Welcome to Tanzania's 'fish city', Mwanza. Sprawled along the shores of Lake Victoria, the terrain of this town looks unwelcoming, but its tatty airstrip is uncannily busy. Enormous cargo planes arrive daily to collect tons of white fish fillets for European consumption.

The booming fishing industry is the pulse sustaining Mwanza's economy. What's the drama in this, you may ask. However Hubert Sauper's Oscar-nominated documentary Darwin's Nightmare does not care much about dramatic rummaging of the subject, and is engrossed instead in a pensive observation of the looming disaster in the town's ecology and economy.

The drama is the Nile Perch, a non-native predatory fish species weighing up to 200kg which was introduced into the lake in the 1960s as an experiment, and which has coupled well with another giant - global capitalism. In the deeper waters of Lake Victoria, the Nile Perch multiplies at a rate that may please the IMF, its advocates and the industries they support. But the voracious fish remains highly elusive to the shallow water fishing technologies of local fishermen, while preying off to near extinction the rich variety of species on which the locals traditionally depended for subsistence.

For Sauper, though, the Nile Perch is only a hellish allegory for the nightmare that emerges from the desolate stories and experiences of people who inhabit the city.

With the economic arrangement fostered by the Nile Perch, Mwanza's denizens include East European pilots, the prostitutes who entertain them, impoverished villagers who have come looking for work, orphans who fight for a fistful of rice, and street kids who take refuge in sniffing intoxicating glue made from the plastic waste of Nile Perch packaging.

Tanzania itself, the film informs us, is anticipating a famine, and locals are unable to afford the leviathan Nile Perch, which goes for export anyhow. Only the sweepings are left over, and in one harrowing scene, we see a woman stacking and drying rotten fish which was rejected for export, heaps of carcasses infested with maggots, which are then sold to the locals who eat the heads fried or smoked.

This shocking image of poverty has not cut ice with everyone, however, and several people have claimed it is merely an objectionable polemic. Sauper's film has been accused of being unbalanced, even untrue, and obscuring the 'positive side' of the global economy which, it is argued, increases in the GDP and expands the middle classes in countries like Tanzania.

But the moral conundrum of this economy does not end there. For what the cargo planes bring to Africa, Sauper argues, is even more devastating. In fact, the idea for the film came when pilots Sauper befriended during the shooting of his previous film Kisangani Diary admitted that they were bringing ammunitions to Africa. Pilots featured in Darwin's Nighmare make the same confession, albeit with great hesitance and ambiguity.

Sauper does not suggest a direct bullets-for-fillets exchange. After all, the arms trade occurs not in Mwanza but in other African stopovers. The argument - Sauper is aware - is extremely tenuous. Asking a group of prostitutes in one scene if they thought the planes could be importing guns, the women scoff at the notion, many Tanzanians have also protested at this suggestion, chastising Sauper for not finding in Tanzania the glories of wildlife beauty and political stability. Even so, Sauper manages to make us think about some of the more oblique ways in which things are related in the globalised world.

All in all, Darwin is a momentous investigation, which refuses to see anything - ecology, economy or politics - in isolation. Excruciating it may be, but the film is not to miss. Catch it at Alliance Francaise, 6PM this Sunday.

Darwin's Nightmare
Director: Hubert Sauper.
2004. 107 min.