Snowfish

Snowfish is a Rare fish from Frigid Cavern, and the cold is part of the recipe: you need Fish Head bait and Winter, though time of day and weather do not matter. The wiki describes a mysterious fish whose scales form intricate snowflake patterns that never melt.

It is mid-sized for the kind of catch it is. Average weight is 32.5 kg, from 20 kg to 94.5 kg, and the maximum ranks 284 of 663 fish. At roughly C$7.67 per kg, an average Snowfish sells for C$249.2, which ranks 467 of 742 documented fish by average value. The standout figure is the base XP: 450, which is higher than its price in coins.

Frigid Shrimp is the other Rare fish in the same cavern, so you may alternate between the two during a Winter session.

Verdict: this one is better for XP than for cash. If you are leveling, Snowfish gives a lot of progress for a Rare, and you can fish for it at any hour in any weather as long as it is Winter and you have Fish Head. If you only care about coin, there are better uses of the season. Keep one for the collection and sell extras.