I put this together, which should work for you:
The data is in a json array file in the project settings and I import it into an array object using the AJAX object.
If you run the project in debug you can check the a_fish object and see that the probabilities have been added correctly.
I've used percentage figures you supplied, but a bonus of this system is that you don't have to use percentages - you can use counts (10 of fish A, 6 of fish B etc) and it will still work fine, meaning that if you change the value for one fish you don't have to recalculate the percentages for the other fish.
Each fish is a separate object as you requested, but if you had a single fish object with each fish as an animation then you could set the animation in a single event directly from the "fish" variable rather than needing an event to check for each fish object.
In Construct 3 there is a probability table feature in the Advanced Random plugin which makes this very easy to do. Sadly there isn't that option for C2.