アユの社会構造と生産 : 生息密度と関連づけて
Nihon Seitai Gakkaishi
SOCIAL BEHAVIOUR AND PRODUCTION OF A SALMON-LIKE FISH, PLECOGLOSSUS ALTIVELIS, OR AYU, WITH REFERENCE TO ITS POPULATION DENSITY
Kyoto Prefecture . Members of our research group have been studing the ecology of this 丘sh from various viewpoillts since 1955 . Thls paper con . cerns the change of the social behaviour and the mode of production . As already reported by our group , the cQmmon habit of th 董 s fish is territorial in the river ・ rapids and schooling in the river ・ pools. But on certain occasions , it showsno territorial behaviour, even in the r 五 ver ・ rapids . By my recent observation , the social type of this
... ocial type of this 飴 h appears to change with reference to the population density , and this change is ac ・ companied by the difference in the body − 1ength distribu匙ion. Five types were recogniz ξ d in its sDcial beha − viour : schooling , sDlitary travelling , non ・ territo ・ r 董 a 至 . solitary residential , solitary territorial and aggregated . The schooling is a social type in which all the members of a group show common behaviours , swimming id the same direction and feedi夏 g in similar manner . Solitary trave11er is an independent passenger , having no home , but it may be united w 三 th a school or may be separated again . Non − territorial resident is a sDlitary dwe11er havin 喜 its own range buし does not sho 珊 the territ 〔 } rial or attacking behaviour to the nearby individuals. In the territQrial sDlitary type each fish has its own territory . An aggregation means that its members are aggregated within a certain area hut do not show the common behaviour , and this type of beha ・ viour is observed when they are resting or sleep . ing, The popultitien of this fish , by our estimation , was far less in 1956 than in 1955 ( abou 亡 one sixth ) , and the socia 夏 relationship seemed to have been inHuenced by it. In 1955, when the density was high , most 丘shes showed the schoo1 ・ ing behaviour and the territorial ones were very scarce . On the contrary , in 1956, when t 車 e density was low, many fishes behaved as settled solitaries and territorial 圭 ndividuals were hot scarce . The stability of the schoo 正 and the territory differs according to the change of populati6n density. In the h 韮 gh dens 正 ty , the schoelin 奮 is the . cDmmon and mQre stable behaviOur type of this fish and no d 圭 fference is s3en in the body − Iength between the schooling and terrtorial ones . In the low denSity , however , the territorial behaviour is more stable , and there occurs a difference in the body ・ length , i. e . the territerial fishes are much larger than the schooling ones . In the case of the sociai structure in which the fishes are stabilizedlwith t 皇 rritoriality , the growth of the non − territorial fishes seems to be checked because the latter cannot 三 nvade the bottom of the 丘rst class , which is defended by territorial fishes .