Answer:
The fish is likely to suffer a heart attack.
Explanation:
A fish has a single circulatory system, where the blood only flows through the heart once per cycle. The heart receives deoxygenated blood, also called O2 Poor-blood, then it's pumped to the gills, and rest of the body.
Since the fish is under extreme pressure having to swim upwards for an extended period, the blood reaching his heart will have too little oxygen and the muscle will eventually fail.