Experimental Release of Juvenile Lobster at Koshiki Islands in South Japan

Abstract

As a part of reasearch on introduction of lobster Homarus americanus into Japan, a preparatory release was achieved in Koshiki Islands near Kyushu in July 1982. Before releasing, 100 concrete blocks were laid on sandy bottom in 4 parallel lines with a 2m space between each block. 97 one year old lobsters (XI-XV stage) were introduced under each block one by one. Monitoring was carried out for 11 days by diving. Survival rate was 2.7% and 25.0% for the lobsters released close to the stony-coral substrate and the others respectively. Movement of lobsters between blocks was rather frequent. Roughly 20-40% lobsters disappeared and 10-15% new ones appeared under the blocks. These facts suggest that the number of lobsters decreased mainly due to aggression by fishes which inhabit the stony-coral substrate.
A net cage (3m×3m×hight 0.3m) without bottom was immersed into the sandy substrate for another experiment. About 500 oyster shells were placed inside the cage. 156 three week old lobsters (IV-V stage) were released into the cage. Survival rate was 68% just after release and 25% agter 9 days. Degree of movement for IV or V stage lobsters was rather similar to that of one year old lobsters.