I approached Fish Mountain from Hwy 230. Hwy 230 starts near milepost 56 on Hwy 62 (Crater Lake Highway) just north of the Union Creek Campground and ends when it meets Hwy 138 (Diamond Lake Highway) near Mt Thielsen. From the cutoff to Crater Lake, travel about 12.2 miles north on Hwy 230 to the turnoff for Hamaker Campgound. If you are traveling from the north Diamond Lake area, it is about 11.4 miles from the intersection with Hwy 138. Do not turn east towards the Hamaker Campground, instead turn west towards the Rogue Umpqua Divide Wilderness on forest road 6560.
Stay on the main track of forest road 6560 as it climbs westward. When you go over a little pass at about 5,000 ft and 4.0 miles from hwy 230, the road crosses the national forest boundary and you leave the Rogue River National Forest and enter the Umpqua National Forest. The road then becomes road 37. Continue down to about 5.9 miles from Hwy 230 and you will see a fork to the right that is uphill and signed as road 800. Take this fork.
Road 800 is not a nice as road 37, but an ordinary vehicle should have no problem following the road all the way to the end at about 14.3 miles from Hwy 230. The road changes to road 870 at about mile 9.5 as it enters the Rogue Umpqua Divide Wilderness. This is a unique road in my experience in wilderness areas. There is no motorized anything allowed in wilderness areas, but this road is through a 400 ft wide corridor for 4.5 miles deep into the wilderness that has been specifically eliminated from the wilderness and is not subject to the wilderness regulations. There are several campsites developed by hunters along this road and there are a couple of trailheads also. Ignore all of that and just drive all the way to the end where there is parking for 5-6 vehicles. The end of the road is the trailhead for both Fish Mountain and Castle Rock that is just north of the parking area.
This area gets lots of snow in the winter and the roads are not accessible from the first snow storm in November until probably late June or early July before the snow melts from the road.
The trailhead for Fish Mountain is at 5,910 ft in elevation. There are 2 trails leaving the parking area. One goes north and looks like ATV’s have gone up the trail a ways, but this trail goes towards Castle Rock and not Fish Mountain. If you look back down the road that you just came up, you can see the summit of Fish Mountain southeast of the parking area. There is a nice trail on the south side of the parking area. Take this trail south.
You only want to stay on the trail about 100 ft or until you can see a way through the brushy trees on your left. You need to climb up to the top of the ridge and follow it all the way to the summit. Initially the brush is a little bothersome, but it lightens up after a few hundred feet up the ridge. Pick the best route that you can with the goal of getting to the top of the south ridge. The west side of the ridge is a lot steeper than the east side, so I either stayed on top of the ridge or used the east side to get around some brushy areas or rocky outcroppings.
As you continue up the ridge, you will start getting some views to the south, including views of Mt McLoughlin and Rabbit Ears. There are a few rocky areas that need to be climbed or skirted. Do whatever you are comfortable with.
There is nothing difficult about finding the summit. Just keep going up until you are at the obvious highpoint. Return to the trailhead the same way you ascended.
The total hike covers about 1.75 miles, gains about 880 ft in elevation, and took me 1.25 hours.
The 10 essentials are always required. There is no water on the trail, so take all you need. A GPS may be handy to find the parking lot if you stray off the ridge on the way down.