Silt is one kind of sediment that can be transported and deposited by water or ice. Sand is another kind of sediment. Silts are like mud or clays. Silts can be fine or coarse grained. The more coarse they are the faster they settle out of solution.
Any kind of silt settling on fish eggs is harmful, and lethal if deposited in enough concentration. Any kind of silt is harmful to fish. Fish living in naturally silty waters, like those of the Homathko system in Bute Inlet, have evolved strategies for coping with and avoiding silty conditions.
Silt can be round-edged or sharp-edged. Silt that has bumped along in water courses for a long time tends to the round-edged while those that settle in high elevation lakes tend to be sharp-edged (not necessarily but generally speaking).
Suspended sharp-edged silt is the most dangerous to fry, or larger fish as well as virtually any living thing; especially invertebrates, insects, etc. Basically, silt is a limiting factor to biological productivity of streams, although it might be terrific for your garden!
A sudden flush of silt in a watershed that is not naturally prone to silty waters can be devastating to fry, resident fish and many other living things. Nature has put some silt into all wild rivers and creeks but that is not a good reason for humans to do it! Case law around the Fisheries Act is very clear that any human-caused sedimentation is a breach of the Act.