Carole, There are three options: the slip lead, the box trap, the catch pole. If a dog is trusting enough to come close and walk into the noose of a slip lead but then panics and tries to bolt, it tightens and chokes him, and he'll be wary of a lead next time. If a stray dog will not walk into the 'noose' of a slip lead, either a box trap or a catch pole is used. The catch pole usually leaves a dog traumatized.
There are 'humane' approaches to using a catch pole in the least traumatizing way, but it takes training, and training costs money. Sometimes the catch pole is the only way to capture a stray dog if the dog is in an area where a box trap cannot be set safely.
I don't know how Grizzley was brought to the shelter, but those are the options.
I hope he is learning that people with leashes can be a good thing to happen, and will find a foster or forever home soon.