Spatial and temporal variability in length of glass eels (Anguillaspp.) 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.

Tropical eels Anguilla spp. recruiting to Réunion Island in the Indian Ocean: taxonomy, patterns of recruitment and early life histories

“…Similarly, A. bicolor bicolor sizes reported for Cimandiri River, Indonesia (45.5 to 52.3 mm: Arai et al 1999a), are markedly lower than those found for this species at Réunion Island (48.0 to 59.0 mm), possibly because of the shorter duration of the metamorphosis stage in Indonesian glass eels (18 ± 4.2 d; Arai et al 1999a). However, all these tropical glass eels are distinctly smaller than temperate species at recruitment (A. japonica, A. Anguilla, A. rostrata, A. australis and A. dieffenbachii : Tesch 1977, Tsukamoto 1990, Chisnall et al 2002. Tropical species may recruit faster after metamorphosis than temperate species.…”

Dispersal pattern of glass eel stage of Anguilla australis revealed by otolith growth increments

“…To drift a distance of 1500 km at speed of 0.14 m s -1 requires approximately 120 d. In practice, the ages of leptocephali at metamorphosis (T m ) from the most adjacent transTasman sites (A5 and A7) differed by an average of only 8 d. Thus, it is highly unlikely that recruitment of glass eels to New Zealand is via the east coast of Australia. Furthermore, information on arrival times and sizes of glass eels suggest that arrival in New Zealand is more likely to be from the north than the west (Jellyman 1987, Chisnall et al 2000. Another possible migration route is transportation by the southwest-flowing portion of the SEC (Jellyman 1987).…”

Variability in spatial and temporal abundance of glass eels(Anguillaspp.) in New Zealand waterways

“…In brief, glass eels were captured by electro-fishing designated reaches of streams at 14-day intervals. As in previous investigations Chisnall et al 2002), the present work separated glass eels into "newly arrived" and "previously arrived" categories on the basis of pigmentation (Strubberg 1913); more lightly pigmented eels, stages 5B-6A23, were classified as newly arrived, and those with more advanced pigmentation, stages >6A23, were classified as previously arrived. To avoid the possibility of resampling glass eels that were already present on a previous sampling occasion, only newly arrived glass eels were used in the following analyses.…”

“…There was no evidence that such yearly variation was directly associated with the El Niño Southern Oscillation phenomenon (ENSO), as the "high" years for both species of 1997 and 1999 were El Niño and La Niña years, respectively. However, it is possible that the ENSO may influence the direction of recruitment to New Zealand (Chisnall et al 2002).…”

“…Previous papers have examined seasonal arrival patterns , and trends in size and pigmentation (Chisnall et al 2002).…”