Welcome back from the Lab and Stream. We have been off chasing fish for the last few months, and we hope you have been too!
In this post we are going to discuss some very interesting research on colour changes in salmonids. Most trout and salmon have beautiful colouration, and I am sure many readers of this blog will be familiar with changes in colour during spawning (think of the flush red cheek of a steelhead, or the bright red and green of a sockeye). However, many fish (including salmonids) also use colour to signal during fights. For example, juvenile salmonids change their colour to signal when they have been defeated in a fight. When one individual has had enough and wants to give up, they darken their skin and their eyes to signal to their opponent. In response to this darkening the winner will stop attacking them, and so the signal is useful for both parties.
But what happens in murky or turbid water, which many trout live in? Does the signal still work? Researchers from the UK performed an experiment aimed at testing how this ‘signal of defeat’ was modified by water turbidity. They allowed pairs of juvenile brown trout to fight in either clear water, water with low turbidity or water with high turbidity. What they found was that as the water turbidity increased, the ‘loser’ of the fight became even darker than those that lost fights in the clear water condition. The researchers noted that the physiology of the losers did not differ according to the water turbidity, so the only explanation was that fish in turbid conditions were actively increasing their ‘signal of defeat’ so that it was more apparent in the turbid water.
This interesting finding underlines how water turbidity can have important effects on a fish’s life. Not only does turbidity influence feeding, predation risk and respiration, but now we know that it also influences signalling.
Citation: L Eaton and KA Sloman. 2011. Subordinate brown trout exaggerate social signalling in turbid conditions. Animal Behaviour 81: 603-608.
Find the paper here