Evan Rees at Stratfor writes:
Over the past four decades, China has gradually abandoned its self-imposed isolation in favor of deep ties with global markets. Though the approach has pushed the Chinese economy to new heights, it has also made the country's supply lines more vulnerable, a reality to which the Chinese military has had to adapt. The seas — not the land — are now the key to China's economic security and regional dominance, and protecting them has become one of Beijing's greatest concerns.
But safeguarding the South China Sea, the most valuable of China's waterways, is no easy task, and Beijing has employed a variety of creative tactics to try to do so. In addition to building up islands and troop numbers alike, China has encouraged its fishermen to venture out into the disputed waters. The civilian fleet, which has spread across the territory staked by the "nine-dash line", defends China's claims as any navy might by harrying and diverting the ships of its competitors...
[T]he Chinese government has made an effort in recent years to build up a small subset of its fishing fleet: the maritime militia. Though these fishermen still complete their normal activities, they do so equipped with light arms, better vessels and monitoring equipment, ready to respond to the needs of China's leaders. Their movements ebb and flow with those of the fisheries, but they spend more time at sea — and in more obscure locations — than the comparatively conspicuous coast guard or naval vessels, giving Beijing a more granular picture of (and some measure of ambient control over) its sprawling maritime domain. As China continues to expand its reach in the contested waters of the Asia-Pacific, the importance of the maritime militia in defending those claims will only grow.
The full analysis is here.