Salmon stomachs help beat the heat

When you think about survival in a warming world, you might picture animals moving to colder places, but sometimes, survival depends on something much more prosaic – the gut. The gut is where food is digested and nutrients are absorbed, supporting growth and daily activity, and the guts of some fish species are larger and more efficient than others. Could those differences explain why some fish grow well in warmer waters while others struggle? That was the question that Harriet Goodrich and colleagues from the University of Tasmania, Australia, set out to answer. Atlantic salmon (Salmo salar) are one of the world's most popular farmed fish and an important food supply. But they prefer cool waters. When the temperature creeps above 18°C, they quickly feel the heat, their appetite drops, their growth slows and farmers face big challenges keeping them healthy. Goodrich and colleagues wanted to find out whether differences in gut size and function might give some salmon an advantage when temperatures rise.To test this idea, the team raised 83 salmon in tanks for a little over 4 months. For most of that time, the fish lived in warm water at 19°C, similar to the summer conditions in Tasmania. Before and after this period, the researchers carefully measured the fish, weighing them, checking their growth and examining their guts. They looked not only at the length of the gut but also at how well different parts of the gut absorbed nutrients.The results were striking. As a group, the salmon ate less in the warmer water, confirming that hot summers are detrimental to salmon appetites. But not all of the fish reacted in the same way. Roughly half of them managed to keep growing, while the others barely put on weight. What set the successful growers apart? They had longer guts. Goodrich and colleagues then looked further. They tested how well the fish's guts absorbed nutrients – a vital step in turning food into usable energy and providing the building blocks for growth. The fish with guts that absorbed nutrients more efficiently grew faster. Together, these findings showed that both the length of the gut and how well it worked influenced which salmon could thrive in the heat. For fish farmers, this could be an important clue for breeding or selecting salmon that will stay healthy and productive as ocean temperatures rise.In the end, Goodrich and her team revealed that the secret to coping with heat may lie in what happens inside fish's guts. Bigger and better guts might just be one of the hidden differences that help these fish keep growing when the heat is on.