Physiological and ecological characteristics of diadromous fish in islands of the Pyukyus

点击翻译按钮获取中文摘要

This study was carried out to clarify the physiological and ecological characteristics of some selected diadromous fish inhabiting sround the island of the Ryukyus.1.Ryukyu-ayu (Plecoglossus altivelis ryukyuensis) : The structure of mature spermatozoon of this species was characterized by a symmetric nucleus and asymmetric arrangement of two centrioles unlike Ayu, P.altivelis. Scales, comb-like teenth, alimentary canals and skeleton developed earlier than Ayu. Upstream migration also occurred earlier (February-March) and smaller in size (av. 32.3 mm) than Ayu.2.Flagtail (Kuhlia marginata) : Based on the histological observations and gonad-somatic index, it was estimated that females spawn repeatedly during the spawning period from April to September. Number of ovarian eggs varied between 70,000 to 360,000 with an increase of BL.After young migrated from sea to rivers at av. 20mm BL,during March to July, females grew faster than males. The average longevity of female and male was estimated 6 and 2, respectively, and the ratio of female to male resulted in 3 : 1. The change in plasma Na and C1 after trasfer to sea water revealed that the fish have the high ability of tolerance to environmental salinity.3.Tilapia (Oreochromis mossambicus) : The species O.mossambicus is the dominant fish occupying the estuaries of Okinawa Island. The average fertile span of this fish was estimated to be 2-9 years. Annual spawing period lasts from May through September, and females may spawn several times during this season. Although the analysis of the verious physiological parameters such as oxygen consumption rates, plasma constituents and plasma hormone levels under the different salinity conditions showed that sea water may be favorable environment to the fish, they might selsct estuaries as their habitat on the basis of the causes of reproductive strategy of the species.