Environmental factors drive different life strategies in rainbow trout

The life strategy adopted by rainbow trout, whether river-dwelling or migrating between freshwater and the sea, is determined mainly by environmental factors such as the availability of food and spawning grounds, and not genetics, research reported in the Journal of Ichthyology has shown¹.

The rainbow trout (Parasalmo mykiss) is a salmonid species that can take on very different life strategies. While the majority are anadromous, meaning they migrate between freshwater rivers and saline marine waters, some spend their entire lives in rivers or brackish estuaries. Little is known about how this difference arises and the comparative influence of environmental and genetic factors.

To address this knowledge gap, ichthyologist, Kirill Kuzishschin, and colleagues at Moscow State University and the Russian Academy of Sciences investigated the population structure and genetic variability of rainbow trout in the Utkholok River in northwestern Kamchatka, which flows towards the Sea of Okhotsk.

Based on monitoring data collected over the last 30 years, the researchers compared the proportion of rainbow trout specimens categorised according to five different types of life strategy: typically anadromous, anadromous, estuarine, riverine-estuarine, and riverine.

The study revealed a high proportion of typically anadromous or anadromous fish compared with all other types across all years.

The researchers observed that large anadromous females tend to spawn with small river-resident males. They infer that this could reduce the likelihood of genetic isolation between groups of fish with different life strategies.

Genetic analysis showed that the Utkholok rainbow trout represent a single, integrated population with a common gene pool, and suggested that the influence of heredity on the formation of different life strategies is therefore only partial.

The main drivers in shaping alternative life strategies appear to be environmental factors such as the availability of spawning grounds, temperature and food availability.

“That means every specimen has a potential to adopt one of the five known life strategy patterns,” says Kuzishschin.

Changes in climate may already be a strong influencing factor for life strategy variation. A cold period in the North Pacific may account for the absence of estuarine and riverine-estuarine fish in 1970–1971, and a warmer period that followed may be linked to a northward shift in feeding areas. Notably, the rainbow trout population appears to have been able to adjust flexibly to large-scale fluctuations of environmental factors over the course of decades.

“We plan to continue studying more riverine systems with different geomorphologies, where all spawning grounds are located in tributaries,” Kuzishschin says. “We are interested in the effects of greater isolation pressure on local stock.”