Brown trout are rebuilding their condition after winter
Anglers often report catching fish early in the season that are below average condition. Brown trout spawn between May and July. During this period they have eaten little and spent considerable energy moving up through the cold fast water to the spawning grounds. Male brown trout can mature as early as two years of age and females at three. They have been known to live over twenty years. During their life they will spawn multiple times.
It is not unreasonable for some fish you catch at this time of year to still be in the rebuilding phase. Some older fish may never recover. A good healthy fishery has a wide range of fish ages and sizes indicating successful spawning in most years. Typically as the weather warms more food will become availble and the trout will fatten up.