Determination of the age of brown and rainbow trout in a range of New Zealand lakes

Abstract

Brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) from eight New Zealand lakes were aged by length-frequency analysis and by examining otoliths, pectoral fin rays and scales. Bimonthly changes in the length frequency of age classes were used to age fish for their first 2 or 3 years of life. Growth rates were highest in spring and summer, when a single wide opaque band was laid down on otoliths and fin rays, followed by a narrow transparent band in winter. Age estimates from otoliths agreed with 94-97% of those from length-frequency methods and with the ages of 21 tagged fish. Otoliths were used to age fish up to 11 years old and were more accurate than fin rays and scales. Although fin rays could be used to age juvenile fish from Lake Coleridge, where there was 78-80% agreement between otolith and fin ray ages, they were of little value for older fish in Lake Alexandrina (28% agreement). Rainbow trout scales from all lakes were difficult to read because of indistinct annuli and the presence of false checks. Mature trout were also difficult to age owing to scale absorption and a cessation of scale growth. The degree of agreement between otolith and scale ages ranged from 83-84% for trout in Lake Coleridge to 32-38% for older rainbow trout in other lakes. This study supports the findings of other researchers that trout scales can provide inaccurate estimates of the age of trout in New Zealand and Australia. It is suggested that otolith examination, supported by length-frequency analysis of juvenile trout, be used to age trout from New Zealand lakes. In many cases, examination of otoliths will be cheaper and quicker than the release and later recapture of fin-clipped or tagged juvenile trout.