STUDIES ON THE DRIFT PATTERN OF DRIFTING FISH EGGS WITH TRACERS IN UPPER YANGTZE RIVER

Investigation of fish early life history is an important technique to calculate spawning sites and estimate spawning size for fish with drifting eggs. But, drift patterns of drifting eggs are greatly different for the diverse hydrological condition in different river reach. The drift pattern of drifting fish eggs was studied in this experiment with drifting tracers, and the classic estimate method of spawning size was tested to evaluate the applicability in upper Yangtze River. The experiment was conducted in the 31 km river reach in the upper Yangtze River from Jiangjin city to Luohuang town on 7th April 2008. A total of 485800 tracers were separated into 6 groups and dyed into red, green, white, black, blue and yellow respectively, the total number of tracers of every group was 80966, 80968, 80966, 80966, 80968 and 80966 in turn. Two releasing sites were set up to test the influence of configuration of river channel on the drifting speed. A total of 58 tracers were collected in the collecting site. The results showed that the mean drifting speeds of tracers released in the same releasing site were significantly different. Drift density of tracers showed a pattern of alternate with peak and low density, and the peak density showed gradual decline as drift time increase. All the 58 tracers were collected in the main stream, it reflected that drift density in the main stream was higher than the slow flow region. So we advise the main stream should be surveyed in the fish early life history investigation. Mean drifting speeds were 1.19 m/s, 1.03 m/s, 1.05 m/s and 1.22 m/s for red, green, white and black tracers respectively. There was no significantly difference for mean drifting speeds released in the two releasing sites. Estimated releasing numbers of red, green, white and black tracers were 8356, 11141, 12533 and 48741, and corresponding for 10.3%, 13.8%, 15.5% and 60.2% of the total number released respectively, so the accuracy of estimation showed greatly declining as drift time increasing. No blue and yellow tracers were collected for the low drift density in the slow flow region. The total estimated number was 16.6% of the real releasing number only. So a coefficient should be added into the calculation formula to compensate the decline of accuracy caused by drift time increase when estimate spawning size in the upperYangtze River.