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

Abstract

The regional and annual variability in the abundance of newly arrived glass eels, Anguilla australis (Richardson) and Anguilla dieffenbachii (Gray), were examined using glass eel densities (n = 100 m -2 ) from electric-fishing samples collected at 14-day intervals over 5 years from 13 streams/ rivers throughout New Zealand. Although the density of shortfin eels exceeded that of longfin eels for any one year, the annual trends for both species were generally similar. Both species behaved independently, but significant factors affecting density were "year" and "region", with the pronounced regional effect for longfin eels attributed to "coast" (east or west), but not "island" (north or south). There was no evidence of declining recruitment for longfin glass eels.

Downstream passage of silver eels at a small hydroelectric facility

“…Recent studies indicated a decline in the numbers of glass eels and elvers recruiting into New Zealand waters (Jellyman, Chisnall, Sykes & Bonnett 2002), similar to elsewhere in the world (e.g. International Eel Symposium 2003; Rosell, Evans & Allen 2005).…”

Anguilla rostrata glass eel ingress into two, U.S. east coast estuaries: patterns, processes and implications for adult abundance

“…High geographic variability in glass eel catch also has been shown for other eel species in multiple regions around the world [ i.e. Taiwan (Tzeng, 1996) and New Zealand (Jellyman et al , 2002)].…”

“…A significant black box exists between when adult American eel leave North American freshwater habitats to when glass eels first arrive back at local estuaries (Tesch, 2003). Unfortunately, for most eel species, glass eel collections are typically of short duration and focus on a single location [although this was not the case for Dekker (1998) and Jellyman et al (2002)], making it difficult to discern patterns of intra‐ or interannual variability.…”

Factors affecting recruitment of glass eels into the Grey River, New Zealand

“…In the absence of a substantial time‐series of glass eel recruitment, it is not possible to resolve this disparity between the species composition of glass and yellow eels. The only available time‐series of recruitment (Jellyman et al ., 2002) covers 5 years, whereas generation times for this species are measured in decades (Jellyman, 1995). In the absence of such time series, cohort analysis was used to model changes in previous recruitment, and it was concluded that there had been a substantial decline in recruitment over the past 20 years.…”