FOR anglers nothing beats catching a big fish. Commercial fisherfolk also prefer to haul in big specimens. Unfortunately, in recent years, research has shown that selectively capturing the largest fish has worrying ecological consequences. In some species the large ones are the healthiest ones, and so the ones most likely to breed successfully. In others they are the oldest, and so the most experienced at eluding predators or securing resources, such as food and breeding sites. In tropical wetlands, such as the Pantanal and Amazon regions of Brazil, the largest fish are also vital in dispersing seeds—and thus maintaining and regenerating habitat.

Trees in these areas fruit most prolifically during the summer, when local rivers burst their banks and flood the land, making those fruit available to fish, which gladly gobble them up. Then, as the fish swim around the floodplain, they pass the seeds inside those fruit, which often remain intact, as part of their faeces. These seeds are thus distributed far and wide. Researchers have found that the most effective distributors are the biggest ones. Because they have bigger bellies they eat more, and because they have wider mouths they are more likely to swallow seeds whole rather than chew them up, as smaller fish might.

Many studies of seed-eating fish have thus looked at large species. Yet small fish also distribute seeds, especially small seeds, according to a review of the scientific literature just published in Biotropica by Raul Costa-Pereira and his colleagues at São Paulo State University. That led Mr Costa-Pereira to speculate about the role of large specimens in populations of smaller fish. This matters, because when stocks of larger species are depleted commercial fishermen often start “fishing down” the food chain to capture smaller ones.

The researchers therefore monitored the seed-eating behaviour of tetra fish, which grow to a length of 12cm, and freshwater sardines, which make 18cm. Since both species patrol areas where trees drop ripe fruit during the flood season, and both are also caught by fishermen when they approach their maximum size, Mr Costa-Pereira speculated that selective fishing of the largest tetra and sardines would reduce the number of intact seeds that the animals would later defecate.

The team collected a sample of fish on the Miranda river flood plain in the Pantanal, measured the lengths of their specimens and the gapes of their jaws with callipers, and then analysed the animals’ gut contents under a microscope. They found that 63% of seeds counted in the guts of the freshwater sardines had been destroyed by chewing, and 22% of those in tetras had suffered the same fate. Crucially, they also found, for both species, that the number of intact seeds did indeed increase with the size of the fish they came from.

Mr Costa-Pereira therefore believes that the same rule applies to small fish species as to big ones—namely, the largest individuals play the biggest role in distributing seeds. This, he suggests, needs to be considered when catch limits are set. The catching of small species, which are used not only as food for people but also to feed animals and as bait, is often less regulated than the catching of large ones. Intensive fishing can rapidly deplete the number of smaller fish. If the biggest individuals go first, there could be repercussions for the wider ecology of a flood plain.