Copying and pasting what Rod posted on another forum:
This batch of fish is specifically for the Chilliwack River, so they are trucked down from the hatchery down to several release locations. Three locations are usually used - Peach Road, Lickman Road, No. 3 Road in Yarrow the head of the canal. They are released in the lower river to slow down their movement once they are back in the river as adults, so we can make sure they are all caught and harvested before they reach the upper fishing boundary. This year's fish average between 60-80g, pretty close to target size. I'm not sure what the optimal size for smoltification, but hatchery raises them as close to smolts as they can be so they'd shoot straight down to the Fraser estuaries and leave to the Pacific Ocean immediately. A few months ago I sat through a presentation on Lower Fraser estuaries and how anadromous salmonids behave in this area and it is quite fascinating. Dave Scott is the main guy who does most of this research. He has determined the timing of arrival of juvenile salmon from different runs, and how long they would stay in the estuaries before leaving. Beside wild salmon, he has also found that hatchery salmon (chinook in the presentation I saw), would arrive near the end of the window after wild juveniles from the same run had spent a few weeks in the estuaries. The hatchery fish would only spend a couple of days in the estuaries and go straight into the ocean. This correspond with the hatchery's goal, to raise them to a larger size, release them slightly later than the out migration of wild juveniles, so they do not overlap. I'm almost certain that similar strategy is being used for steelhead in this case.