Ecological study on the alien fish in Okinawa prefecture

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Distribution patterns of the alien freshwater fishes in the Ryukyu Islands (Yonaguni, Iriomote, Ishigaki, Miyako, Minami-daito, Okinawa, Tokashiki, Kume, Zamami, Iheya, Izena, Okinoerabu, Tokunosima, Amami-ohsima, tanegashima Islands) were investigated from 2003 to 2005. Twenty-tow alien species were recognized in this study period.The zill tilapia, Tilapia zillii is a species intentionally introduced as bait for the Skipjack angling in Okinawa Prefecture. This species has recently been found to breed in natural rivers and pond of the Ryukyu Islands. The growth and life history of this species were studied between October 2004 to October 2005 at the Haebaru reservoir on Okinawa Island, Japan. Analysis of the marginal increment of 723 otoliths showed an annulus was formed between May to July. Maximum age was 6 years for both sexes, and patterns by the von Bertalanffy formula growth were described as Lt=214.4{1-exp[-0.16(t+1.45)] for males and Lt=97.5{1-exp[-0.61(t+0.08)] for females.The morphological development of Tilapia zillii was also described.