Parents influence their offspring in one form or another, whether it is the initial input of genes or ongoing involvement throughout development. Importantly, parental contribution can impact the evolution of a species. But what if parents are prevented from contributing to their offspring's future? How would that affect the evolution of a population? Sonya Auer, from the University of Glasgow, UK, and colleagues explain that Scottish Atlantic salmon (Salmo salar) are no longer able to reach many of their spawning grounds thanks to dams that have been installed by humans. So, to ensure that salmon populations are not lost, people now capture adult fish as they return from the sea ready to spawn, collect the eggs and sperm and place the fertilised eggs upstream in the spawning areas where they need to hatch. However, during this process, an essential ecological component is lost: adult salmon usually die after spawning, leaving their carcasses as nutrients for the ecosystem. Auer and colleagues wondered whether this loss of nutrients can alter the survival, growth and metabolic rates of their young and, therefore, the evolutionary trajectory of the salmon in Scotland's River Conon catchment.
Auer and her team took advantage of this human-modified ecosystem by adding nutrients in the form of dried hatchery salmon pellets – which are essentially salmon carcasses – to five streams that form part of the river catchment during the breeding season. Simultaneously, they transferred 3000 eggs from 30 different salmon families into each of the pellet-enriched streams and five other streams that remained untreated. The team also took fertilised eggs from each of the salmon families to grow and develop in the lab so that they could measure the metabolic rates of these lab-reared siblings at the age of 2 months. In addition, Auer and colleagues collected insects, the preferred meal of juvenile salmon, from each of the 10 study streams a couple of months after the young salmon should have hatched to assess how the added nutrients influenced the food available to the developing juveniles. Unsurprisingly, the increased amount of nutrients provided by the makeshift corpses boosted the insect population 2.5-fold. But could this insect bounty influence the development of salmon young?
First, the team explored this by capturing young salmon from the streams to measure their size and obtain fin clips for DNA analysis to determine which families the juveniles belonged to. The insect glut indeed led to heftier juveniles. Intriguingly, the surviving salmon in the pellet-supplemented streams included individuals from most of the original families, whereas several of the families in the unsupplemented streams died out. And when the team compared the maximum metabolic rates of the lab-reared siblings, it was clear that the siblings from families that survived in the unsupplemented streams had higher maximum metabolic rates than those of the families that perished. These families with higher maximum metabolic rates are likely to be more dominant and can obtain and protect better territories, which should be beneficial in a habitat where food is scarce and competition fierce. Conversely, as the family death rate in the supplemented river was low and there was significant diversity in the maximum metabolic rates measured in the laboratory siblings, the team suspects that the variation of maximum metabolic rates of the fish in these streams was greater, possibly as a result of more food and less intense competition.
It seems that nutrients provided by the rotting corpses of salmon parents can ease the squabbling over food among young, resulting in vaster genetic diversity. Yet, the circumstances created by the damming of the River Conon catchment have influenced the evolution of salmon by favouring individuals that are better able to defend prime territories.