Mortality and growth of juvenile brown and rainbow trout in a lake inlet nursery stream, New Zealand

Density and growth of mixed populations of 0+ brown and rainbow trout, and population changes of the latter, were studied in Scotts Creek, a spawning/nursery inlet stream of Lake Alexandrina. Stream rearing capacity and the potential for interspecific competition between resident 0+ brown and rainbow trout were assessed. The 0+ trout populations were dominated by rainbow trout with maximum density of rainbow fry being 7.7 fry m ‐2 ; whereas thai: of brown fry usually was less than 0.16 frym ‐2 . Observed loss of 0+ rainbow trout over much of the summer was constant whereas mortality increased. Loss for the first six months of life was c. 96%. Mean length at the end of the summer growing season (April) was c. 114 mm and 69 mm for 0+ brown and rainbow trout, respectively. Size‐selective emigration for large fish affected observed growth of 0+ rainbow trout in the autumn and early winter. Maximum biomass of 0+ fish ranged between 19.2 + 3.5 gm ‐2 and 26.0 ± 4.5 giti ‐2 over the three summers. Brown trout obtained a size advantage over rainbow trout through earlier emergence, but they were too few in number to exert a competitive influence over rainbow trout.