Biology of the Shortfinned Eel Anguilla obscura in Lake Te Rotonui, Mitiaro, Cook Islands

Abstract

Lake Te Rotonui, a shallow depression lake in the center of Mitiaro Island, southern Cook Islands, contains freshwater eels despite having no surface connection to the sea. During a survey of the eel population in July 1988, all of the 287 eels captured using fyke nets and gaffs were Anguilla obscura, although it is possible that A. megastoma and perhaps A. marmorata also occur in small numbers. Ages of eels were found from burnt otoliths; it was assumed that otolith zones were formed annually, although this could not be validated. Growth rates were slower than those of other tropical eel species, being similar to those of temperate species. Eels fed exclusively on Oreochromis mossambica, which was abundant in the lake. The relatively slow growth in the presence of abundant food may be due to high and stressful summer water temperatures. From length and age frequency distributions, it is suggested that recruitment of glass-eels into the lake is intermittent and via submarine outfalls. A review of the limited larval information suggested that A. obscura spawns to the east of Tahiti, with larvae transported west and south by the South Equatorial Current.