Morphometric and External Injury Characterization of Rainbow Trout ( O. mykiss ) Used for Stocking in the Alpine River Inn (Bavaria, Germany)

Stocking fish is a common practice in fisheries management, particularly for species of high conservation value or economic importance. Despite its widespread use, systematic evaluation of fish stockings remains scarce. This includes non‐native species such as rainbow trout ( Oncorhynchus mykiss ), which are intensively stocked into European water bodies for recreational angling. Rainbow trout used for stocking into the German River Inn are sourced from various hatcheries, likely contributing to differences in their physical condition which may affect post‐stocking survival. This study provides a baseline assessment of the physical condition, focusing on morphometric traits and external injuries, of rainbow trout prior to stocking in this large alpine river. We analyzed 150 individuals from five aquaculture facilities using morphometric measurements and standardized injury assessment protocols. Rainbow trout showed a high level of morphometric plasticity associated with their origin. All examined fish had visible external injuries, with the most frequent ones being tears and amputations on the caudal and dorsal fins, as well as scale loss. Up to 11 of 22 possible injury types were identified in the most severely affected individuals, and specific external injury patterns could be related to specific hatcheries. These findings offer important baseline data on the physical condition of stocked rainbow trout and provide a foundation for interpreting future studies on their performance in the wild.