UK broadcast 2 November 2009, 4.56 million viewers (18.1% audience share)

Fish, the most diverse group of vertebrate animals, thrive in the world's rivers, lakes and oceans. Slow-motion footage reveals the behaviour of some of the fastest fish in the sea, sailfish and flying fish. The latter gather in large numbers to lay their eggs on a floating palm frond, which sinks under the weight. The eggs of weedy sea dragons, found in the shallow waters off southern Australia, are carried by the male. In the fertile seas of the western Pacific, competition is fierce. A sarcastic fringehead defends its home, an old shell, from a passing octopus and a rival. In Japan, mudskippers have carved a niche on the rich mudflats. Freshwater fish are also featured. Tiny gobies are filmed climbing Hawaiian waterfalls to colonise the placid pools upstream, while in East Africa, barbels pick clean the skin of the resident hippos and feed on their rich dung in return. Wrasses perform the cleaning duties on coral reefs, but jacks also remove parasites by scratching against the rough skin of silvertip sharks. Clownfish, whose life cycle is filmed in intimate detail using macro cameras, are protected by the fronds of an anemone, but other species seek safety in numbers. A shoal of ever-moving anchovies proves too difficult a target for sea lions. Sometimes, predators have the edge: ragged tooth sharks are shown attacking sardines trapped in shallow waters off South Africa. Life on Location looks at the efforts of underwater cameramen to capture the sailfish and flying fish sequences.