River connectivity implies free passage for migrating fish, yet not all fish individuals migrate in spite of full connectivity. Such partial migration can be driven by a variety of factors, and fish migration propensity can have delicate implications for conservation and passage solutions. Using European cyprinids as model organisms and a PIT system to monitor migration, this presentation describes seasonal changes in predation risk (P) and growth opportunity (G) as drivers of seasonal migration between lakes and their connected streams. Although the P/G ratio convincingly describes seasonal timing of migration at the population level, individual migration probability and behaviour vary substantially with fish individual e.g. body condition, animal personality, migration history, or size- and predator-specific risk of predation. Fitness tradeoffs in migration decisions can affect survival and consequently phenotypic and genotypic composition in migratory fish populations, and such selective forces can be imminent also for fish passage solutions that do not consider phenotypic diversity in the migratory fish populations they are aimed to aid.