May to July during 2000–2004, there were 11 multi-disciplinary integrated surveys carried out at the spawning ground of the southern Shandong Peninsula by Research Vessel Beidou within the area 120°00￠E–124°00￠E, 33°30￠N–37°00￠N. The main goal of the investigation was to study the anchovy Engraulis japonicus eggs and larvae abundance distribution and their relationship between the environments. The horizontal distribution of eggs and larvae were sampled by the large zooplankton net with 80 cm diameter, 270 cm length and 0.50 mm (38GG) bolt silk. Surface horizontal trawl was deployed at each grid station last for 10 min as the ship moved at the speed of 3.0 n mile/h. Layered vertical trawls were carried out when the relative high density of eggs and larvae at the surface was found. The drift experiment station was determined by the high abundance and living rate of eggs. In the drift experiment, a 1-m high, 0.8-m wide cross-sail was put into the sea at 2-3 m depth and floated freely with current. The position of this drift sail was recorded each 0.5 hour or 1 hour by GPS. The horizontal trawl was done around the drifter every 2 hours. The temperature profile was obtained by SBE19 CTD at the same time. The upper mixed layer, thermocline and lower mixed layer were determined according to the measured temperature profile. Then layered vertical trawl in different layers by large zooplankton net was done with down speed of 1 m/s. Samples were preserved in 5 % formaldehyde solution immediately after sampling and analyzed at laboratory. In fact the horizontal trawl speed could not be accurately control due to the wind wave and current, so the amount of eggs or larvae each station was actual number according to the sampling of the haul. The density (ind./m3) of eggs or larvae were normalized by the formula, density of eggs or larvae = amount of eggs or larvae each net (ind.)/filtered water volume (m3).Several results were concluded. 1) In the second half of each May, the anchovy in the spawning ground of the southern Shandong Peninsula begin to spawn. The spawning adults increase gradually in the early June. Vigorous spawn happen in the second half of June. 2) In this area, there is a stable high density of eggs of anchovy near 35°N section every year while the high density area range and distribution in the north of this area vary much larger. As the high density area formed the averaged egg amount is very high. There are rare eggs of anchovy in the southwestern waters of this area. The distribution of eggs has tightly relationship with physical factors su