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Hybrid salmon have emerged near Vancouver Island in Canada, possibly due to environmental changes in the waterways where they hatched.

at Fisheries and Oceans Canada in Nanaimo and his colleagues analysed the genes of salmon surveyed mostly within the Strait of Georgia, which separates Vancouver Island from the mainland, between 2013 and 2019. They found that samples from what they assumed to be聽young Chinook salmon (Oncorhynchus tshawytscha) had unusual genetic markers. More sampling and genetic analysis revealed that these fish聽were hybrids, crossed with coho salmon (Oncorhynchus kisutch), another species found in the region.

Many of the hybrids look like a mix of the two species, says Araujo. Some have grey-lined gums, intermediate between the Chinook鈥檚 black gums and pale coho gums, for example.

Hybrid salmon have been documented before, but very rarely. In the latest study, the team found a persistent, low-level presence of hybrids, most of which were found near the mouth of the Cowichan river, an important spawning ground for the two species. The team says nearly 5 per cent of the fish sampled that are thought to have originated in the Cowichan are hybrids.

Araujo and his colleagues think these hybrids could be a result of the impact of climate change on the Cowichan. In recent years, dry conditions have lowered the river鈥檚 water level, delaying the start of the Chinook salmon鈥檚 late summer spawning. This increases the chance that the coho 鈥� which arrive and spawn in the autumn聽鈥� will overlap with the Chinook spawning, possibly resulting in interbreeding.

Araujo says there have been anecdotal reports of coho and Chinook salmon sharing Cowichan spawning grounds at聽the same time, 鈥渨hich is not聽really what is expected鈥�.

Logging operations along the river are thought to have reduced the amount of suitable habitat for salmon spawning, which further encourages the two species to cross paths.聽As more streams in the region聽shrink in an increasingly warm world, such hybrids may become more prevalent, the researchers say.

The team found some hybrids that reached adulthood and were able to reproduce, but interbreeding could cause problems as the hybrids may be聽less well adapted to their environment, says Araujo.

at the University of California, Davis, says he isn鈥檛 convinced that hybridisation is directly linked to human activities. These two species have been in the same rivers for 鈥渢housands and thousands of years鈥�, says Miller, and have endured many low-water years with little separation in spawning.

If hybridisation does have a聽negative impact on salmon populations, it could have wider聽consequences, too. Salmon are a food source for聽a聽variety of animals, and are聽also聽both economically important to the area and deeply significant in Indigenous cultures in the region.

Araujo hopes that future monitoring programmes can use the prevalence of these hybrids as an ecosystem health warning sign, since they 鈥渕ight be an indicator of problems鈥� in the river system, like low water levels.

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