Same thing happens with meat. There is a huge reduction of variety, even at the local fishmonger/butcher, compared to continental Europe (I'm sure it is coming though)
A limited choice is available all the time everywhere, but the variety is not anymore.
In 2014 UK fishing boats landed 424ktonnes of fish into UK ports, while we imported 720ktonnes of fish, mostly from outside the EU.
For example, if you visit a supermarket in the UK or the USA at any time of the year, you can always find, say, fresh strawberries, despite the fruit only being locally in season for a month or two.
There is very little genuinely "seasonal" produce, meat or fish in supermarkets and I think that this is a major factor in the lack of variety.
Out of interest, if you went to an Indian market every three months, would you find all 100 different varieties of fish on offer every time, or does it change?
- first, fish, as has been said about the US in this thread, is not eaten much inland (though some fresh-water fish is), due to distances and heat and spoiling, and while India has a large coast, it has an even larger interior.
- second, I doubt there are even close to 100 varieties of fish available (in markets) at the coasts, maybe 20 to 40; those who know more, feel free to correct me, I don't live near the coast (though did long ago), and don't eat fish.
- third, I don't think that in any of the types of markets, I mentioned, all the same varieties would be available all year round (and kiranas don't sell fish or meat at all, except some may, but only tinned). It is a mix of some staples and some seasonal items.
E.g. I've visited Goa (a coastal state) a few times, and at least at the beach restaurants / shacks, where they lay out the fish and other seafood varieties for inspection by customers, I don't remember seeing more than 20 or so items - kingfish, mackerel, red snapper, pomfret, crabs, a couple of types / sizes of prawns, squid, shark, that's almost it. One place that I went to a few times, had seafood special nights (the owner fished as a hobby), where there were a few more kinds, like barracuda, tuna, mussels, etc. These were in general touristy areas though. Local markets and restaurants may have more varieties.