EFFECT OF AGE AT MATURITY ON LIFE-SPAN AND GENERATION TIME OF NEW-ZEALAND COMMON SMELT, RETROPINNA-RETROPINNA (RICHARDSON)

Errors in ageing may seriously affect calculations of the generation t ime of fish, particularly short-lived species. Ages of common smelt, R etropinna retropinna, from the Waikato River system in the North Islan d of New Zealand were estimated from counts of increments observed in sagittal otoliths. Ages were verified in the laboratory using the oxyt etracycline marking method. The estimated mean deposition rate was one increment per day. The ages of 86 smelt sampled at sites on the Waika to River system ranged from 2 days to 18 months. The birth dates of di adromous fish occurred primarily during austral autumn and winter; whe reas lacustrine-reservoir fish hatched during the spring and summer, w ith a few fish originating from winter spawning. The generation time o f the majority of smelt was approximately 1 year rather than 2 as was previously described from analyses of length frequency distributions. Short generation times help explain the rapid increases in the abundan ce of smelt when they are introduced into new habitats and also the ra pid changes in the meristic character, the mean number of rakers on th e first gill arch.