A trout swimming in the lab behind the D-shaped obstacle, approaching a drifting food pellet (indicated by red arrow). Photo credit: Jimmy Liao.
Despite appearances, life in the fast lane may not be as tumultuous as it might seem at first. Many fish species choose to reside in fast-flowing sections of river as the turbulence provides them with an almost free ride when sheltering downstream of rocks in the flow. ‘Prevailing theory suggests that many species exploit hydrodynamic refuges to minimise the cost of locomotion while foraging’, says Jimmy Liao from the University of Florida, USA. But he was curious to find out how much exertion it takes for fish sheltering behind rocks to catch a snack. Explaining that a lifetime of reading fly-fishing magazines had convinced him that fish use less energy when foraging in choppy water than in more sluggish sections, Liao, with Jacob Johansen and Otar Akanyeti, put the anglers’ perceived wisdom to the test.
Slowly increasing the water flow in an artificial stream in the lab to 68 cm s−1, Liao and Johansen provided individual trout with a D-shaped column to shelter behind while measuring how much oxygen they consumed. Then they tricked the hungry trout into nipping out from behind the obstacle to snap at a tasty treat drifting past on a line, before wrenching it from the surprised fish's lips; ‘They would sometimes chase it upstream’, he chuckles. However, when Liao compared the fish's oxygen consumption with that of fish swimming freely in an unobstructed flow, he realised that the fish that were ensconced behind the D-shaped obstacle had to work 65% harder when snapping at the lure than fish that were feeding in a freely flowing stream. And, when Liao and Johansen monitored the fish's reactions as tempting food pellets drifted toward the animals, the sheltering fish struggled to intercept their treats as the flow increased, barely capturing any pellets at the fastest flow (84 cm s−1), compared with the unobstructed fish, which still captured 60% of the pellets. Even though trout save energy when surfing in the wake of an obstacle, venturing out of the wake to feed requires more effort and their success rate plummets in faster flowing sections of water.
Taking these lessons into account, Akanyeti and Liao built a computer model of the energetics of a fish feeding in a river to help them understand when it is best for fish to take advantage of shelter and when the benefits of refuging no longer outweigh the expense of foraging from behind a rock. Running the simulated river at flows ranging from still water up to 84 cm s–1, the team found that sheltering behind a rock was more beneficial in slower flows, while freely swimming fish appeared to have the upper hand in torrents of faster water; ‘Our model predicts that individuals living in flows greater than 50 cm s–1 should avoid refuges while foraging’, says Akanyeti. The simulation also suggested that toggling between hiding behind rocks for part of the day and venturing into faster smooth water at other times to feed benefits the fish most, allowing them to take advantage of both worlds.
Although it seems that life is more costly for refuging trout than anglers had presumed, Liao suspects that the shrewd fish also make the most of other energy-saving shortcuts that we have yet to discover. He hopes that engineers designing slipways alongside dams for migrating fish and ecologists restoring river beds can learn from the life experiences of trout. ‘I'd like to see us more often asking the fish themselves what they want; which means ground-truthing designs based on fish behaviour’, he says.
