Do you indicate when you merge?

It's not legally required for you to indicate, but it is best practice because other drivers might not have noticed that the lane is merging (i.e. the road is narrowing) and therefore may not be expecting you to move left or right. In an urban area, it's common for two lanes to merge into one.

Who is responsible for merging?

Decency. Merging laws dictate that drivers in both lanes of traffic are responsible for merging safety. This means that if you're merging you must pay attention and safely judge when to move over, and if you're in the through lane, then it's your responsibility to help create room for the merging vehicle.