Once you pass this sign, it isn't very far to the turnoff that will lead south towards Stewart Benchmark peak. This is not far after you go through a creekbed after coming down the hill into Horseshoe bend (may be dry later on). Click on the pic a couple times for highest resolution which will enable you to read the sign.
Lahoton Trout Natural ISA stands for Instant study area. It contains an area of over 12,000 acres and was established in 1973 and is not truly a wilderness area even though some call it that. Its main purpose is to help preserve this threatened species of fish and do so by maximizing its natural spawning areas. I was able to find a pdf by googling it but cannot copy to share here. If this is of interest to you, please take the time to learn more about this area.