No fish or dolphins after 5 days

Started a Java realm with a couple of friends. After about 5 days, all the dolphins and fish in the oceans and rivers have disappeared. In the last two days, no new fish or dolphins seem to have spawned or are present in any ocean or river. To verify I then traveled around 7500 blocks of ocean and back, but still did not meet a single fish.

(Note: World download available but I don't know what file to add)