I imagine sterilising the ones you've caught is the easy part. How do you actually catch them all though?
They claim to have sterilised 150000 dogs. At the beginning of the programme, you could reasonably assume that every stray dog you see needs to be processed. Later on in the programme, you'll be releasing a lot of dogs that have already been processed, which seems like a lot more effort for the last 10% compared to the first 10%.
In these programs it is standard to mark the animal in some way. In NYC, for instance, you clip the top of one ear of a cat after it's been processed.
So you catch ten animals, immediately let go any ones that have been marked, process the last one or two -- still more effort to catch, but less effort to treat.
They could just not bother to catch them if they see ones with clipped ears. Also, catching them might be quite simple if you attract them with treats and treat them decently.
They claim to have sterilised 150000 dogs. At the beginning of the programme, you could reasonably assume that every stray dog you see needs to be processed. Later on in the programme, you'll be releasing a lot of dogs that have already been processed, which seems like a lot more effort for the last 10% compared to the first 10%.