If you were to catch a Chinook salmon, it might be red or it might be silver-white and this is because red Chinook salmon take up pigments called carotenoids. Carotenoids come with benefits in the animal kingdom: they can enhance how attractive colourful signals are to mates and even help boost the immune system. Time and time again, carotenoids have been shown to be beneficial for animal fitness. How is it, then, that the white Chinook salmon, which have little to no carotenoids, persist in the wild?
This question engaged Sarah Lehnert and her colleagues from the University of Windsor and the Department of Fisheries and Oceans, Canada, who wanted to know whether there was a hidden cost associated with carotenoids making fish bright and rosy red. The two colour variants in Chinook salmon are present throughout the entire life cycle; the red salmon produce vivid red eggs while the white salmon eggs are uncoloured. Salmon eggs are particularly susceptible to predation because salmon parents do not care for their offspring after spawning. Might the red eggs from the carotenoid-rich red salmon be more vulnerable to predation than the colourless eggs of the carotenoid-deficient white salmon?
Lehnert's research group decided to test whether white Chinook salmon eggs enjoyed a survival benefit over their red counterparts when faced with predators. As rainbow trout naturally prey on salmon fish eggs, the team simultaneously released white and red Chinook salmon eggs into tanks full of hungry trout. They then monitored how many of the eggs were snapped up by the famished fish. The researchers found that the red eggs were more likely to be eaten first by the trout. In fact, the red eggs were twice as vulnerable as the white eggs and they were consumed much faster.
The vulnerability of the red eggs to predatory fish may therefore allow the white Chinook salmon to persist in the population, despite the fact that red fish are likely to benefit from being carotenoid rich. Before the team's study, researchers knew very little about how the two colours of Chinook salmon were maintained in the population. Their findings show that white salmon have a knack for surviving early in life, as flashy colourful carotenoids are in fact quite costly for Chinook salmon eggs.