Why? "Rōvyz uēhossa" is nominative plural and "big squid" is plural, too. (One squid, two squid, three squid, same as with one fish, two fish, three fish) "kastra qaedrī" is accusative plural and "blue whales" is plural, too. Everything looks just fine.

If I am not mistaken, the singular would be: Rōvyz uēs kastor qaedri izūgas.

Rōvys kastyz uēhossa qaedrī izūgis.

Solar plural declension has this special case where it's normally "-ys", but it becomes "-yz" before a vowel, "h", or voiced consonant. This is why they change around a little.