The relative abundance, movement, and growth of rainbow trout (Salmo gairdneri) and brown trout (Salmo trutta) in the Rangitikei River, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science with Honours in Zoology at Massey University
The null hypothesis tested was that rainbow and brown trout populations do not move between naturally defined sections of the Rangitikei River. It was found to be true for adult brown trout but false for rainbow trout. Recaptures of tagged brown trout demonstrated that the majority of these fish living in the mid-reaches do not make seasonal movements between river sections. Brown trout dwelling in the lower reaches were smaller than mid-reach brown trout. This difference, and the lack of tag returns indicating movement between the two sections, supports the hypothesis. Recaptures of tagged rainbow trout demonstrated that the majority of these fish migrating from the mid-reaches in autumn and winter travel to the headwaters where they remain the following summer. Those rainbow trout which were recaptured in the headwaters after moving from the mid-reaches tended to migrate earlier in the winter than those captured, then later recaptured, in the mid-reaches. Similarities in the size of rainbow trout spawning migrants captured in the lower reaches and the mid-reaches suggested that both groups spent at least their second and third years in the same area of the river, but low numbers of tag returns meant that no firm conclusions regarding rainbow trout movement between the mid and lower reaches could be made. Limited data concerning movement during the summer period suggested that some rainbow and brown trout move within sections but evidence of individuals remaining in one place for extended periods was found also. Reported behaviour of both species of trout in response to seasonal physiological changes and agonistic pressure, allied with stream bed morphology probably accounted for the observed distribution of young of the year, year one, year two and adult trout in the river. Upstream migrating adults, of both species counted at two traps, were found to respond to fluctuations in water flow and were probably affected by moon phase so that migratory activity was saltatory. Rainbow trout tended to migrate earlier in the winter than brown trout. The movement of female brown trout followed the male brown trout migration but similar differences were not observed in the rainbow trout migrants.