Seasonal changes in spatial patterns of Oncorhynchus mykiss growth require year-round monitoring

Growth and movement of juvenile salmonids influence the expression of individual life history traits and production of adults at the population scale. We individually marked and recaptured juvenile Oncorhynchus mykiss over the course of a year in Murderers Creek, a semi-arid tributary to the South Fork John Day River in Northeast Oregon. We tagged O.mykiss in three reaches with differing stream gradient, stream temperature and fish density. Mean growth rates differed significantly among reaches and seasons with a significant interaction between reach and season. Reaches with high growth rates shifted across Murderers Creek among seasons. Stream reaches with high growth during the winter had low growth during summer and vice-versa. The proportion of individuals moving at the reach scale during summer was low (2.6%), suggesting that individuals did not track resources at the reach scale. The spatio-temporal variation in growth indicates that monitoring stream salmonids across multiple seasons is necessary to accurately characterise the production of different stream reaches.