Overlooked aspects of the Salmo salar and Salmo trutta lifecycles

The salmonid lifecycle has been studied for over a 100 years. Our literature search indicated that the Atlantic salmon ( Salmo salar ) and brown trout ( Salmo trutta ) are among the most studied of fish species. By reviewing both their anadromous and non-anadromous lifecycles, we show that there is a growing body of evidence of considerable variation in many aspects of their lifecycle. However, variation in migration patterns and life history strategies are still poorly studied and not well understood, such as juvenile autumn migration, repeat spawning, marine migrations, straying and homing. Growing evidence supports a group of downstream autumn migrants in both species, which may represent as much as 25–40% of the spring class. Some males and females mature sexually as parr very early in life. They probably contribute to genetic variation and stability to populations in a changing environment and are likely very common in many rivers, but rarely considered. Information on marine migrations have been restricted by available methods, and particularly for brown trout, this may have resulted in underestimating straying and long-distance migrations. Repeat spawning is another understudied aspect of the salmonid life history but should be viewed as an opportunity to understand ecological and evolutionary dynamics. We conclude that both brown trout and Atlantic salmon appear to have aspects of their lifecycle overlooked, and that the description of their lifecycle should acknowledge the variation we observe in natural systems as well as the flexibility between strategies.