Size and age of migrating New Zealand freshwater eels(Anguillaspp.)

Abstract

Migrating adult freshwater eels, A. australis Richardson and A. dieffenbachii Gray, were examined during the autumnal migration from the Makara Stream (1971-1973), Lake Onoke (1971, 1972) and Lake Ellesmere (1972, 1974-80). Males of A. australis migrated from the Makara Stream at a mean length of 465 mm and from Lake Onoke at a mean length of 447 mm. The difference between the mean lengths at migration of A. australis males from Lake Ellesmere between 1972 (mean 443 mm) and 1980 (mean 425 mm) were significantly different. Females of A. australis migrated from the Makara Stream, Lake Onoke and Lake Ellesmere at mean lengths of 737, 764 and 615 mm, respectively. Aging was by examination of burned otoliths. Males of A. australis from each area migrated after spending an average of 14 yr in fresh water. A. australis females migrated from the Makara Stream at an average age of 19 yr, from Lake Onoke at 23 yr and from Lake Ellesmere at 24 yr. A. dieffenbachii males and females were larger and older at migration than A. australis of the same sex. Males of A. dieffenbachii from the Makara Stream migrated at a mean length of 623 mm after spending an average of 23 yr in fresh water; females may attain a large size before migration. A. dieffenbachii females migrated from the Makara Stream at a mean length of 1063 mm and from Lake Ellesmere at 1156 mm; ages ranged from 25-60 yr. There was a higher proportion of male to female A. australis in Lake Ellesmere and the Makara Stream than in Lake Onoke. In Lake Ellesmere the high percentage of migrating males (83-98%) recorded each year between 1972-1980 showed reversal of the sex ratio from that reported earlier. In the Makara Stream there was a higher proportion of male to female A. dieffenbachii (17:1) and in Lake Ellesmere female A. dieffenbachii outnumbered males (11:1). Migrating A. australis males from Lake Ellesmere were slightly heavier than those from the Makara Stream and Lake Onoke. Computed weights of A. australis females of the same length from each area were similar. Migrating A. dieffenbachii were heavier than A. australis migrants of the same length. Changes in the length-frequency and sex composition of migrating A. australis eels in Lake Ellesmere are coincident with extensive commercial eel fishing. These changes are probably related to the selective fishing for eels > 380 mm in the non-migrant population.