Occurrence, Growth and Food of Longheaded Eagle Ray, Aetobatus flagellum, in Ariake Sound, Kyushu, Japan

Abstract

The longheaded eagle ray, Aetobatus flagellum, has recently increased significantly in numbers in Ariake Sound. It is assumed that it feeds on bivalves and so, to prevent predation by eagle rays on bivalves, a ‘predator control program’ aimed at reducing the ray population has been in place since 2001. We examined their occurrence, age, growth and food in Ariake Sound to obtain data on the ecology of the eagle ray and provide basic information on their potential impact on bivalve stocks in Ariake Bay. The eagle ray is a seasonal visitor to Ariake Sound, increasing in numbers from April, and peaking during the summer. None were captured during surveys in December and February. Their movement pattern around the bay differed according to sex. Pregnant females were caught in the estuary during August and September. Females grew to a larger size than males and apparently lived longer. The maximum ages were 19 years for females and 9 years for males. Growth until two years was similar in both sexes, but after 2 years females grew larger. The eagle ray fed only on bivalves, especially Ruditapes philippinarum and Atrina pectinata, very important fishery species farmed in Ariake Bay.