Maternal inheritance influences homing and growth of hybrid offspring between wild and farmed Atlantic salmon

The occurrence of domesticated organisms in nature is a major biological issue. Many can survive and reproduce with wild conspecifics, but little is known about the ecology of the hybrid offspring. Here, we investigated recapture, homing and growth of the released offspring of farmed (domesticated) and wild Atlantic salmon Salmo salar as well as their crosses. The fish were hatchery-reared for 1 yr and released as smolts in the River Imsa, Norway, the home river of the wild fish. The released fish migrated to the ocean. Many disappeared, but between 0.6 and 2% were sampled on their return as adults 1 or 2 yr later. Homing to the river of release was low for farmed salmon (88% strays). Among hybrids, having a wild mother provided increased homing accuracy (36% strays) relative to having a wild father (64% strays). Offspring of the local River Imsa salmon exhibited the best homing (28% strays). The growth capacity of the farmed salmon appeared to be not fully utilized in nature, as there was little difference in maximum size between wild and farmed salmon. However, minimum size at maturity was larger for farmed than wild conspecifics. Inheritance from a farmed mother increased minimum size of the hybrids versus that of the pure farmed salmon. Thus, maternal inheritance from domesticated, farmed salmon decreased homing and increased minimum size at adulthood of the offspring, making domesticated salmon a risk for locally adapted salmon in nature.