Yes you can, IF fish fertilizer is your choice. IME it would be vary unusual for mushroom compost to have a high level of nutrients in it unless they were artificially added. Time and the activity of the soil bacteria are needed to make those nutrients available to the plants.

Your plants are far more likely to be benefiting from all the stuff you have added over the past years. Assuming a good level of beneficial soil bacteria the compost from this year will serve you well NEXT year.

Dave

FWIW, the clay soil probably has more nutrients than the compost. Compost is not a very rich fertilizer.

I live down the road from many mushroom houses. HUGE business here. Their mushroom compost is BLACK GOLD. I use it for everything. Plants of all types love it. I only use a couple feedings of fish/kelp on my tomatoes all year and they thrive. And those feedings are while seedlings. I won't feed a thing from here on out and my plants will be 10+ feet tall again. I'm also adding compost evey year.

How do the trees and shrubs and wild berries and wild flowers in the woods and fields grow? Who fertilizes these places? How do these plants all thrive without Blue Miracle Grow salts? Years and years of compost enriched soil is the answer. And if you need a boost this year by all means use your natural fish emulsion. Good Growing!

Fish/Kelp/Compost all work great together...;-)

Mushroom compost is not necessarily "Organic" because inorganic fertilizers are often included to feed the mushroom growth along with the organic nutrient sources. So much fertilizer is left over that it can burn seedlings.

http://extension.oregonstate.edu/news/story.php?S_No=28&storyType=garde

I have 10 plants out in mushroom compost(I also mixed in some surrounding soil). When I planted them I also mixed in some espoma tomato tone. I also watered them with miricale grow the first 2 weeks.(once each week) The other 2 weeks we had had a LOT of rain, and they have been in standing water quite a bit of the last 2 weeks.
I only have 2 plants that aren't looking right, and I'm not sure if it's over fertilizing or too much water. The rest look great.
I would fertilize a couple smaller plants if I were you, and give it a week or so, and follow thier progress.
Fred

Mushroom compost IS called "black gold" by many organic gardeners. But it is NOT fertilizer. The NPK of mushroom compost comes out at 0.5-0.-0.5. But it becomes active with those wonderful micro-organisms when added your soil as an soil amendment. Over the years I have added hundreds of pounds of mushroom compost to my Georgia red-clay 1/4 acre garden. Not that color of soil matters, however, now my garden is nothing but beautiful black soil. But it still needs fertilizer.