Feed me: the pacu tends to munch on fruit and carry seeds inland during floods. Credit: M. SMITH/SPL

When it comes to studying seed dispersal in forests, mammals and birds are the usual suspects. But in Brazil's Pantanal, the largest freshwater wetlands in the world, fish have been found to have a remarkable role in distributing the seeds of tropical plants. Fishing practices are not only threatening the fish, say the researchers who report the find, but the forests as well.

In the usual methods of animal seed dispersal, primates, rodents and birds either eat fruit and ingest seeds contained inside or get seeds from the plant stuck to their bodies. Later, the seeds are either defecated intact, or fall off. But in recent years ecologists have found seeds in the digestive tracts of more than a hundred fish species as well.

In the Pantanal, plants including palms and legumes tend to release their fruits during a time of year when massive flooding is common, and waters encroach over thousands of square kilometres. The fruits fall from the trees into water and the pacu (Piaractus mesopotamicus), one of the most common fish in the Pantanal, migrate deep inland during such floods and munch on the fruit; local fishermen often catch them by putting fruit on their lures. Mauro Galetti, at S£o Paulo State University in Brazil, and his team looked to see whether these fish were carrying intact seeds, which they could defecate in areas that dry out when the floodwaters recede.

The team explored the guts of 70 fish collected at Fazenda Rio Negro, a ranch run by Conservation International in the Nhecol£ndia region, an ecologically diverse area of the Pantanal that teems with wildlife. They report in Biotropica that there was a positive correlation between fish size and the number of intact seeds in the stomach: more than 141 seeds from the tucum palm were found in the largest individuals1.

Fish spread

Galetti and his team also conducted a four-year study of 54 vertebrate fruit eaters, ranging from tapirs and monkeys to toucans and pacus, and watched 23 fruiting plants for hundreds of hours, to determine which animals ate what. They then collected the faeces of all these animals and tested whether the seeds found would germinate under natural conditions. From the data collected, Galetti says, it seems that the tucum palm relies almost entirely on pacu services for seed dispersal. ?[It is] amazing that for some plant species, pacu appear to be the main dispersers,? says Galetti.

This is a worrying find, because pacu populations are declining rapidly. The largest fish, which are the best swimmers and the most capable of dispersing seeds furthest during a flood, are also the most likely to be captured by fishermen and taken to the dinner table. Ironically, conservation laws may be making the situation worse. In Brazil, larger fish are the ones that fishermen are allowed to catch, and only pacu that are smaller than 40 centimetres long are legally protected. ?Fishery management like this is probably detrimental to forests since large fruit-eating fish are the best dispersers,? Galetti adds.

?This is the first report that I know of suggesting links between aquatic and terrestrial ecosystems where fishing may be impacting tropical forests,? says ichthyologist Bill Pine at the University of Florida in Gainsville. Monitoring the typical furry and feathery seed dispersers no longer seems to be enough: fish are important for these forests too.

?I think the Amazon and African jungles need to be extensively studied for ecosystems like this,? says Galetti. ?Fish seed distribution is probably a lot more common than we realise.?