Comparison of physiological smolt status in descending and nondescending wild brown trout (Salmo trutta) in a Danish stream

Abstract

Abstract – Downstream movement of a wild population of brown trout was examined in a small Danish stream in relation to morphological and physiological smolt status from March to May. Downstream movement was monitored in a Wolf‐type trap covering all possible passage routes in the stream. Trout caught in the trap were classified as parr, pre‐smolt or smolt based on morphological criteria and compared with trout randomly caught by electrofishing upstream of the trap. Representative gill samples from trap‐caught and electrofished trout were analysed for gill Na+, K+‐ATPase activity and used as a measure of physiological smolt status. Only a few parr occurred in the trap. Few pre‐smolts occurred in the trap evenly in March and early April. In late April, pre‐smolt movement peaked. By comparison, the main downstream movement of smolts occurred in distinct peaks through late March and April. The majority of fish caught in the trap were judged as pre‐smolts or smolts based on morphological criteria's and they were characterised by relatively high gill Na+, K+‐ATPase activity compared with trout judged as parr. Trout caught by electrofishing upstream the trap, were classified as parr, pre‐smolts and smolts early in the season (March). During and after the main smolt‐run in April the distribution of the remaining trout in the brook became skewed in favour of pre‐smolt and parr. The study suggests that smolting trout initiate downstream movement once having reached a certain physiological smolt condition (judged by increased gill Na+, K+‐ATPase activity).