Seasonal changes in abundance of brown trout (Salmo trutta) and rainbow trout (s. gairdnerii) assessed by drift diving in the Rangitikei river, New Zealand

Abstract

Numbers and approximate sizes of brown trout (Salmo trutta L.) and rainbow trout (5. gairdnerii Richardson) were estimated by snorkel divers at 6 sites in the middle reaches of the Rangitikei River, North Island, New Zealand, over 14 months. The results showed that different species and sizes of trout varied in abundance with time. The species of fingerling trout (6-12 cm FL) could not be identified because of their small size and shoaling behaviour. Rainbow trout abundance varied seasonally and was greatest in January and April (between 18 and 60 fish per kilometre) when fish between 23 and 38 cm FL were the most abundant size class. Brown trout abundance showed much less variation with time (between 5 and 36 fish per kilometre at most sites). Also in contrast to rainbow trout, the majority of brown trout were > 38 cm FL, and in June, when the greatest density was observed (56 fish per kilometre), 70 redds were seen at the same site. Two sites were dived within a 48 h period to test the variability of the method. Comparisons between the 3 dives at each site revealed no significant differences between the numbers offish in different species and size classes.