Double Haul -
Yes hatchery fish hurt wild fish. The question is how much? and in what way? and are the costs (impacts)worth the benefits? You asked about the summer program on the Stillaguamish so let's look at the specifics of the summer steelhead program on the North Fork Stillaguamish and its interaction with the Deer Creek fish.
Generally the three areas where hatchery impacts can be the greatest on the wild stocks are: 1) additional fishing pressure with potential wild fish mortalities (kept fish and/or hooking mortality), 2) genetic impacts from hatchey fish spawning with wild fish, and 3) lost of productive of natural spawners due to mal-adapted hatchery spawning with the wild fish.
Without a doubt having hatchery summer steelhead in the North Fork increases fishing pressure. The regulations requires the release of all wild steelhead thus the concern from the increased fishing pressure would confined to hooking mortalities. With Deer Creek itself closed to all fishing the returning adults have sanctuaries in which to escape the fishery. Adults begin entering Deer Creek in July and by early September most of the population is out of the North Fork thus out of the fishery - most of the returning adults would be exposed to only a month or two of fishing pressure. On the other hand because of the habitat problems in both Deer Creek and the North Fork stream temperatures are high than I or the fish would like. These higher temperatures means that hooking mortality could higher than normal. Bottom line there is some impact on the wild population from hooking mortality though that hasn't seen to have limited the populations from bouncing back from the horrible habitat problems in Deer Creek in the 1980s. They have rebound from less than returns of less than 100 fish a year in the late 1980s so that the returns in recent years (since 1996) have been in the 500 to 1,000 fish range.
The other 2 impacts are related and depends on the amount of hatchery fish that are spawning with the wild fish. In the case of the North Fork the hatchery fish are planted well upstream of Deer Creek (Whitehorse is more than 10 miles upstream) and the Deer Creek fish spawn well up in Deer Creek (above the Canyon). The limited sampling of adult steelehad in Deer Creek itself have not found any hatchery steelhead. This is support by the genetic information that is available (the steelhead (juveniles to limit the impacts on the population) were sampled over 3 years in the early 1990s. What was found was the Deer Creek fish were unique (different than any other popualtions) with good genetic diversity with little or no evidence of interactions with hatchery fish.
In summary the evidence under current conditions is that the Deer Creek fish are experiencing some limit hooking mortality with no apparent genetic or productive impacts from hachery and wild fish spawning together. That could change of course of conditions were to change (higher stream temperatures may push mortalities higher or some catastrophic event may cause hatchery fish to divert into Deer Creek). The question for each of us to consider and answer for ourselves is whether these "minor" impacts or risks worth the fishing oportunities.
What can be done with the hatchery program is look at the spawn timing to minimize the potential interactions with the wild fish. The native summers in most of Puget Sound spawn from early March to mid May. The hatchry stocks (Skykomish) currently spawn in January and Febraury. It appears that potential interactions are small. The other thing to consider is the magnitude of the releases. Are the number of smolts released so large that the number of hatchery adults escaping the fisheries greatly outnumber the wild population. In this case the season structure on the North Fork (switching from fly fishing to regular gear the first of December) means that the most of the hatchery summer fish are caught and killed (exploitation well over 90%) before they have had a chance to spawn. The number of fish being release doesn't seem to be producing many fish that survive the fishery.
I suspect in this case most would agree that the benefits (the fishing supported by the hatchery fish) are greater than the wild fish costs (limited hooking mortality and low likelyhood of hatvhery/wild genetic interactions). This type of evaluation process what is needed to answer your question and needs to be done case by case as every situation has its own unique twists.
Tight lines
Smalma