Bigeye Houndshark

Bigeye Houndshark is the heavyweight of Collapsed Ruins for its rarity, though it is only Unusual. To catch it you need Fish Head bait at Night in Clear weather, in Spring or Autumn. The wiki describes a slender shark with enormous eyes made for hunting in dim light, which fits the night requirement.

The size is the headline: 800 kg minimum, 1,650 kg average and up to 5,250 kg. That puts its maximum 107th of 663 fish with a recorded maximum, a very high spot for such a low rarity. The average sale, C$1,650, is 279th of 742 documented fish, so it pays better than most Unusual catches, but the base XP is a plain 150.

Longnose Chimaera is the only other Unusual fish in the same zone. If you are in Collapsed Ruins already, this is an easy target worth keeping an eye out for, mainly because its weight turns into a solid sale. It is not a reason to travel there, though. Treat it as a pleasant surprise on a night session, and sell it unless you need it for a collection.