Occurrence Patterns and Ontogenetic Intervals of Eutaeniichthys gilli （Gobiidae）in Obitsu-gawa River Estuary, Tokyo Bay, Central Japan

Abstract

Occurrence patterns and ontogenetic intervals based on the development of swimming and feeding functions were investigated on the endangered goby, Eutaeniichthys gilli, collected from Obitsu-gawa River estuary in the inner Tokyo Bay. A small seine net and two set nets were used for the samplings at the main stream from May 2005 to April 2006 and at the creek on the tideland from July 2009 to June 2010. Individual numbers of E. gilli collected were 94 ranging from 2.8 to 34.5 mm BL with the mean ± SD = 7.7 ± 3.9 mm BL and 7 from 6.4 to 33.9 mm BL with 17.1 ± 12.3 mm BL at the tidal flat in the lower and middle streams, respectively, although no specimen was collected at the upper stream. On the other hand, 1,127 individuals of 10.1-39.5 mm BL with 27.5 ± 7.2 mm BL（n = 464）were collected from the creek. Based on the development of swimming- and feeding-related characters of 3.5-39.1 mm BL（n = 100） cleared and stained specimens, the following three swimming and feeding phases were recognized: caudal fin propulsion from 3.5 to 5.0 mm BL, whole body propulsion from 5.0 to 10-11 mm BL, and functional, juvenile swimming over 10-11 mm BL; sucking and biting from 3.5 to 5-6 mm BL, improved sucking and biting abilities from 5-6 to 10-12 mm BL, and functional, juvenile feeding over 10-12 mm BL. Based on the occurrence patterns and functional development, the spawning ground is considered to be formed in the tideland including the creek of Obitsu-gawa River estuary, and the hatched larvae are drifted and dispersed toward the sea. The forehead tidal flat at the lower stream is one of nurseries for early stage larvae. No nursery and growing habitats are formed in the main stream, but the juveniles of about 8 mm BL and larger migrate shoreward to and grow in the tideland of the Obitsu-gawa River estuary. This study revealed that Obitsu-gawa River estuary is almost the only major spawning ground and provides nursery and growing habitats for E. gilli in the inner Tokyo Bay.