I’d guess given technology advances you’d expect prices to go down, so the fact that they stay flat is actually indicating that fish are becoming harder to find. Similar to how if the price of something stays the same then it’s becoming cheaper because of inflation.

Also, fish are not a long term investment. The price only depends on how hard they are to find combined with demand to eat them sometime soon. If you could buy a fish now with the expectation of selling it later when stocks dry up then I think you’d see a lot more upward pressure.