The idea with seasons in the game would give players something different to focus on every 4 days to keep the game fresh and interesting. Also to focus efforts on certain species they wish to trophy for personal, trophy location cards and now qualifications.
For me personally I'm still yet to get a Frog, Ide & Perch for my Mosquito trophy card. If there were seasons I could focus on a certain species that has been alluding me for some time. I could chase the missing Ide during Summer for example.
Seasonal fish species get an additional 10% chance of bite rate & 10% increased chance of a trophy.
(along the same lines as a 10% bonus you get with certain fishing rods)
3 month season = 90 days = 90 IRL (In Real Life) hours = 4 days
4 days = 1 Season
IRL Days 0-4 - December, January and February are WINTER
IRL Days 5-9 - March, April, May are SPRING
IRL Days 10-14 - June, July and August are SUMMER
IRL Days 15-19 - September, October, November are AUTUMN
The offset approach not only falls inline with with 24 game hours = 1 IRL hour.
It also allows the game seasons to be staggered with real life. So those of us who cannot play this game everyday and may miss out on a season they were focusing on, will get the opportunity again in another 2 weeks time.
Spring - Atlantic Salmon
Summer - Pink Salmon
Autumn - Taimen Trout
Winter - Pike
In addition to this the weather should reflect the season patterns:
The wetter season lasts 5.9 months, from 14 May to 11 November, with a greater than 25% chance of a given day being a wet day.
The chance of a wet day peaks at 35% on 21 June.
The drier season lasts 6.1 months, from 11 November to 14 May. The smallest chance of a wet day is 16% on 20 March.
With the addition of more focused weather patterns could also give players the ability to target certain species.
Certain fish species do better in rain while others in full sunshine.
*** Forgive any inaccuracies with data. I'm relying on Google for Russian seasonal information.
But hopefully you get the general idea.