Spatial and temporal variability in length of glass eels (Anguilla spp.) in New Zealand

Abstract

The spatial and temporal trends in length and pigmentation of glass eels for the shortfinned eel (Anguilla australis (Richardson)) and the longfinned eel (A. dieffenbachii (Gray)), were investigated by electric‐fishing 13 New Zealand streams/rivers on both the east and west coasts, at 14‐day intervals between July and December, from 1995 to 1998. A total of 11 479 glass eels were individually identified, measured, and assigned a pigmentation stage. On the basis of pigmentation, 67% were described as “newly arrived”. Modal pigmentation stage advanced over the arrival season for both species, and on arrival, longfinned glass eels were usually more pigmented than shortfinned glass eels. For any given time, North Island glass eels were consistently more advanced in pigmentation than were their South Island counterparts. Few very early pigmentation stages were recorded from the North Island samples, probably because sample sites were above estuaries and harbours. Longfinned glass eels were generally longer than shortfinned glass eels from the same river. Length of both species declined during the arrival season. Regional differences in mean length provides some indication of arrival patterns—predominantly from the north in an anti‐clockwise dispersal around New Zealand. Recruitment also showed annual variation, which appeared to be influenced by the El Niño Southern Oscillation, with the arrival route appearing to be more strongly from the north‐west during the El Niño phase (persistent westerlies) and from the north‐east during the La Niño phase (more persistent north‐easterlies). The mean length of new arrivals increased in the north and in the south‐west between 1995 and 1997, and decreased in all regions between 1997 and 1998, which could have been the result of changes in the duration of migration. Possible reasons for this include spatial and temporal changes to spawning, and oceanic variations in production.