Larval competition for food between wild and hatchery‐reared ayu, Plecoglossm altivelis Temminck et Schlegel, in culture ponds

To examine the larval competition between wild and hatchery ayu in the culture ponds, mixed rearing of 580 000 wild and 520 000 hatchery larvae was carried out in two 25‐m 3 ponds for 3 months, in contrast to the control in which 860 000 wild larvae were reared in another pond. The number of wild larvae in the mixed‐rearing treatments decreased rapidly 20 days after the start of mixed rearing, in contrast to hatchery larvae. Mortality of wild larvae was almost 100% at the end of the experiment (3 months), while the hatchery larvae showed the usual survival rate of 15–16%. In the control pond, however, 16% of the wild larvae survived. The wild larvae grew much slower (0.10mmday ‐1 ) than the hatchery larvae (0·26 mm day ‐1 ) in the mixed‐rearing ponds, whereas the wild larvae in the control pond showed almost the same growth rate (0·24 mm day ‐1 ) as hatchery larvae. On day 6 the gut fullness of wild larvae was only 30% of that of the hatchery larvae in the mixed‐rearing experiments. On day 46 the wild larvae occurred deeper in the mixed‐rearing ponds than the hatchery larvae. This depth difference in vertical distribution appeared to cause a disadvantage for the wild larvae in the competition with hatchery larvae, since the food was supplied at the surface. Thus, the wild larvae starved and died.