Life in the fast (and slow) lanes for salmon

Life history variation is fundamental to the long-term persistence of populations and species, as it ensures their ability to adapt to changing environments. Many important studies have focused on life history variation between habitats, but variation maintained within a habitat has often been neglected.

Solving this puzzle at the landscape level is essential to understanding the spatial scales at which adaptation and persistence of populations in nature occur. This motivated a team of researchers from Kyoto University to study variations in the life history of masu salmon. The results were published in Journal of Animal Ecology.

“We wanted to understand how life history variation is distributed within and between habitats across heterogeneous landscapes,” says first author Takeya Shida.

Previous research has demonstrated that salmonids inhabiting large temperate watersheds are an ideal subject for this type of study. In salmonids, “fast-living” individuals achieve accelerated growth early in the growing season – initiating gonad development in the spring and reaching maturity in the fall of a given year – while “slow-living” individuals postpone the maturation decision until the following year.

With this knowledge, the research team examined the hierarchical structure of salmon life history variations across a large temperate watershed. They predicted that each habitat would retain some degree of variation, but that the tendency toward fast living would be more prevalent in downstream reaches while slow living would be more prevalent upstream.

The team established six study reaches, or segments, along a certain stream and studied seasonal changes in water temperature and food resources, as well as the seasonal growth of juvenile salmon and the age of mature fish.

They then quantified the relative contributions of differences in mature fish ages between and within habitats to overall mature fish age diversity across the watershed, performing separate calculations for males and females.

The results largely confirmed the team’s hypothesis. Fast-living individuals tended to dominate downstream reaches—with warmer temperatures and abundant aquatic invertebrate prey—while slow-living individuals were more prevalent upstream, with colder temperatures and fewer aquatic prey but relatively abundant terrestrial invertebrate inputs into streams.

However, scientists also found considerable variation in the ages of mature fish in all areas except the most downstream and upstream reaches. This suggests that life history variation at the watershed scale depends more strongly on variation within habitat than on variation between habitats.

“It is fascinating that the balance of diversity within and between habitats can be influenced not only by environmental factors within the stream, but also by terrestrial invertebrate prey from the surrounding riparian forests,” says Shida.

As habitat homogenization continues, this within-habitat variation may become more important in supporting adaptation to environmental changes such as global climate change.

“Recently, we have seen a loss of diversity within species much faster than species diversity,” says team leader Takuya Sato. “This study reminded us of the importance of managing salmonid resources in a way that preserves this diversity.”