Growth and age validation of Rainbow Trout, Oncorhynchus mykiss (Walbaum)' in Lake Eucumbene, NSW

Abstract

Growth rates of finclipped rainbow trout (Oncorhynchus mykiss) released into Lake Eucumbene, New South Wales, at an age of 0+ years were examined for each year between 1986 and 1988. Traps and nets yielded significant numbers of marked fish for validation of age estimation and length backcalculation from scale reading. Poor recognition of the marks by anglers was confirmed, indicating that survival of the fish was higher than was implied by angling returns. The diffuse nature of the second annulus in scales provided initial difficulty in ageing, particularly because of a similarly diffuse false check formed during the second year. Care was required in separating the annuli after asymptotic growth was reached (3 years), Growth rates in Lake Eucumbene were comparable with those found in studies in Tasmania and Victoria. Back-calculated lengths were in agreement with those from fish of known ages. The presence of finclipped fish aged 2 + and 3 + years in a spawning population confirmed that stocked fish contributed to the spawning stocks. The study reconfirms the importance of validation of ageing techniques in fisheries investigations.