Stream Gradient‐Related Movement and Growth of Atlantic Salmon Parr during Winter

Abstract There has been considerable focus on winter studies of Atlantic salmon Salmo salar parr during the last two decades. However, a lack of knowledge exists about the linkage between the physical conditions, including ice, and parr behavior in flow environments during the cold season. In this study, the movement and growth of Atlantic salmon parr were studied during winter in two stream sections with different gradients (0.3% and 1.8%) in a small natural river. Passive integrated transponder (PIT) technology was implemented by using both fixed antennae and mobile tracking devices. In the low‐gradient section, the formation of surface ice dominated and created stable conditions, whereas in the high‐gradient section, the formation of anchor ice and anchor ice dams occurred periodically and produced a dynamic environment. The results indicated a relationship between parr movement and river gradient. Movement by parr was negatively correlated with time from autumn to late winter but increased as spring approached. The level of movement was considered low (median = 0.9 m), but larger movements (up to 125.6 m) were recorded, indicating individual variation. Furthermore, parr inhabiting the low‐gradient section with the static ice formation exhibited larger movement than parr in the high‐gradient section with dynamic ice formation. Parr showed high site fidelity to both high‐ and low‐gradient sections but were less attracted to pool habitats. Finally, from late autumn to spring, parr demonstrated a specific growth rate close to 0.0%, indicating suitable conditions in both sections. As this study implies, the complexity of physical conditions during winter may lead to variability in individual fish response (i.e., movements). However, the results also imply that winter is not a limiting factor in parr performance if fish have access to suitable cover, such as areas with low substrate embeddedness.