Complex effects of body length and condition on within-tributary movement and emigration in stream salmonids

Movement patterns of stream fish vary individually, but little is known about how a set of individual characteristics affect movement at multiple spatial scales. We investigated the effect of body length and condition (i.e. weight relative to length) on emigration from a tributary and movement within the tributary during summer in Japanese salmonids (white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou) using mark-recapture and PIT antenna technology. Emigration from the tributary was influenced more strongly by body length than by body condition, whereas movement within the tributary (> 20 m) was influenced by body condition and their interaction with body length. Specifically, larger individuals in better body condition were more likely to stay locally (≤20 m), but smaller individuals in better condition were more likely to move in the tributary. We discuss benefits and costs of movement that vary with individual characteristics (i.e. body length and condition) and spatial scales. In one instance (charr between June and July), survival rates were lower in smaller individuals, which also were more likely to emigrate from the tributary, suggesting that emigration might have been facilitated by the mortality cost of staying in the tributary. This study indicates that stream fish movement is shaped by complex mechanisms that differ by spatial scale. Although complex, two study species often responded similarly, indicative of emerging regularities across species in determinants of multi-scale stream fish movement.