Variation in the sex ratio, size and age of longfinned eels within and among coastal catchments of south- eastern Australia C. T. WALSH *†‡, B. C. P EASE * AND D. J. B OOTH † *NSW Fisheries, P. O. Box 21, Cronulla, NSW 2230, Australia and †University of Technology, Sydney, Department of Environmental Sciences, Westbourne Street, Gore Hill, NSW 2065, Australia (Received 6 June 2003, Accepted 16 February 2004) Longfinned eels Anguilla reinhardtii were captured by both fishery-dependent and independent sampling methods from three rivers in New South Wales, south-eastern Australia. Sex ratios, catch per unit effort and population age and total length structure were examined in three zones (fresh water and upper and lower tidal) in the Hacking, Hawkesbury and Clarence Rivers. Females were found in relatively high proportions in all zones, ranging from 97% in a fresh- water (non-tidal) site down to 59% in a tidal site. Males were found primarily in tidal zones (only two of the 677 longfinned eels caught in non-tidal fresh water were males), with the greatest proportions being found in the brackish upper tidal areas. The mean number of fish captured per trap was higher in the fresh water and upper tidal zones than in the lower tidal zones. The mean  S.E. age, 179  03 years, and age range, 5–52 years for females were significantly higher than those of males 122  04 years; range 5–22 years, which is typical of other anguillid species. Longfinned eels captured in fresh water were found be signifi- cantly larger and older than those in tidal zones due to the almost exclusive predominance of females. # 2004 The Fisheries Society of the British Isles Key words: age; Anguilla reinhardtii; relative abundance; sex ratio; size. INTRODUCTION Catadromous Australian longfinned eels Anguilla reinhardtii Steindachner enter fresh water as sexually undifferentiated glass eels, where they develop into yellow eels, and later undertake a spawning migration back to the Coral Sea as silver eels (Beumer, 1986). During the yellow eel phase, this predominantly tropical to sub-tropical species (Schmidt, 1928) occupies a wider range of habitats within coastal catchments than any other fish species in eastern Australia. It is also during the yellow eel life-history stage that sexual differentiation of the gonads takes place (Colombo & Grandi, 1996; Walsh et al., 2003). While there is extensive literature available on the distribution of the sexes of some anguillid species, no such studies have been done on A. reinhardtii. ‡Author to whom correspondence should be addressed. Tel.: þ61295278411; fax: þ61295278576; email: walshc@fisheries.nsw.gov.au JournalofFishBiology (2004) 64, 1297–1312 doi:10.1111/j.1095-8649.2004.00392.x,availableonlineathttp://www.blackwell-synergy.com 1297 # 2004TheFisheriesSocietyoftheBritishIsles