A fisherman yanks a fish vertically out of the water with an acceleration of 2.5m/s^2 using very light fishing line that has a breaking strength of 22N. The fisherman unfortunately loses the fish as the line snaps. What can you say about the mass of the fish? It was too big doesn't cut it.

So, I've done this.

f=ma
22N= m(2.5m/s^2)
22N/ 2.5m/s^2= m
8.8kg=m

But this doesn't seem right. Should I add or subtract the force of gravity to a or am I totally on the wrong track. Any help is appreciated. Thank you.

You're correct, you've worked out the mass if the fisherman and fish were in free-fall!

Simply add the acceleration due to gravity onto the acceleration that the fisherman is providing.